This Document Contains Chapters 19 to 24 Chapter 19 Precambrian Earth and Life History Chapter Outline 19.1 Introduction 19.2 What Happened During the Hadean? GEO-FOCUS 19.1: The Faint Young Sun Paradox – An Unresolved Controversy 19.3 The Foundations of the Continents—Shields, Platforms, and Cratons 19.4 Archean Earth History 19.5 Proterozoic Earth History 19.6 Origin and Evolution of the Atmosphere and Hydrosphere 19.7 Life—Its Origin and Early History 19.8 Resources in Precambrian Rocks Key Concepts Review Learning Objectives Upon completion of this material, the student should understand the following. • The Precambrian encompasses more than 88 percent of all geologic time, yet we know less about its history than we do about more recent intervals of geologic time. • During the earliest part of the Precambrian, called the Hadean, Earth accreted, differentiated into core and mantle, formed some crust, and was struck by a planetesimals that led to the formation of the Moon. • The Archean Eon was a time during which several small continental nuclei began to evolve, and the most common rocks were granite-gneiss complexes and greenstone belts. • During the Proterozoic Eon, an essentially modern style of plate tectonics developed, and crust that formed during the Archean amalgamated into a large craton we now call Laurentia. • Proterozoic rocks consist of sandstone-carbonate-shale assemblages, red beds, banded iron formations, glacial deposits, and a variety of others. • The Precambrian atmosphere evolved from one that lacked free oxygen and an ozone layer, but even at the end of the Precambrian, the atmosphere had much less free oxygen than it does now. Chapter Summary • Geologists use a threefold division of the Precambrian—the Hadean, Archean, and Proterozoic. • Each continent has an ancient, stable craton made up of a Precambrian shield and platform. The Canadian shield in North America is made up of several subunits. • Archean rocks are mostly granite-gneiss complexes and subordinate greenstone belts. One model for the origin of greenstone belts holds that they formed in back-arc marginal basins. • The amalgamation of Archean cratons and continental accretion along their margins account for the origin of a large landmass known as Laurentia. • Many geologists think that Archean plates moved faster than plates do now because Earth possessed more radiogenic heat. • Sandstone-carbonate-shale assemblages deposited on passive continental margins and in intracratonic basins are the most common Proterozoic-age rocks. • Widespread glaciers were present during the Paleoprotoerozoic and Neoproterozoic. • Earth's earliest atmosphere lacked free oxygen, but it was rich in carbon dioxide. It was derived by the release of gases during volcanism, a process called outgassing. Meteorite and comet impacts and outgassing yielded the hydrosphere. • Deposition of widespread banded iron formations between 2 and 2.5 billion years ago and the first red beds about 1.8 billion years ago indicate that some free oxygen was present in the atmosphere. • Energy such as lightning and ultraviolet radiation acting on chemical elements present on early Earth may have yielded the first living things. Some investigators think that RNA molecules were the first molecules capable of reproduction. • All known Archean fossils represent prokaryotic bacteria. Stromatolites formed by photosynthesizing bacteria may date from 3.5 billion years ago. • Endosymbiosis practiced by prokaryotic cells was probably responsible for the first eukaryotic cells. • The oldest megafossils are carbonaceous impressions, probably of algae, in rocks more than 2 billion years old. • The Neoproterozoic Ediacaran faunas include the oldest well-documented animal fossils. None had durable skeletons, so their fossils are not common. Enrichment Topics Topic 1. Origin of the Moon. Just after the Earth was fully formed, about 4.5 billion years ago, an asteroid as large as the planet Mars struck. Metallic materials from the asteroid were heavy and stayed with the metals of the Earth’s core. The rocky material was lighter and flew into space, encircling the planet in a thin ring. The debris came together violently, pebbles smashed to become rocks, rocks crashed into boulders and finally the Moon was born. Initially, it was so hot that it was entirely molten. As it cooled, it separated into layers including a crust, a mantle, and possibly a small metal core. In the crust, minerals light in color and density rose to the top of the lava ocean to form the lunar highlands, while the darker denser minerals sank, later serving as the source of the mare basalts. This theory, known as the impactor theory, accounts for why the Moon's density is relatively low—after the impact, the metals were left in the Earth. It explains why the Earth's orbital speed is slightly higher than it should be; the asteroid hit the Earth off-center. It also gives a reason that the Moon has no atmosphere; things were so hot after the impact that the gases volatilized (the Earth’s atmosphere primarily formed from gases that came from the mantle during later volcanic eruptions; the Moon has been geologically dead since early in its history). In addition, the Moon is moving away from the Earth at about 3 cm per year; it is likely then that they started together. Imagine a moonrise 4 billion years ago when the Moon was more than 40 times larger than it is today! Topic 2. Precambrian Plate Tectonics. Scientists know plate tectonics operated in the Precambrian because of the presence of ophiolites and other characteristic rock sequences. Precambrian tectonics was different from tectonics today because heat flow was much higher due to the radioactive decay of isotopes with short or medium length half-lives such that they are now completely decayed. Heat flow at 3600 Ma (million years ago) was about three times that of today and at 2600 Ma was about twice that of today. Continental crust at the beginning was a chemical differentiate and there was not very much of it. This would have resulted in higher geothermal gradients, thinner lithosphere, higher convection rates, and therefore higher plate motion rates, smaller and more mobile plates, and perhaps more hotspots. Topic 3. The Origin of Life. For decades scientists have thought that life originated in the primordial soup that was the Precambrian ocean, ignited perhaps by a bolt of lightning. Now many are questioning this hypothesis and are suggesting that life began in hydrothermal vents, hot springs found in the ocean depths. In one new variation of this hypothesis, the walls of tiny chambers in iron sulfide vent chimneys act as early cell walls, dodging the problem of how a cell could form if it couldn’t keep itself together and how it could form a cell membrane to keep itself together if it couldn’t form. When fundamental chemical ingredients such as carbon dioxide and hydrogen are present with iron sulfide at the hot, high pressure conditions of the vents, they begin cycles that look like metabolism. In addition, many proteins that drive basic biochemical reactions rely on smaller iron sulfur cofactors. These basic chemical ingredients are so common that life may be just about inevitable elsewhere in the universe. Science News, April 26, 2003 v163 i17 p264(3). Common Misconceptions Misconception 1: Earth, although possibly old by human standards, is really very young. Also, as a consequence, the landscape has always looked pretty much the way it does now. Fact: Earth is actually much older than most people believe. Hopefully, this will be made clear in this course, and this will be one idea which the students will carry away with them. The landscape which we see today is in fact, geologically very young, and only the most recent which Earth has had through a seemingly endless continuum of change. Misconception 2: Life originated just once in Earth history. Fact: It is likely that life originated multiple times and was wiped out as conditions changed. For example, had Earth been populated with life forms at the time the giant planetesimal that resulted in the formation of the Moon struck the heat would have wiped it out. Obviously life would then have originated at least one more time. Lecture Suggestions 1. To present a vivid image of the Precambrian world, have the students try to envision a world with nothing living on the land surfaces. An image of a very stark landscape is easy to imagine; think of the images sent back to Earth from the Mars Rover! 2. Paleontologists deal with an incomplete fossil record and must recognize the limitations of their data base. Because of this, there are several types of conclusions and interpretations they might make. First, and the most sound, are conclusions based on strong evidence that has no contradictory information. Second, are conclusions that are very likely to be right and are reasonable interpretations. The third and fourth types require more of a leap of faith. Speculation may be made if there is a good but fairly weak body of evidence. Finally, reasonable interpretations represent anyone’s best idea, but these interpretations are untestable and may be considered at best creative thinking. 3. Banded iron formations represent a significant economic resource. Ninety percent of the world’s iron ore (1 billion tons yearly) comes from these types of deposits. From the period of 1.8 to 2.0 billion years, it is estimated that 1014–15 tons of iron was precipitated. 4. Proterozoic rift basins are not only interesting from a tectonic and sedimentologic standpoint but also from an economic standpoint. Mid to Early Proterozoic rift basins represent the first major presence of sediment-hosted sulfide deposits and are a major source of the world’s lead and zinc. At deposits such as Sullivan in British Columbia, there is evidence that sulfide minerals were precipitated from hydrothermal vents and accumulated at the sea floor. These differ from black smoker type deposits because the Proterozoic deposits are spatially associated with rift basin growth faults but generally not with volcanism. 5. The late Stephen Jay Gould’s book Wonderful Life contains excellent commentaries on science, evolution and extinction. Gould discusses contingencies in the geologic record or the “what if” factor. The fauna of the Burgess Shale contained many species that may have been from phyla that went extinct. What if those phyla had survived and the ones that gave rise to modern life gone extinct; what would this world be like today? Consider This 1. According to the plate tectonic theory, what should be the orogenic-shield-craton structure of all of the continents? Answer: According to plate tectonic theory, continents typically feature orogenic belts at their margins where tectonic activity creates mountain ranges. Within the interior, these regions evolve into shields, which are exposed Precambrian crystalline rocks. The most stable parts of continents are cratons, encompassing both the ancient shields and the surrounding sedimentary platform areas. 2. The Belt Basin in northern Idaho and western Montana was the site of the accumulation of a phenomenal amount of silt- and clay-sized sediment. Have students calculate the amount of time it would take to deposit 42,000 feet of sediment, given deposition rates ranging from 1 to about 5 mm/year. Answer: To deposit 42,000 feet (12,801,600 mm) of sediment at deposition rates between 1 and 5 mm/year, calculate the time as follows: So, deposition would take between approximately 2.56 and 12.8 million years. 3. Why is lead not a major economic commodity in deposits older than Mid-Proterozoic? Answer: Lead is not a major economic commodity in deposits older than Mid-Proterozoic because lead tends to migrate or be leached away over billions of years due to its high mobility. Additionally, significant lead deposits typically form in younger geological settings with specific mineralization processes. Older rocks often have experienced extensive geological alterations that dilute lead concentrations. 4. How has the distribution of banded-iron formations affected the development of the automobile industry in the United States? Answer: The distribution of banded-iron formations (BIFs) has significantly impacted the U.S. automobile industry by providing a primary source of iron ore, essential for steel production. Regions rich in BIFs, like the Lake Superior area, have become key steel manufacturing hubs. This availability of high-quality iron ore has supported the growth and development of the automobile industry by ensuring a steady supply of raw materials. Internet Sites, Videos, and Demonstration Aids Internet Sites 1. University of California Museum of Paleontology http://www.ucmp.berkeley.edu/ 2. Smithsonian National Museum of Natural History http://www.mnh.si.edu/ 3. The Origin and Evolution of Life http://cmex.ihmc.us/VikingCD/Puzzle/Evolife.htm Videos 1. Earth’s Catastrophic Past. Insight Media. 2. Miracle Planet. Insight Media. 3. Earth Revealed #10: Geologic Time. Annenberg/CPC Collection. Slides 1. JLM Visuals Physical Geology, 2 CD Rom set. 2. Fossils of the Precambrian and Lower Paleozoic I and II. JLM Visuals. 3. Life of the Precambrian and Lower Paleozoic. JLM Visuals. 4. Precambrian Era. Wards Geoscience. 5. Answers to Figure-Related Critical Thinking Questions ❯❯ Critical Thinking Question Figure 19.6 What kind of unconformity is present between the Precambrian and Cambrian rocks, and what is the duration of the hiatus? Answer: An unconformity is a gap in the rock record between Cambrian times (~550 m.y. ago) and the pre-Cambrian (anything earlier). An unconformity is a surface in the rock record, in the stratigraphic column, representing a time from which no rocks are preserved. In Figure 19.20, the gap is about 2.5 billion years. ❯ Critical Thinking Question Figure 19.7 How does pillow lava form? Answer: In the 1960s, divers found Hawaiian lava flows entering the Pacific forming basalt pillow structures. Pillow lava forms by cooling in water. We now know that these structures are common in the upper surface of the newly formed ocean crust – part of the ophiolite complex, forming at divergent boundaries in ocean basins. The outer part touching the water cools and hardens, while the inner portion is still molten. This inner portion is still fed by flowing lava and that pressure pushes the outer crust apart to form new pillows, etc. ❯❯ Critical Thinking Question Figure 19.17 Which of these processes, photochemical dissociation or photosynthesis, was the most important in contributing free oxygen to the Precambrian atmosphere? Answer: Photochemical dissociation created a protective barrier to harmful UV radiation to assist the development of life on Earth. That initial life was “pond scum” – an algal mat that polluted its own environment with a poison gas, oxygen, through photosynthesis! O2 built up in the ocean water and contributed to the formation of hematite in those basin sediments (as a banded iron formation) but in time was released into the atmosphere to react with iron on land (red beds). BIF is not found after ~1.8 Ga because of the success of photosynthesis in establishing oxygen in the Precambrian atmosphere: so – yes – photosynthesis is the more important of the two in contributing O2. ❯❯ Critical Thinking Question Figure 19.20 Stromatolites were widespread during the Proterozoic Eon, but now they have a restricted distribution. Why? Answer: Stromatolites are a major constituent of the fossil record for about the first 3.5 billion years of life on earth, peaking about 1.25 billion years ago. They subsequently declined in abundance and diversity. It’s thought that one reason for the decline is the evolution of more complex organisms which grazed on stromatolites. Another explanation is the change in Earth chemistry. Suggested Answer to Selected Short Answer Question (Answers to question 6 and question 7 provided in the appendix to the text) 8. What are the most common types of Archean rocks and how are they thought to have formed? Suggested Answer: Archean rocks comprise the granitic-genesis complex. Granitic gneiss and granitic plutonic rocks are the most common, both of which were probably derived from plutons emplaced in volcanic island arcs. Sedimentary rocks include conglomerate, carbonates, and chert. The most common types of Archean rocks are igneous and metamorphic rocks, such as granite, basalt, and gneiss. They are thought to have formed from early volcanic activity and subsequent metamorphism during the Earth's formative years, when the planet was still cooling and developing its crust. Chapter 20 Paleozoic Earth History Chapter Outline 20.1 Introduction 20.2 Continental Architecture: Cratons and Mobile Belts 20.3 Paleozoic Paleogeography 20.4 Paleozoic Evolution of North America 20.5 The Sauk Sequence 20.6 The Tippecanoe Sequence 20.7 The Kaskaskia Sequence 20.8 The Absaroka Sequence 20.9 History of the Paleozoic Mobile Belts GEO-INSIGHT 20.1: The Devonian Old 20.10 What Role Did Microplates and Terranes Play in the Formation of Pangaea? 20.11 Paleozoic Mineral Resources Key Concepts Review Learning Objectives Upon completion of this material, the student should understand the following. • Six major continents were present at the beginning of the Paleozoic Era, and that plate movement during the Paleozoic resulted in continental collisions leading to the formation of the supercontinent Pangaea at the end of the Paleozoic. • The Paleozoic history of North America can be subdivided into six cratonic sequences, which represent major transgressive-regressive cycles. • During the transgressive portions of each cycle, the North American craton was partially to completely covered by shallow seas in which a variety of detrital and carbonate sediments were deposited, resulting in widespread sandstone, shale, reef, and coal deposits. • Mountain-building activity took place primarily along the eastern and southern margins (known as mobile belts) of the North American craton during the Paleozoic era. • In addition to the large scale plate interactions during the Paleozoic, microplate and terrane activity also played an important role in forming Pangaea. • Paleozoic-age rocks contain a variety of important mineral resources. Chapter Summary • Tables 20.1 and 20.2 summarize the Paleozoic geologic history of the North American craton and mobile belts, as well as global events and sea-level changes. • Most continents consist of two major components: a relatively stable craton over which epeiric seas transgressed and regressed, surrounded by mobile belts in which mountain building took place. • Six major continents and numerous microcontinents and island arcs existed at the beginning of the Paleozoic Era; all of these were dispersed around the globe at low latitudes during the Cambrian. • During the Early Paleozoic (Cambrian-Silurian), Laurentia was moving northward, whereas Gondwana moved southward and began to cross the South Pole as indicated by tillite deposits. • During the Late Paleozoic, Baltica and Laurentia collided, forming Laurasia. Siberia and Kazakhstania also collided and were then sutured to Laurasia. Gondwana continued moving over the South Pole and experienced several glacial-interglacial periods, resulting in global sea-level changes and transgressions and regressions along the low-lying craton margins. • Laurasia and Gondwana underwent a series of collisions beginning in the Carboniferous and during the Permian the formation of Pangaea was completed. Surrounding the supercontinent was a global ocean, Panthalassa. • Geologists divide the geologic history of North America into cratonic sequences that formed as a result of cratonwide transgressions and regressions. • The Sauk Sequence began with a major marine transgression onto the craton. At its maximum, the Sauk Sea covered the craton except for parts of the Canadian shield and the Transcontinental Arch, a series of large, northeast-southwest trending islands. • The Tippecanoe Sequence began with deposition of an extensive sand unit over the exposed and eroded Sauk landscape and was followed by extensive carbonate deposition. In addition, large barrier reefs surrounded many cratonic basins, resulting in evaporite deposition within these basins. • The basal beds of the Kaskaskia Sequence that were deposited on the exposed Tippecanoe surface consisted either of sandstones derived from the eroding Taconic Highlands or of carbonate rocks. • Most of the Kaskaskia Sequence is dominated by carbonates and associated evaporites. The Devonian Period was a time of major reef building in western Canada, southern England, Belgium, Australia, and Russia. • The Mississippian Period was dominated, for the most part, by carbonate deposition. • Transgressions and regressions over the low-lying North American craton, probably caused by advancing and retreating Gondwanan ice sheets, resulted in cyclothems and the formation of coals during the Pennsylvanian Period. • Cratonic mountain building, specifically the Ancestral Rockies, occurred during the Pennsylvanian Period and resulted in thick, nonmarine detrital sediments and evaporites being deposited in the intervening basins. • By the Early Permian, the Absaroka Sea occupied a narrow zone of the south-central craton. Here, several large reefs and associated evaporites developed. By the end of the Permian Period, this epeiric sea had retreated from the craton. • The eastern edge of North America was a stable carbonate platform during Sauk time. During Tippecanoe time, an oceanic-continental convergent plate boundary formed resulting in the Taconic orogeny, the first of three major orogenies to affect the Appalachian mobile belt. • The newly formed Taconic Highlands shed sediments into the western epeiric sea, producing a clastic wedge that geologists call the Queenston Delta. • The Caledonian, Acadian, Hercynian (Variscan), and Alleghenian orogenies were all part of the global tectonic activity that resulted in the assembly of Pangaea. • The Cordilleran mobile belt was the site of the Antler orogeny, a minor Devonian orogeny during which deep-water sediments were thrust eastward over shallow-water sediments. • During the Pennsylvanian and Early Permian, mountain building occurred in the Ouachita mobile belt. In addition to the mountain range the Ouachita orogeny produced, this tectonic activity was partly responsible for the cratonic uplift in the southwest, resulting in the Ancestral Rockies. • During the Paleozoic Era, numerous microplates and terranes – such as Avalonia, Iberia-Armorica, and Perunica – existed and played an important role in forming Pangaea. • Paleozoic-age rocks contain a variety of mineral resources, including petroleum, coal, evaporites, silica sand, lead, zinc, and other metallic deposits. Enrichment Topics Topic 1. Pangaea’s Parent. Pangaea formed as Earth’s continents came together to create one supercontinent and began to break up about 180 million years ago. But where had the continents that formed Pangaea come from? The Wilson or supercontinent cycle states that the continents drift from one side of the Earth to the other, where they collide, form a supercontinent, break up, and then drift around only to meet up on the other side. Of course, the rocks that would contain the record of the supercontinent before Pangaea would be quite old and altered. Nonetheless, scientists have named Pangaea’s parent Laurentia. Converging flow patterns in the mantle bring the continents together with a bang. The continental crust insulates the mantle beneath, causing it to get really hot. Hot mantle rises as a superswell and eventually rips apart the supercontinent above. Laurentia, which fits this model well, came together quickly about 1.9 billion years ago as seven microcontinents. Enormous volumes of red granites and rhyolites, which are anorogenic, meaning not related to mountain building, developed as the lower crust melted. The source of heat for the igneous rocks was the superswell. http://www.ucmp.berkeley.edu/cambrian/cambtect.html Topic 2. A Chronology of Paleozoic Sea-Level Changes. Stratigraphic sections yield the fine-scale history of sea-level fluctuations for the entire Paleozoic. In all, 172 eustatic events, from a few tens of meters to approximately 125 meters have been documented. Science 3 October 2008, vol. 322 no. 5898. http://www.sciencemag.org/content/322/5898/64.abstract#aff-1 Topic 3. Paleozoic Oxygen Levels. The modern atmosphere contains 21% oxygen. But this has not always been the case. Around 300 million years ago oxygen concentrations were much higher and have been linked to gigantism in some animal groups, especially insects. The higher oxygen levels were the result of the colonization of the land by plants. With higher oxygen levels, wildfires would have been abundant. Fortunately, oxygen levels stabilized at a lower concentration about 50 million years ago. http://www.sciencedaily.com/releases/2010/08/100802091125.htm Common Misconceptions Misconception 1: The Grand Canyon is billions of years old because the rocks exposed in it are that old. Fact: The oldest rocks exposed in the bottom of the Grand Canyon are quite old. The Vishnu complex originated as sediments and volcanic rocks which may have been deposited on the sea floor between 1.8 and 2 billion years ago. However, the downcutting by the Colorado River, which produced the Canyon now seen, began only a few million years ago. Misconception 2: Oceans and seas are underlain by oceanic crust. Fact: True ocean basins are created by the density and isostatic positioning of oceanic crust. But many regions that are covered by water are underlain by continental crust. During the transgressions of the Paleozoic, seas covered much of North America’s continental crust. Lecture Suggestions 1. Point out the importance of river deltas in the growth of trading and cultural centers. Modern deltas such as on the Nile, Yangtze, and Mississippi Rivers have prominent cultural development. The Queenston Delta formed in much the same way, as part of a fluvial system. The rivers responsible for this delta flowed from a growing highlands area into the Proto-Atlantic Ocean. 2. Reinforce the importance of time-stratigraphic markers by talking about the significance of Ordovician age bentonite deposits in the Appalachians. These regionally extensive bentonite deposits are interpreted to be volcanic ash layers related to island arc volcanism during the early mountain building stages of the Appalachians. 3. Before the concept of accreted terranes became so important to interpreting the tectonic development of an area, geologists did not have a good theoretical foundation to explain the diverse geology of the Appalachians. Read John McPhee's excellent book In Suspect Terrane for an interesting account of the development of geologic thought about the Appalachian Mountain chain. 4. Discuss with students how geologists working in the Appalachians have managed to unravel such a complicated geologic story. 5. Take this one step further. Discuss how geologists have put together the geologic history of North America or the world. How can we know anything at all about events that happened so very long ago? Consider This 1. Have students try to list the types of data that geologists might collect to piece together ancient mountain ranges that are now separated by a vast oceanic region. How confident can geologists be that the Appalachians, Caledonides, Alleghenian, and Hercynian orogenies were all closely related in time and space? Answer: Geologists might collect data such as fossil distributions, rock types, structural geology, and geochronology to piece together ancient mountain ranges. They can be quite confident about the relatedness of the Appalachians, Caledonides, Alleghenian, and Hercynian orogenies due to their similar age, geological features, and evidence of past continental connections. 2. Knowing what events occurred with the amalgamation of Pangaea in the Paleozoic, have students speculate on what might happen should the Atlantic and/or Pacific Oceans close in the future. What types of new mountain ranges might form? What would happen to old orogenic zones? Answer: If the Atlantic or Pacific Oceans were to close, new mountain ranges could form due to the collision of continents, similar to the Himalayas from the India-Eurasia collision. Old orogenic zones might become reactivated or further deformed, possibly leading to renewed tectonic activity or reorganization of existing mountain belts. 3. The Paleozoic lasted a few hundred million years. How much did the early Cambrian Earth resemble the modern Earth? What changes occurred during the Paleozoic that allowed Earth to better resemble the modern planet? Answer: The early Cambrian Earth was quite different from today, with fewer continents and different atmospheric conditions. During the Paleozoic, significant changes such as the assembly of Pangaea, the development of complex life, and the evolution of modern oceanic and continental configurations helped Earth resemble its modern form. Internet Sites, Videos, and Demonstration Aids Internet Sites 1. University of California Museum of Paleontology http://www.ucmp.berkeley.edu/ 2. Grand Canyon Geology http://www.kaibab.org/geology/gc_geol.htm Videos 1. Making of a Continent: Corridors of Time. Wards. 2. Making of a Continent: The Land of Sleeping Mountains. Wards. Answers to Figure-Related Critical Thinking Questions ❯❯ Critical Thinking Question Figure 20.8 What were the sources that supplied detrital sediments to the Sauk Sea during the Cambrian Period? Answer: In the Michigan Basin area: that would include the erosion of the Northern Michigan Highlands formed during the Penokean Orogeny (after the Huronian of Dorr & Eshman, 1971). Much of the Sauk Sea sediment in Laurentia came from the erosion of the Canadian Shield. ❯❯ Critical Thinking Question Figure 20.10 Do you think there is a difference between the organisms living in the fore-reef area and those in the back-reef area and lagoon? Explain. Answer: Wave action is greater in the fore-reef area than in the quieter back-reef area; therefore, tolerances of that wave energy will create the differences of organisms – one from the other. ❯❯ Critical Thinking Question Figure 20.15 How do you think the reefs controlled the regional facies in this area? Answer: In the Devonian Reef Complex of Western Canada; the reef core became the organic coral limestone resisting wave action. What did break off this reef generated clastic deposits on the fore- and back-reef area as fossiliferous limestone in the fore-reef area and fine calcareous mud in the lagoon – grain sizes differences owing to wave energy differences. ❯❯ Critical Thinking Question Figure 20.22 Why is so much oil found in reef structures? Answer: Living coral reefs are very porous, often interconnecting. In lithification to coral limestone much of this porosity is preserved. Even in recrystallization, porosity may be enhanced by secondary solution structures. Good oil reservoirs depend on good porosity. Suggested Answer to Selected Short Answer Question (Answers to question 7 and question 9 provided in the appendix to the text) 10. Creative Thinking Visual Question: This close-up (Figure 1) of a Devonian red rock from a building in Glasgow, Scotland, shows a distinctive sedimentary structure. Identify the sedimentary structure, indicate what type of environment you think it was deposited in, and give the name of the formation this rock comes from. Suggested Answer: During the Devonian Period, a section of northwestern Europe collided with a landmass made up of parts of present-day North America and Greenland. This created thick deposits of sand and mud, which were often stained red by oxidized iron minerals present. These poorly sorted sediments represented deposits of continental origin rather than marine deposits. The distinctive sedimentary structure in the Devonian red rock from Glasgow is likely cross-bedding, indicating deposition in a shallow, fluvial or aeolian environment with shifting sand dunes or river channels. This rock is from the Old Red Sandstone formation, which is known for such sedimentary features and its deposition in ancient continental environments. Chapter 21 Paleozoic Life History Chapter Outline 21.1 Introduction 21.2 What Was the Cambrian Explosion? 21.3 The Emergence of a Shelly Fauna 21.4 The Present Marine Ecosystem 21.5 Paleozoic Invertebrate Marine-Life GEO-INSIGHT 21.1: Trilobites—Paleozoic Arthropods 21.6 Vertebrate Evolution 21.7 Fish 21.8 Amphibians—Vertebrates Invade the Land 21.9 Evolution of the Reptiles—The Land Is Conquered 21.10 Plant Evolution Key Concepts Review Learning Objectives Upon completion of this material, the student should understand the following. • Animals with skeletons appeared abruptly at the beginning of the Paleozoic Era and experienced a short period of rapid evolutionary diversification. • During the Paleozoic Era, the invertebrates experienced times of diversification followed by extinction, culminating in the greatest recorded mass extinction in Earth's history at the end of the Permian Period. • Vertebrates first evolved during the Cambrian Period, and fish diversified rapidly during the Paleozoic Era. • Amphibians first appear in the fossil record during the Late Devonian, having made the transition from water to land; they became extremely abundant during the Pennsylvanian Period when coal-forming swamps were widespread. • The evolution of the amniote egg allowed reptiles to colonize all parts of the land beginning in the Late Mississippian. Chapter Summary • Table 21.4 summarizes the major evolutionary and geologic events of the Paleozoic Era and shows their relationships to one another. • Multicelled organisms presumably had a long Precambrian history during which they lacked hard parts. • Invertebrates with hard parts suddenly appeared during the Early Cambrian in what is called the "Cambrian explosion." Skeletons provided such advantages as protection against predators and support for muscles, enabling organisms to grow large and increase locomotor efficiency. Hard parts probably evolved as a result of various geologic and biologic factors rather than from a single cause. • Marine organisms are classified as plankton if they are floaters, nekton if they swim, and benthos if they live on or in the sea floor. • Marine organisms are divided into four basic feeding groups: suspension feeders, which consume microscopic plants and animals as well as dissolved nutrients from water; herbivores, which are plant eaters; carnivore-scavengers, which are meat eaters; and sediment-deposit feeders, which ingest sediment and extract nutrients from it. • The marine ecosystem consists of various trophic levels of food production and consumption. At the base are primary producers, on which all other organisms are dependent. Feeding on the primary producers are the primary consumers, which in turn are fed on by higher levels of consumers. The decomposers are bacteria that break down the complex organic compounds of dead organisms and recycle them within the ecosystem. • The Cambrian invertebrate community was dominated by three major groups—the trilobites, brachiopods, and archaeocyathids. Little specialization existed among the invertebrates, and most phyla were represented by only a few species. • The Middle Cambrian Burgess Shale contains one of the finest examples of a well-preserved soft-bodied biota in the world and provides us with an important glimpse of not only rarely preserved organisms but also the soft-part anatomy of many extinct groups. • The Ordovician marine invertebrate community marked the beginning of the dominance of the shelly fauna and the start of large-scale reef building. The end of the Ordovician Period was a time of major extinctions of many invertebrate phyla. • The Silurian and Devonian periods were times of diverse faunas dominated by reef-building animals, whereas the Carboniferous and Permian periods saw a great decline in invertebrate diversity. • Chordates are characterized by a notochord, dorsal hollow nerve cord, and pharyngeal slits. The earliest chordates were soft-bodied organisms that were rarely fossilized. Vertebrates are a subphylum of the chordates. • Fish are the earliest known vertebrates, with their first fossil occurrence in Cambrian rocks. They have had a long and varied history, including jawless and jawed armored forms (ostracoderms and placoderms), cartilaginous forms, and bony forms. It is from the lobe-finned fish that amphibians evolved. • The link between crossopterygian lobe-finned fish and the earliest amphibians is convincing and includes a close similarity of bone and tooth structures. New fossil discoveries, however, show that the transition between the two groups is more complicated than originally hypothesized and includes several intermediate forms. • Amphibians evolved during the Late Devonian, with labyrinthodont amphibians becoming the dominant terrestrial vertebrate animals during the Carboniferous. • The Late Mississippian marks the earliest fossil record of reptiles. The evolution of an amniote egg was the critical factor that allowed reptiles to completely colonize the land. • Pelycosaurs were the dominant reptile group during the Early Permian, whereas therapsids dominated the landscape for the rest of the Permian Period. • In making the transition from water to land, plants had to overcome the same basic problems as animals—namely, desiccation, reproduction, and gravity. • The earliest fossil record of land plants is from Middle to Upper Ordovician rocks. These plants were probably small and bryophyte-like in their overall organization. • The evolution of vascular tissue was an important event in plant evolution as it allowed nutrients and water to be transported throughout the plant and provided the plant with additional support. • The ancestor of terrestrial vascular plants was probably some type of green algae based on such similarities as pigmentation, metabolic enzymes, and the same type of reproductive cycle. • The earliest seedless vascular plants were small, leafless stalks with spore-producing structures on their tips. From this simple beginning, plants evolved many of the major structural features characteristic of today's plants. • By the end of the Devonian Period, forests with tree-sized plants up to 10 m tall had evolved. The Late Devonian also witnessed the evolution of gymnosperms (flowerless seed plants) whose reproductive style freed them from having to stay near water. • The Carboniferous Period was a time of vast coal swamps, where conditions were ideal for the seedless vascular plants. With the onset of more arid conditions during the Permian, the gymnosperms became the dominant element of the world’s forests. • A major extinction occurred at the end of the Paleozoic Era, affecting the invertebrates as well as the vertebrates. Its cause is still the subject of debate. Enrichment Topics Topic 1. Gigantic Insects. Record high oxygen levels in the late Paleozoic allowed gigantic insects to evolve. Dragonflies had wing spans as wide as hawks, millipedes were a few feet long, and cockroaches were as large as cats. Normally, insect size is limited by their inefficient respiratory system, in which oxygen is delivered through tracheal tubes. With more oxygen in the atmosphere, enough of the gas could be delivered through the insect’s body for it to survive. In beetles, the tracheal tubes leading to the legs limit the size the animal can get. This is just one example of the evolution and extinction of gigantism in organisms. http://www.sciencedaily.com/releases/2007/08/070810194908.htm Topic 2. Cause of the Permo-Triassic Extinction. Some of the planet’s mass extinctions, most notably the extinction that ended the reign of the dinosaurs, have been attributed to catastrophic causes. Yet there has been no conclusive evidence found for what caused the extinction that ended the Paleozoic. This extinction was the Earth’s worst; about 90 percent of marine species became extinct. Recent research suggests that climate change was the cause. The extinction was not sudden, but gradual. Animal species started dying out 10 million years before the Permian-Triassic boundary when the extinction rate increased and extinctions continued for another 5 million years. In addition, many geologists do not detect any signs of a meteorite impact such as a crater or an extraterrestrial chemical signature in the rocks. Those who favor the climate change model suggest that this was likely caused by the breakup of Pangaea, which rearranged land and lowered sea levels. Decaying plants were exposed to the air and oxygen levels that had dropped from 21 percent to 16 percent or lower (this would be like breathing at 14,000 feet elevation today). In addition, the rifting of Pangaea would have caused widespread volcanic eruptions, which are apparent in Siberia, resulting in increased carbon dioxide in the atmosphere and increased greenhouse effect. The New York Times, January 25, 2005. Topic 3. Oldest Known Toothache. As Paleozoic reptiles adapted to living on land the change in diet to meat and plants led to oral and dental disease. This is 200 million years earlier than the previously first known evidence of an ancient toothache. http://www.sciencedaily.com/releases/2011/04/110418114202.htm Common Misconceptions Misconception 1. Life evolved pretty quickly during the Precambrian and Paleozoic. Fact. Life remained was unicellular for billions of years. Evolutionary processes started to pick up when multicellular organism evolved but even then evolution was relatively slow. Even the Cambrian explosion took place over millions of years. Misconception 2. By the end of the Paleozoic, Earth looked pretty similar to what it does today. Fact. Of course, it would depend on where you were, but lots of the planet looked very different. Flowering plants make up so much of our current flora, but they had not yet evolved. Animals were much different than today. Reptiles were on the rise but there were no mammals. Ocean habitats were much more similar than the land habitats but there were no marine mammals or birds. Lecture Suggestions 1. The late Stephen Jay Gould’s book Wonderful Life contains excellent commentaries on science, evolution, and extinction. Gould discusses contingencies in the geologic record or the “what if” factor. The fauna of the Burgess Shale contained many species that may have been from phyla that went extinct. What if those phyla had survived and the ones that gave rise to modern life gone extinct; what would this world be like today? 2. Another book, also by Gould, is Eight Little Piggies. This volume contains essays dealing with evolution and extinction and the relationships of humans to these important themes. This would be a useful reference for the first two “Consider This” questions given below. 3. It is important for students to understand not just the concept of evolution but also co-evolution. What cause and effect relationships determined the progression of life? Discuss the concept of the food chain and levels of predators. What does each level on the food chain do for the organisms on the level above or below? 4. Students are fascinated by mass extinctions. You can have them list the possible causes of mass extinctions and then go through each extinction to see what organisms died out and what clues there are as to what caused the extinction. 5. Students love fossil hunting! If you have a location near you, try to arrange a field trip or give the students the sites whereabouts and send them off to make their own discoveries! Consider This 1. This chapter introduces the important concept of extinction. Given the number of species that have gone extinct throughout geologic time, is it really important to try to save every endangered species or are humans just trying to come to terms with their own guilt? Answer: Saving endangered species is crucial because each species plays a role in maintaining ecosystem stability and biodiversity. While extinction is natural, human-induced losses can destabilize ecosystems, which can have far-reaching consequences beyond mere guilt. 2. Is there any way to determine which species should be protected and which may not be important in the long run? Are people too focused on “charismatic megafauna,” a.k.a. big, pretty animals, rather than species that may be more important to protect? What makes a species important? Answer: To determine which species to protect, scientists assess their ecological roles, such as their impact on ecosystem functions and interactions. While charismatic megafauna often receive more attention, protecting species that maintain critical ecological processes is also essential. A species is important if it contributes to ecosystem health, stability, or function. 3. Why was it necessary for plants to populate the land before animals were able to? Discuss the role that plants play in the establishment of animal habitats and food sources. What types of animals might have been able to inhabit the land without plants being there? Answer: Plants were essential for land colonization because they provided the foundation for terrestrial ecosystems by producing oxygen, forming soil, and offering food sources. Animals could not have established stable habitats without plants, as they needed plant-derived resources. Early land-dwelling animals might have included simple herbivores or detritivores that could feed on decaying organic matter before more complex ecosystems developed. Internet Sites, Videos, and Demonstration Aids Internet Sites 1. Paleozoic Era Paleobiology http://www.fossilmuseum.net/Paleobiology/Paleozoic_paleobiology.htm 2. The Burgess Shale Site http://www.burgess-shale.bc.ca 3. Strange Creatures—A Burgess Shale Fossil Sampler http://paleobiology.si.edu/burgess/index.html 4. A Guide to the Eight Orders of Trilobites http://www.trilobites.info/ Videos 1. Making of a Continent: Corridors of Time. Wards. Demonstration Aids and Slides 1. Evolution of Life on Earth, slide set, Educational Images, Ltd. 2. Paleozoic Fossil Collection, Science Stuff 3. Deck of 50 Fossil Cards, Science Stuff 4. Paleozoic Era, color slide set. Ward’s Natural Science. Answers to Figure-Related Critical Thinking Questions ❯❯ Critical Thinking Question Figure 21.7 What role did the transgressing seas during the Tippecanoe Sequence play in terms of opening up new ecologic niches for the marine invertebrate community, and what was the contribution of the invertebrates to the lithology of the Tippecanoe Sequence? Answer: Reefs became the dominant ecosystem during the Ordovician and as the sea invaded farther on to the continent, more coral reefs and their diversity populated the geologic record along with the limestone facies. ❯❯ Critical Thinking Question Figure 21.8 Why does a reef make a good oil reservoir? Answer: Reefs have a high degree of porosity in which oil tends to collect. A reef makes a good oil reservoir because its porous and permeable limestone structure can store significant amounts of oil and gas. The reef also acts as a natural trap, where oil migrating from surrounding rocks accumulates and gets trapped by non-permeable cap rocks above, preventing its escape and allowing for efficient oil extraction. ❯❯ Critical Thinking Question Figure 21.13 Note that the greatest extinction event in Earth history occurred at the end of the Permian, yet the Late Cretaceous mass extinction is the one most people are familiar with. Why do you think most people have heard of the Late Cretaceous mass extinction but not the one at the end of the Permian? Answer: Perhaps dinosaurs are more lovable than trilobites? In popular media, science takes a back seat. Extinctions in the oceans are not as dramatic as extinctions to a major group of land animals that opened the way for greater mammalian evolution leading to hominid development. The Late Cretaceous mass extinction is more widely known because it is associated with the dramatic disappearance of the dinosaurs, a topic of significant public interest and media coverage. In contrast, the Permian extinction, while more severe, involved less charismatic organisms and occurred over a longer, more complex timeframe, making it less prominent in popular culture. ❯❯ Critical Thinking Question Figure 21.27 What were the conditions that made the Late Carboniferous (Pennsylvanian) Period so favorable for the proliferation of amphibians and seedless vascular plants? Answer: The Cyclothems of Pennsylvanian provided an ever shifting shore line and the sashay back and forth of hydrogeologic conditions in warm climates. These conditions were optimal for large plants that grew out of saturated substrate (swamps). This vegetated community created niches for vertebrates to leave the water at a time when oxygen was increasing, insects were flying – in short resources were increasing on land. ❯❯ Critical Thinking Question Figure 21.29 Although the transition from reptiles will not be discussed in detail until the next chapter, what is one characteristic in common between pelycosaurs and therapsids that indicates the pelycosaur to therapsid to mammal lineage is a correct evolutionary lineage? Answer: Leg placement and size (4, ~equal length) for both pelycosaurs and therapsids, are more characteristic of an early mammalian placement rather than the unequal length of the thecodonts. ❯❯ Critical Thinking Question Figure 21.34 What is it about the life cycle of the gymnosperms that gave them an advantage over the seedless vascular plants in terms of the climatic changes taking place during the Permian Period, when the continents came together to form the supercontinent Pangaea, and the subsequent Triassic Period? Answer: As conditions were becoming drier or seasonally cooler, the gymnosperm seed was protected from conditions less favorable to growth than the seedless plants. Gymnosperms had an advantage over seedless vascular plants during the climatic changes of the Permian and Triassic periods because their seeds provided protection and nourishment to the developing embryo, allowing them to survive harsh conditions and variable climates. Additionally, gymnosperms were better adapted to dry environments with their specialized pollen for reproduction without the need for water, which was crucial as Pangaea's climate became more arid. Suggested Answer to Selected Short Answer Question (Answers to question 7 and question 8 provided in the appendix to the text) 6. Discuss how changing geologic conditions affected the evolution of invertebrate life during the Paleozoic Era. Suggested Answer: The Paleozoic was characterized by major continental changes and upheavals. Major marine transgressions were occurring in all of the six major continental land masses. These geologic changes opened up huge marine shallow ecosystems that could be inhabitated and exploited. This marshalled in a dramatic explosion of invertebrate life. Changing geologic conditions during the Paleozoic Era, such as shifting plate tectonics, varying sea levels, and climate changes, significantly impacted invertebrate evolution. These conditions led to habitat changes, driving diversification and adaptation. For example, the formation and breakup of supercontinents influenced marine and freshwater environments, leading to the evolution of new invertebrate species and the expansion of existing ones into new ecological niches. Chapter 22 Mesozoic Earth and Life History Chapter Outline 22.1 Introduction 22.2 The Breakup of Pangaea 22.3 Mesozoic History of North America 22.4 What Role Did Accretion of Terranes Play in the Growth of Western North America? 22.5 Mesozoic Mineral Resources 22.6 Life of the Mesozoic Era GEO-INSIGHT 22.1 Dinosaurs GEO-INSIGHT 22.2 Mary Anning’s Contributions to Paleontology 22.7 Mesozoic Climates and Paleogeography 22.8 Mass Extinctions—A Crisis in the History of Life Key Concepts Review Learning Objectives Upon completion of this material, the student should understand the following. • The Mesozoic breakup of Pangaea profoundly affected geologic and biologic events. • Most of North America was above sea level during much of the Mesozoic Era. • A global rise in sea level during the Cretaceous Period resulted in an enormous interior seaway that divided North America into two large landmasses. • Western North America was affected by four interrelated orogenies that took place at an oceanic-continental convergent plate boundary. • Terrane accretion also affected the Mesozoic geologic history of western North America. • Marine invertebrates that survived the Permian extinction event diversified and repopulated the Mesozoic seas. • Land-plants changed markedly when flowering plants evolved during the Cretaceous. Chapter Summary • Table 22.2 and 22.3 summarize many Mesozoic geologic and biologic events. • We can summarize the breakup of Pangaea as follows: 1. During the Late Triassic, North America began separating from Africa. This was followed by the rifting of North America from South America. 2. During the Late Triassic and Jurassic periods, Antarctica and Australia—which remained sutured together—began separating from South America and Africa, and India began rifting from Gondwana. 3. South America and Africa began separating during the Jurassic, and Europe and Africa began converging during this time. 4. The final stage in Pangaea's breakup occurred during the Cenozoic, when Greenland completely separated from Europe and North America. • The breakup of Pangaea influenced global climatic and atmospheric circulation patterns. Although the temperature gradient from the tropics to the poles gradually increased during the Mesozoic, overall global temperatures remained equable. • An increased rate of seafloor spreading during the Cretaceous Period caused sea level to rise and transgressions to occur. • Except for incursions along the continental margin and two major transgressions (the Sundance Sea and the Cretaceous Interior Seaway), the North American craton was above sea level during the Mesozoic Era. • The Eastern Coastal region was the initial site of the separation of North America from Africa that began during the Late Triassic. During the Cretaceous Period, it was inundated by a transgressing sea, which, at its maximum, connected with a sea transgressing away from the north to create the Cretacous Interior Seaway. • Mesozoic rocks of the western region of North America were deposited in a variety of continental and marine environments. One of the major controls of sediment distribution patterns was tectonism. • Western North America was affected by four interrelated orogenies: the Sonoma, Nevadan, Sevier, and Laramide. Each involved igneous intrusions, as well as eastward thrust faulting and folding. • The cause of the Nevadan, Sevier, and Laramide orogenies was the changing angle of subduction of the oceanic Farallon plate beneath the continental North American plate. The timing, rate, and to some degree, the direction of plate movement were related to seafloor spreading and the opening of the Atlantic Ocean. • Although the structural features of North America’s western margin are associated with activity along an oceanic-continental convergent plate boundary, it is thought that more than 25% of the western margin originated from the accretion of terranes. • Mesozoic rocks contain a variety of mineral resources, including coal, petroleum, uranium, gold, and copper. • The marine invertebrate survivors of the Permian mass extinction diversified and gave rise to increasingly diverse Mesozoic marine invertebrate communities. • Land-plant communities of the Triassic and Jurassic consisted of seedless vascular plants and gymnosperms. The angiosperms, or flowering plants, evolved during the Early Cretaceous, diversified rapidly, and were soon the most abundant land plants. • Dinosaurs evolved during the Late Triassic, but were most abundant and diverse during the Jurassic and Cretaceous. The two distinct orders of dinosaurs, based on pelvic structure, are Saurischia (lizard-hipped) and Ornithischia (bird-hipped). • Bone structure, predator-prey relationships, and other features have been cited as evidence of dinosaur endothermy. Although there is still no solid consensus, many paleontologists think that some dinosaurs were endotherms. • That some theropods had feathers indicates that they were warm-blooded and provides further evidence of their relationship to birds. • Small pterosaurs were probably active, wing-flapping fliers, whereas large ones may have depended on soaring to stay aloft. At least two pterosaur species had hair or feathers, so they, and perhaps all pterosaurs, were probably endothermic. • The fish-eating, porpoise-like ichthyosaurs were thoroughly adapted to an aquatic life, whereas the plesiosaurs with their paddle-like limbs could most likely come out of the water to lay their eggs. The marine reptiles known as mosasaurs were most closely related to lizards. • Birds probably evolved from small theropod dinosaurs. The oldest known bird, Archaeopteryx, appeared during the Jurassic; however, few other Mesozoic birds are known. • The earliest mammals evolved during the Late Triassic, but they are difficult to distinguish from advanced cynodonts. Details of the teeth, middle ear, and lower jaw are used to distinguish the two. • Several types of Mesozoic mammals existed, but most were small and their diversity was low. Both marsupials and placentals evolved during the Cretaceous from a group known as eupantotheres. • Because the continents were close together and climates were mild during much of the Mesozoic, animals and plants occupied much larger geographic ranges than they do now. • Among the victims of the Mesozoic mass extinctions were dinosaurs, flying reptiles, marine reptiles, and several groups of marine invertebrates. A meteorite impact may have caused these extinctions, but some paleontologists think that other factors also contributed. Enrichment Topics Topic 1. Birds From Dinosaurs. The accepted dogma among paleontologists is that birds evolved from dinosaurs. Birders like to think that they’re seeing modern relatives of dinosaurs through their binoculars. Many articles have been written about this connection. You can find some at this website: http://www.birding.com/birdsdino.asp Topic 2. The Asteroid Extinction Hypothesis. The most widely accepted explanation for the end of the Mesozoic mass extinction was proposed in 1980 by the father-and-son team of Luis and Walter Alvarez. Walter was a geologist who thought the secret to the end of the dinosaurs’ reign might lie in a clay layer found right at the K-T boundary. He asked his chemist and Nobel laureate father, Luis, how to determine the amount of time it took for the clay to be deposited, and together they came up with a plan. Since iridium comes in from outer space in cosmic dust at a low but known rate, they could use the amount of iridium in the clay layer to determine the amount of time it took to deposit it. What they found shocked them; the amount of iridium in the clay layer was nearly 100 times what they expected! They knew that much iridium could only have come from an extraterrestrial source, like an asteroid. This led to their hypothesis, which was published the following year. Over the past two decades many other researchers have found other data to fill in additional details. The hypothesis as it stands is this: the K-T extinction was triggered when the planet was struck by a giant asteroid, 10 km (6 miles) in diameter. Heat from the impact raised the temperature of the atmosphere to that of a kitchen oven on broil. The enormous quantities of dust and gas that were kicked up coalesced into tektites that fell to the ground. Animals roasted and forests burned. Dust and smoke enshrouded the planet, blocking out the sun so the temperature went from burning hot to freezing cold; photosynthesis was temporarily shut down. The only life to survive this holocaust were the small animals that burrowed into the ground or lived in fresh water and plants with seeds that could survive for long periods in the soil. Marine species were decimated, although deep sea animals survived fairly well. One decade after the Alvarez hypothesis was first proposed, the meteorite’s impact crater was found. It is a 180 km diameter hole buried beneath nearly 2 km of sediment near the northern coast of the Yucatan Peninsula, Mexico. While it is certain that the dinosaurs and other life went extinct 65 million years ago, and it is also certain that a giant asteroid struck at that time, the link between the two events is not accepted by all scientists. Topic 3. The Flood Basalt Extinction Hypothesis. Another camp blames mass extinction on a volcanic eruption so massive it discharged incessant quantities of poisonous gases into the atmosphere and brought about tremendous acid rains for years. The eruption was at the Deccan Traps in India, where approximately 500,000 km2 (200,000 mi2) of lava came out of a huge set of volcanoes at about the same time as the K-T extinctions. Either or both catastrophes could have caused the mass extinctions, or they could have been the last straw in a decline in animal and plant populations that was already taking place for another reason, such as changing climate. Since 65 million years is a long time in the past to be searching for evidence, it’s possible that we will never know for sure what killed the dinosaurs. Common Misconceptions Misconception 1. Dinosaurs and humans coexisted. Fact: Only in cartoons. Dinosaurs were long gone, about 63 million years gone, before the evolution of hominids. The only evidence given by creationists that the two coexisted are footprints—dinosaur footprints and shapes that are not human at all. Richard Dawkins discusses this: http://www.youtube.com/watch?v=qvHII6Vv06s Misconception 2. We humans have the capability of causing the extinction of all life on Earth. Fact: All the multiple megatons of nuclear weapons of all the world’s powers represent probably only one ten thousandth the energy released by the 10 km (estimated) bolide that impacted Earth, causing the mass extinctions at the end of the Cretaceous Period. Life and Earth survived this event. People may be able to bring about our own extinction and that of many other higher forms of life. We probably wouldn’t touch the diversity of bacteria and even insects. Cockroaches will likely prevail. Lecture Suggestions 1. To illustrate how magmas rise in the crust to form large batholith bodies, conduct this demonstration. Put two differently colored immiscible liquids such as colored Karo syrup and colored lamp oil in a jar. It is best to use about equal amounts of the two liquids and use a long cylindrical jar. After pouring the two liquids in the jar, allow them to separate and then invert the jar. The students should see teardrop shaped blobs of the lower density liquid rise to the surface. Point out that many granite plutons such as those associated with the Sierra Nevada batholith have this shape. 2. Accreted and suspect terranes have been the subject of several books by John McPhee. All of the following provide useful information and anecdotes for lectures about the tectonics of western North America: Basin and Range and Rising from the Plain are primarily about the western states; In Suspect Terrane deals mainly with the development of understanding the tectonic evolution of the Appalachians. 3. Demonstrate folding or thrust faulting by using structural models. Compressional tectonics can be effectively demonstrated using stacks of paper or layered clay. Remind students that compressional tectonics greatly thicken and shorten the continental crust. 4. There is much good information written about the Cretaceous mass extinction. Have the students divide into three groups and research the following theories for this extinction event: 1) meteorite impact, 2) voluminous volcanism, 3) climate change. The groups should be prepared to debate this topic and present sound scientific arguments for their cases and against the others. An impartial outside committee, such as a college speech class might be brought in to evaluate the debate. 5. The breakup of Pangaea is the vital part of Earth’s story during the Mesozoic. It is important to reinforce that this story is continuing. Mountain building events that began in the western United States are still happening. Have students compare the length of time spanned by mountain building in the western United States to the length of time covered by all of the Appalachian orogenies. Consider This 1. Why are the best dinosaur fossil localities where they are? Why aren’t dinosaur fossils found in other areas of the country with Mesozoic age rocks, such as Idaho, Georgia, and Mississippi? Answer: The best dinosaur fossil localities are often found in sedimentary basins where conditions were favorable for fossilization, such as rivers and floodplains. Areas like Montana and Wyoming had the right environments for preserving dinosaur remains. In contrast, regions with Mesozoic rocks like Idaho, Georgia, and Mississippi may have had different sedimentary conditions or were less likely to preserve fossils due to erosion or other geological factors. 2. How would the climate of western North America have varied in the past because of microplate accretion? Think of oceanic influences on climate. Answer: The climate of western North America would have varied due to microplate accretion, which influenced oceanic currents and atmospheric conditions. Accretion could lead to changes in oceanic patterns, affecting regional climate with potential shifts between wetter and drier periods and impacting temperature and precipitation patterns. 3. Have the students formulate a story or hypothesis as to what would happen on Earth if another similar-sized meteorite impacted today. Would humans survive? What habitats would be destroyed? Would there be global warming or cooling? Answer: A meteorite impact of similar size today could cause massive destruction, including habitat loss, fires, and dust clouds blocking sunlight, leading to global cooling. Human survival would depend on preparedness and adaptability, but significant disruptions to ecosystems and climate would likely result in widespread environmental and societal impacts. Internet Sites, Videos, and Demonstration Aids Internet Sites 1. Top 10 Misconceptions About Dinosaurs http://paleobiology.si.edu/dinosaurs/info/misconceptions/main.html 2. The Mesozoic Era http://www.ucmp.berkeley.edu/mesozoic/mesozoic.html 3. What Killed the Dinosaurs? http://www.ucmp.berkeley.edu/diapsids/extinction.html Videos 1. Planet Earth #1: The Living Machine: The Theory of Plate Tectonics. Annenberg/CPB Collection. 2. Earth Revealed: Evolution on Earth: CPB/ Annenberg. 3. On the Trail of the Thick-skulled Dinosaur. JLM Visuals. 4. Dinosaurs: The Video, PBS Home Video 5. Walking with Dinosaurs, PBS Home Video 6. Hidden Worlds: Life in Triassic Park, PBS Home Video 7. End of the Dinosaurs. Insight Media. Demonstration Aids 6. Evolution of Life on Earth, slide set, Educational Images, Ltd. 7. Mesozoic Fossil Collection, Science Stuff 8. Deck of 50 Fossil Cards, Science Stuff 9. Mesoozoic Era, color slide set. Ward’s Natural Science. Answers to Figure-Related Critical Thinking Questions ❯❯ Critical Thinking Question Figure 22.1 Why would increased seafloor spreading, resulting in higher heat flow and an increase and rapid expansion in oceanic ridges during the Cretaceous result in worldwide transgression onto the continents? Answer: As ocean basins expand in spreading centers, tectonically quiet boundaries of continental crust would continue to cool and subside into the mantle. As such, the subsidence would lower coastal elevations and ocean water levels would surge inland – on to the continent. ❯❯ Critical Thinking Question Figure 22.2 What are the differences in terms of tectonic setting and depositional environments between the evaporites that formed in the Gulf of Mexico during the Mesozoic Era and those that formed in North America during the Paleozoic Era? Answer: Paleozoic evaporites: Michigan, Ohio, Appalachian basin examples Tectonics: gradually subsiding basins EoD: restricted marine basins with salt saturation increasing with increase in evaporation & lack of circulation, resulting in massive salt deposits. These basins stabilized and were capped with cyclothemic continental deposits. Early Mesozoic evaporates: in the Gulf of Mexico Tectonics: the opening of the Gulf as Pangaea is starting to breakup; normal faulting in some areas (tension) EoD: isolated restricted basins separated by fault blocks and briefly expanding basins with sediment restrictions. These basins continued to expand and became the ocean basins as we know today; particularly the Atlantic /Gulf basin. ❯❯ Critical Thinking Question Figure 22.15 From this photo, how can you tell these crossbeds are the result of wind deposition? Answer: The Navajo Sandstone contains high-angle crossbeds from wind-blown dunes: this differs from low-angle cross sets found in fluvial environments. ❯ ❯ Critical Thinking Question Figure 22.22 Other than their pelvises, are there other anatomical features you can use to differentiate the two main groups of dinosaurs? Answer: Saurischian (lizard hipped) dinosaurs: generally large, bipedal or quadrapedal with front teeth Ornithischian (bird hipped) dinosaurs: generally smaller, with no front teeth and in some cases bony projections from head or back; there are other differences. Suggested Answer to Selected Short Answer Questions (Answers to question 7 and question 8 provided in the appendix to the text) 10. Creative Thinking Visual Question: This view (Figure 1) in Capitol Reef National Park, Utah, shows excellent exposures of several sedimentary rock formations that were deposited during the Triassic and Jurassic periods. a. What accounts for the reddish color of some of the rock layers? b. The rocks visible on the skyline belong to the Navajo Formation, which is exposed over a large area in the southwest. It is composed of well-sorted, well-rounded quartz sandstone, it has tracks of land-dwelling animals, including dinosaurs, and the sandstone has cross-beds up to 30 m high. How do you think it was deposited? Suggested Answer: The Navajo Sandstone is a remnant of a huge sand sea called the Navajo Sand Sea. The original deposition comprised towering sand dunes of almost pure quartz sand that dwarfs the modern Sahara. The deposition shows a record of cross-bedding which is a record of the migration of dunes under the influence of wind or water. The sedimentary sandstone was laid down as loose sediment - sand, clay and silt - between 210 and 160 million years ago, during the Jurassic period, when dinosaurs were the dominant species. After being deposited, the sediments were covered with more sediment and pushed down into the earth, where with pressure and elevated temperature, the loose sediments were pressed and baked into rock. During the last 80 million years, forces deep within the earth uplifted the Colorado Plateau, the overlying rocks and sediments were eroded off and canyons were formed. The various colors of the sandstones were formed by intruding minerals. In the case of the red sandstone, the mineral is iron oxide. a. The reddish color of some rock layers is due to the presence of iron oxide (rust) within the sediment, which forms in arid or semi-arid environments where oxidation occurs. b. The Navajo Formation, with its well-sorted, well-rounded quartz sandstone and large cross-beds, was likely deposited in a vast desert environment with shifting sand dunes. The cross-beds indicate strong wind action, while the tracks of land-dwelling animals suggest it was a region that supported terrestrial life. Chapter 23 Cenozoic Earth and Life History Chapter Outline 23.1 Introduction 23.2 Cenozoic Plate Tectonics 23.3 Cenozoic Orogenic Belts 23.4 Paleogene and Neogene Evolution of North America GEO-FOCUS 23.1: Geology along the Oregon Trail in Nebraska GEO-INSIGHT 23.1: The Columbia River Basalts 23.5 The Pleistocene and Holocene Epochs 23.6 Cenozoic Mineral Resources 23.7 Paleogene and Neogene Life History Geo-Focus: A Messel Pit Fossil Site in Germany 23.8 Pleistocene Faunas Key Concepts Review Learning Objectives Upon completion of this material, the student should understand the following. • The Mesozoic breakup of Pangaea continued during the Cenozoic, accounting for orogenies in two major belts. • The North American Cordillera experienced deformation, the origin of mountains, volcanism, uplift, and deep erosion. • An epeiric sea briefly occupied North America's interior lowlands. • Thick sedimentary deposits accumulated along the Gulf and Atlantic Coastal Plains • Cenozoic uplift and erosion account for the present-day Appalachian Mountains. • Pleistocene continental glaciers covered vast areas of the Northern Hemisphere continents. • Cenozoic rocks contain several resources, such as oil and gold. • Marine invertebrates such as foraminifera and mollusks were abundant during the Cenozoic. • Mammals diversified and eventually gave rise to today's familiar mammals. Chapter Summary • Cenozoic tectonism was concentrated in the Alpine-Himalayan and circum-Pacific belts. • The Cenozoic evolution of the North American Cordillera included deformation during the Laramide orogeny, extensional tectonics that yielded basin-and-range structures, extensive intrusive and extrusive igneous activity, and uplift and erosion. • One model for the Laramide orogeny involves near-horizontal subduction of the Farallon plate beneath North America, resulting in fault-bounded uplifts in the area of the present-day Rocky Mountains. • As the North American plate drifted westward, it collided with the Pacific-Farallon Ridge, at which time subduction ceased and the continent became bounded by large transform faults, except in the Pacific Northwest where subduction continues. • Sediments eroded from Laramide uplifts were deposited in intermontane basins and in the Great Plains, whereas a wedge of sediments pierced by salt domes is found on the Gulf Coastal Plain. • Cenozoic uplift and erosion were responsible for the present topography of the Appalachian Mountains. As the Appalachians eroded, much of the sediment was deposited on the Atlantic Coastal Plain. • Vast glaciers covered about 30% of Earth's land surface during the Pleistocene. About 20 warm-cold Pleistocene climatic cycles are recognized from evidence found in deep-sea cores. • Cenozoic mineral resources include sand and gravel, placer deposits of gold, some evaporite minerals such as borax, and oil and natural gas. • Marine invertebrate groups that survived the extinctions at the end of the Mesozoic continued to diversify, giving rise to the present-day marine fauna. • The Paleocene mammalian fauna was composed of Mesozoic holdovers and several new orders and by Eocene time, most living mammal orders had evolved. • Shrew-like placental mammals that evolved during the Late Cretaceous were the ancestors for the placental mammalian orders that evolved during the Cenozoic. • Among the hoofed mammals (artiodactyls and perissodactyls), adaptations include modifications of the teeth for grinding vegetation and changes in their limbs for speed. • The evolutionary history of horses is particularly well documented by fossils, but scientists also know much about the evolution of other hoofed mammals, as well as elephants, whales, and some carnivores. • Primates, which evolved during the Paleocene, differ from other mammalian orders on the basis of overall skeletal structure and mode of locomotion, an increase in brain size, stereoscopic vision, and a grasping hand with opposable thumb. • The primates are divided into two suborders: prosimians, the oldest primate lineage and include lemurs, lorises, tarsiers, tree shrews and anthropoids, which include the New and Old World monkeys, apes, and hominids (humans and their extinct ancestors). • The hominid lineage begins at nearly 7 million years ago with Sahelanthropus tchadensis, followed by Orrorin tugenensis at 6 million years, and then two species of Ardipithecus, at 5.8 and 4.4 million years ago, respectively. These early hominids were succeeded by the australopithecines, a fully bipedal group that evolved in Africa 4.2 million years ago. • The human lineage began about 2.5 million years ago in Africa with the evolution of Homo habilis, superseded by Homo erectus, sometime between 1.8 and 1.6 million years ago, which was the first hominid to migrate out of Africa, spreading to Europe, India, China, and Indonesia. • Although the transition from H. erectus to H. sapiens is still unresolved, the most famous of all human fossils are the Neanderthals, who inhabited Europe and the Near East between 200,000 and 30,000 years ago and differed mainly from present-day humans in being more robust and having a long, low skull with heavy brow ridges. • The Cro-Magnons, highly skilled hunters and cave painters, were the successors of the Neanderthals and lived from about 35,000 to 10,000 years ago. Modern humans succeeded the Cro-Magnons and have spread throughout the world, as well as having set foot on the Moon. • Modest in comparison to the mass extinction events at the end of the Paleozoic and Mesozoic eras, the Pleistocene extinctions, nevertheless, had a profound effect on large terrestrial mammals. Two competing hypotheses have been advanced to explain these Pleistocene extinctions: climatic change and prehistoric-overkill. Enrichment Topics Topic 1. The Cascade Range. Beneath the Pacific Northwest, the Juan de Fuca plate is subducting beneath North America. Above it lies a volcanic arc, the Cascade Range. Its most famous member, Washington’s Mount St. Helens, exploded in 1980 and killed 57 people. But the eruption of St. Helens was not a large one; the mountain’s 1480 eruption was five times larger. Mount Rainier, also in Washington, doesn’t tend to explode; it is prone to large mudflows, and some nearby towns are built on old ones. Large mudflows have made it to the Seattle and Tacoma areas and could again. Mount Shasta, in northern California, is even more dangerous but not near any large population centers, and the other volcanoes pose lesser threats. Although these mountains could kill a lot of people and do a lot of damage, earthquakes may represent an even greater danger. While the Pacific Northwest is not known for its earthquakes – in fact, there have been no recent ones – many appear in the geological record. The last earthquake was about 300 years ago, when it is likely that the entire subduction zone ruptured at once, resulting in a quake that seismologists say would have measured about 9 on the Richter scale (the largest actually measured quake ever was an 8.6). While it was 700 years between that quake and the previous one, the two before that had intervals of only 300 years. Tsunamis generated by such large quakes would be 15 to 25 feet high, and even higher in some places. The rarity of these quakes is thought to be caused by the locking of the fault zone, which allows stresses to build up so high that the eventual rupture is enormous. National Geographic, May 1998 v193 n5 p6(32). Topic 2. Could Global Warming Trigger a European Ice Age? The temperate climate that exists across Western Europe and especially the British Isles could come to a dramatic end at the hands of global warming. The reason for the current relatively mild climate is the Gulf Stream, which brings warm water from the equator up the east coast of North America, across the Atlantic, and then splits going north along Britain and south along Western Europe. This circulation occurs because water in the North Atlantic becomes cold and saline (icebergs, which are fresh water, leave behind water that’s saltier than the sea), which means it is dense and so it sinks. The Gulf Stream is pulled northward by the sinking water. However, if global warming keeps North Atlantic surface water from forming icebergs and becoming very cold, it will be less dense than the water beneath it. This will shut off the Gulf Stream and will actually cool Western Europe. Perhaps even enough to start a new ice age. Topic 3. From Where the Whale? Paleontologists look at the skeletal morphology of modern organisms and ancient fossils to find similarities or differences. Molecular biologists look at genes to see how many the modern and fossil organisms have in common. Until recently, these two groups had very different answers to the question of where whales came from. They agreed that cetaceans evolved from four-legged animals who roamed the land, with nostrils near the tips of their noses and teeth of several sizes and types. They moved into the sea during the Eocene, not quite 50 million years ago. But the scientists disagree on other points. The paleontologists thought that ancient whales were members of an extinct group of highly-specialized ungulates (hoofed mammals) called mesonychians. Molecular biologists favored artiodactyls (even-toed ungulates such as cows, pigs, camels, deer, and hippos) as the ancestors of cetaceans. Recently discovered complete cetacean fossils, 49 million years old, have brought paleontologists over to the side of the molecular biologists. This peace-making fossil was wolf-sized Pakicetus attocki, a meat eater, similar to modern dogs but with a more powerful tail, longer snout, and smaller eyes. Pakicetus was a landlubber, adapted to walking and running on land, but it also may have fed while wading in streams. Pakicetids are thought to be ancestral to cetaceans because of the structure of their ear bones and anklebones. Common Misconceptions Misconception 1. Mammals were virtually unknown at the time the dinosaurs went extinct. Fact. Mammals evolved from mammal-like reptiles during the Late Triassic so two-thirds of their evolution took place during the Mesozoic Era. Misconception 2. Humans evolved from apes. Fact. Humans and apes share a common ancestor. Both humans and apes diverged from Old World monkeys sometime during the Miocene. Lecture Suggestions 1. Remind students that the last 65 million years of earth history is the only time that they might feel reasonably comfortable in if they were to be able to travel through time. Most of the continents were relatively close to their present positions. Animals and plants, though strange, were somewhat familiar. 2. Have students take sides (for and against) on the prehistoric overkill hypothesis. For those who are for, how does this relate to what is happening with the human caused extinctions that are happening today? For those who are against, do they think humans are unable to cause extinctions? 3. So much geological activity in North America takes place far inland from plate boundaries. What is the geological history of this region? How is this region active today? 4. The Yellowstone Hotspot is atypical in so many ways. Have the class figure out what information they would need to decipher the geologic history of the hotspot. 5. Find the location where you live on Figure 23.4. What is the major geological province in which you live? What is the general geologic history of that area? Consider This 1. How much extension has taken place in the Basin and Range during extensional tectonics? How much shortening and thickening took place during Sevier compressional tectonics? How would geologists calculate these figures? Answer: The Basin and Range province has experienced significant extension, with stretching resulting in thinning of the Earth's crust by 20-30% in some areas. During the Sevier orogeny, shortening and thickening of the crust were on the order of several kilometers. Geologists calculate these figures through mapping geological structures, measuring strain, and analyzing rock deformation. 2. Is the term “the Age of Mammals” really a fair assessment of the Cenozoic? What would be a better name? Answer: The term "Age of Mammals" might be misleading as it emphasizes only mammals. A better name could be the "Cenozoic Era of Mammalian and Avian Evolution," reflecting the significant evolutionary advancements in both mammals and birds, along with other ecological changes. 3. Consider the world impact of a new ice age. What would happen to sea ports? Which major cities of the world would be plowed over by ice? How would agricultural and desert land shift? How would worldwide distribution of vegetation change? Answer: A new ice age would likely submerge sea ports under ice, particularly in northern regions. Major cities like New York, London, and Moscow might be impacted by glacial advance. Agricultural zones would shift southward, while deserts could expand. Vegetation patterns would change as glaciers advance and retreat, affecting global ecosystems. Internet Sites, Videos, and Demonstration Aids Internet Sites 1. Becoming Human, Institute of Human Origins http://www.becominghuman.org/ 2. The Midwest U.S. 16,000 Years Ago http://www.museum.state.il.us/exhibits/larson/ 3. The Evolution of Man, BBC http://www.bbc.co.uk/sn/prehistoric_life/human/human_evolution/ 4. La Brea Tar Pits http://www.tarpits.org/ Videos 1. Secrets of the Dead: Search for the First Human, PBS 2. NOVA: Mystery of the Megaflood, PBS 3. Exotic Terrane: Geological Discoveries in the Pacific Northwest, Bullfrog Films 4. Walking with Prehistoric Beasts, PBS Home Video 5. The Geology of the United States. Insight Media. 6. Prehistoric America. PBS DVD. Slides 1. Evolution of Man. Wards Geoscience. 2. Submarine Canyons and Deep Sea Fans, slide set, Educational Images, Ltd. 3. Satellite Imagery – Earth from Space, slide set, Educational Images, Ltd. 4. Interpretation of Roadside Geology, slide set, Educational Images, Ltd. 5. Continental Glaciation. Wards Geoscience. 6. Missoula Flood. Slide sets of 20 or 36. Geophoto Publishing, Lindon, Utah. 7. Rocky Mtn. Park. Set of 22 slides. Geophoto Publishing, Lindon, Utah. 8. Yellowstone Geysers. Set of 20 slides. Geophoto Publishing, Lindon, Utah. 9. Mono Lake. Set of 15 slides. Geophoto Publishing, Lindon, Utah. 10. Yosemite. 25 slides. Geophoto Publishing, Lindon, Utah. 11. Evolution of Life on Earth, slide set, Educational Images, Ltd. 12. Evolution of Man, color slide set. Ward’s Natural Science. 13. Cenozoic Era, color slide set. Ward’s Natural Science. Demonstration Aids 11. Evolution of Life on Earth, slide set, Educational Images, Ltd. 12. Cenozoic Fossil Collection, Science Stuff 13. Deck of 50 Fossil Cards, Science Stuff Answers to Figure-Related Critical Thinking Questions ❯❯ Critical Thinking Question Figure 23.16 Why were there large lakes in places such as Death Valley, California, during the Pleistocene? Answer: Glacial Chapter: these were Pluvial (“rainfall”) lakes: occurring at a time when rainfall was unusually prevalent in what is today a dry climate. When glacial ice dominated, ~30% of the Earth’s landmasses, climate change brought “rain” to what is today our southwest desert. Death Valley, like other deserts in the SW, is an internally-drained basin – streams flow in but water leaves only by evaporation (ground water not an issue…). Obviously with a humid climate, these basins (like Lake Bonneville) became quite extensive. ❯❯ Critical Thinking Question Figure 23.32 With features that allow for climbing in trees like a gorilla, and also walking upright on the ground like a hominid, should “Ardi” be considered a "missing link?" Why do you think "missing link" is not a good term to use in evolution? Answer: One can make the argument for this fossil being placed between “gorilla” and “hominid.” The term “missing link” is more of a press release term: Aha – look at our latest find! The paucity of fossil evidence for the evolutionary pattern of life is the norm. We will continue to fill in the gaps with each new fossil found. The beauty of Science as a path of discovery is that it never completes the whole truth – there is still more out there (or maybe not!). Suggested Answer to Selected Short Answer Question (Answers to question 7 and question 8 provided in the appendix to the text) 10. Creative Thinking Visual Question: The image below (Figure 1) shows Mount Shasta in California, which is one of a dozen or so large volcanoes in the Cascade Range. What kind of volcano is shown in this image? If you were to sample a lava flow from Mount Shasta, what kind of volcanic rocks would you expect to find? In addition to lava flows, what other materials are found in volcanoes such as this? Suggested Answer: The volcanoes of the Pacific Northwest and the Cascade Range provide a scenic backdrop for a beautiful part of the United States. Mount Shasta is a compound stratovolcano that has been built by repeated eruptions during the past 200,000 years. Although the mountain itself is relatively young, it has been built atop older basalts and andesites whose ages indicate that volcanism has been taking place at the site of the present cone for at least the past 600,000 years. A stratovolcano is a tall, conical volcano composed of many layers of hardened lava, tephra, and volcanic ash. These volcanoes are characterized by a steep profile and periodic, explosive eruptions. The lava that flows from them is highly viscous, and cools and hardens before spreading very far. The source magma of this rock is classified as acidic, or high in silica to intermediate (rhyolite, dacite, or andesite). Mount Shasta is a stratovolcano, characterized by its large, conical shape and explosive eruptions. Sampling a lava flow from Mount Shasta would likely reveal andesitic volcanic rocks, known for their intermediate composition. Besides lava flows, stratovolcanoes like Mount Shasta also produce volcanic ash, pumice, and pyroclastic deposits from explosive events. Chapter 24 Geology in Perspective Chapter Outline 24.1 Introduction 24.2 Geology and the Environment 24.3 A Final Word Learning Objectives This very short chapter pulls together the concepts from the previous 23 chapters and relates them to the future of the students. Some students will have jobs that will be aided by having knowledge of geology. These students will be city planners, city council members, members of planning boards, developers, contractors, and engineers. Other students will use their geologic knowledge to enhance their enjoyment of nature, such as when visiting national parks. Still others – hopefully all – will be informed citizens when it comes to making decisions regarding global warming, acid rain, ozone depletion, and nuclear waste disposal. It is in this arena that the critical thinking skills developed here and in other aspects of secondary education should be utilized. Chapter Summary Perhaps the most important lesson you can learn from physical and historical geology is that Earth is an extremely complex, ever-changing planet. We tend to view our planet from the perspective of a human lifetime and commonly overlook the fact that Earth has changed markedly over geologic time. And our dynamic planet continues to change. Geology seeks to understand that change. The field itself constantly evolves as new information and methods of investigation become available. • Earth is a dynamic planet that has changed throughout its 4.6-billion-year history and it continues to change. Change on Earth is sometimes very fast but often much slower than humans can witness. • Earth systems, subsystems and cycles that were discussed in this book include the rock cycle, the hydrologic cycle, and the tectonics cycles of the lithosphere, • Changes in one system, subsystem or cycle will affect others. For example, a rise in global temperature will melt glaciers, raise sea level, and affect coastal erosion and storms, and ultimately the populations of people that live along the coast. • Satellites provide us with the ability to see Earth from a global perspective and to view changes over time. • Critical thinking involves evaluating the supporting evidence for a particular point of view. With critical thinking skills you can more effectively evaluate the arguments about global warming, ozone depletion, groundwater contamination and many other environmental issues. • Global warming and cooling are part of a larger cycle that has included glaciations and interglacial periods. Human effects on climate are superimposed on those natural cycles. • Environmental geologists are concerned with studying and resolving the problems that human occupation has caused to the environment. With the human population at 7 billion and climbing, the role humans play on global problems and the consequences of environmental issues on human populations are increasing. • The recent increase in atmospheric carbon dioxide adds to the natural greenhouse effect. Human activities, such as burning fossil fuels and deforestation, add carbon dioxide to the atmosphere, which contributes to global warming. • Geologic hazards such as earthquakes, landslides, volcanic eruptions, and floods are threats to humans. During 2010 and 2011 enormous earthquakes in Haiti, New Zealand and Japan bought about massive damage. Smaller hazards like soil creep also damage human development. • It is important to look at Earth and the environment over periods of time longer than those more typically relevant to humans. Enrichment Topics Topic 1. Scientific Thinking. Remind the students that science is different from other ways of thinking. Scientists use scientific method to create and test hypotheses and formulate theories. Topic 2. Science Can Address Controversial Topics. Have the students pick a topic that may be somewhat controversial but that is relevant to the topics presented in this book. They can then outline the scientific evidence that addresses that topic. A class discussion or short term paper could follow. Topic 3. Affects of Human Population Increases on Hazards. Have the students project an increase in human population of 3 billion onto the environmental problems we have today. Also, have them address the problem of natural hazards and how they may affect a larger human population. Common Misconceptions Misconception 1. It is necessary to have a Ph.D. degree to obtain a job in geology. Research in geology is only done by people with the Ph.D. degree. Fact. As has been true for a long time, many employment opportunities in geology are at the master’s degree level, and many with bachelor’s degrees are finding satisfying jobs. Research is conducted at all levels, and even undergraduate students can do original, frontier-pushing research. Misconception 2. Earth Science is not as scientific as the other sciences. Fact. As students will now be able to see, Earth Science uses all of the other sciences to better understand our planet and all of its systems. Earth Science is more integrative than many of the other sciences and requires a firm knowledge of these other fields. Lecture Suggestions 1. Although this is the last chapter, some important ideas are presented here. Humans have become extremely important geologic agents—more important than any other single agent that operates on a human timeframe. The students need to understand that their actions—individually and as a human collective—have far-reaching implications in time and space. 2. This is a great time for a wide variety of class discussions. Natural hazards, environmental problems, and human population growth are all good ideas for topics. Solution Manual for The Changing Earth: Exploring Geology and Evolution James S. Monroe, Reed Wicander 9781285733418
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