Exam 2 (Q&A)

(CH.11) What is a Precambrian shield? Where is one located in North America?

A Precambrian shield is the Precambrian portion of a craton that is exposed at Earth's surface. An example is the Canadian Shield that makes up much of Canada.

(CH.9) How does the angle of subduction beneath a mountain chain relate to the rate at which the plate and the continent are moving toward each other?

A change in the rate of plate movement usually causes a change in the angle of subduction. Rapidly moving continental plates cause the descending seafloor plate to go down at a low angle, whereas slow moving or static continental plates cause steep angles of descent on the downgoing plate (a gravitational effect).

(CH.9) What are failed rifts, and how are they important to our understanding of the breakup of continents? (Hint: Refer to Figure 9-3.)

A failed rift is a rift that projects inland from a continental margin but fails to divide the continent into two separate landmasses. A three-armed rift usually contributes two of its arms to the composite rift, while the third becomes a failed rift. Before it ceases to be active, the failed rift forms a graben or system of grabens that projects inland from the new continental margin formed by the other two arms. When we reassemble continents now bordering the Atlantic Ocean, ancient three-armed rifts become apparent.

(CH.10) How do glaciers promote chemical weathering?

A glacier accelerates chemical weathering by grinding up the rocks beneath it (mechanical weathering), which exposes more surface area to chemical weathering agents.

(CH.10) What is a negative feedback? What is a positive feedback? Give an example of each kind of feedback.

A negative feedback is a consequence of some change that retards that change. A positive feedback is a consequence of some change that accelerates that change.

(CH.8) Compare the three basic kinds of faults.

A normal fault is one along which the rocks above have moved downward in relation to the rocks below as a result of tensional forces. A thrust fault is a low-angle reverse fault along which the rocks above have moved upward in relation to the rocks below as a result of compressional forces. A strike-slip fault is a high-angle fault along which the rocks on one side move horizontally in relation to rocks on the other side as a result of shearing forces.

(CH.8) What happens to slabs of subducted lithosphere?

A slab of subducted lithosphere eventually breaks away from the plate that is being subducted. It absorbs heat from the asthenosphere around it very slowly. Remaining cold and dense, it sinks deep into the asthenosphere, where it eventually melts. The mafic magma formed when the slab melts is less dense than the ultramafic asthenosphere, and so it rises through convection.

(CH.12) How does the history of North America illustrate continental accretion?

About 2 billion years ago, the Wopmay orogen formed along the western margin of the Slave Province of northeastern Canada. Between 1.95 and 1.85 billion years ago, North America was assembled through suturing of at least six microcontinents into a sizable craton. South of these Archean terranes, a broad zone of crust formed shortly thereafter by igneous activity and by sedimentation along an island arc. The Grenville orogeny, which occurred between about 1.2 billion and 1.0 billion years ago, was another step in the accretion of the North American continent.

(CH.12) What major biological events occurred near the end of Proterozoic time?

Acritarchs began to experience high rates of origination of species and species extinctions near the end of Proterozoic. Also, animals began their rapid evolutionary adaptive radiation.

(CH.8) What is apparent polar wander? Draw pictures of Earth showing how the movement of a continent can produce apparent movement of the north magnetic pole.

Apparent polar wander refers to the apparent movement of the Earth's magnetic pole to its current position from a position much farther south, in the Pacific Ocean. The term "apparent" is used because the poles "wander" only if one assumes that the Earth's crustal plates are fixed.

(CH.13) Why do geologists know more about the life that colonized early Paleozoic seafloors than about the life that floated and swam above those seafloors?

Because seafloor dwellers left an extensive fossil record in the seafloor sediment, both skeletal fossils and trace fossils such as burrows.

(CH.11) The composition and configuration of Earth's crust changed more profoundly in the course of Archean time than during any later interval of Earth's history. Using the Visual Overview on pages 250-251 and what you have learned in this chapter, describe major changes in the Archean crust and explain how they relate to one another and to changes in Earth's deep interior.

By Archean, the Earth's crust was solid and was cooling down. However, higher internal Earth temperatures caused higher rates of mantle convection and thus more and faster plate movements than today. Cratons were forming for the first time by subduction and remelting. Sediments accumulating on craton margins (shelves) were caught up in plate tectonic deformation and deformed into greenstone belts. The high rate of rifting prevented large continental masses from forming during Archean.

(CH.13) Review the history of sediment deposition along the eastern margin of North America during early Paleozoic time and relate this history to plate movements.

Cambrian was notable for the progressive flooding of continents. As the seas began to encroach on Laurentia, siliciclastic sediments were eroded from the eastern margin of North America and accumulated as the innermost belt of sediments around the continental margin. Seaward of this belt were broad carbonate platforms. As the seas continued to move inland, limestones came to lie on top of nearshore sands. This pattern would have occurred as the tectonic plates drifted apart. During Ordovician, collisions between Laurentia and several islands that had occupied the ocean between Laurentia and Baltica and Greenland produced the Taconic orogeny, and the depositional pattern changed drastically. Carbonate deposition ceased, and flysch deposits were laid down on top of the carbonates in deep water. Near the end of Ordovician, flysch gave way to molasse in the form of shallow marine and nonmarine clastics. In the Taconic orogeny, the margin of the carbonate platform that bordered Laurentia was wedged into a subduction zone. Several islands of an igneous arc collided with Laurentia. The rocks of these small landmasses form exotic terranes that are now embedded in the eastern margin of North America.

(CH.12) What is curious about the Neoproterozoic rocks known as cap carbonates? (Hint: Refer to Earth System Shift 12-2.)

Cap carbonates rest directly upon all three tillite formations, which represent different episodes of widespread Neoproterozoic glaciation. These tillites are glacial deposits, yet the cap carbonates are warm water limestone deposits. The cap carbonates must represent very rapid global warming perhaps triggered by rapid release of carbon dioxide into Earth's atmosphere through some mechanism like submarine volcanic eruptions or melting of methane hydrates.

(CH.10) What controls the ratio of magnesium to calcium in the ocean? How have changes in this ratio influenced the mineralogical composition of limestone during Phanerozoic time?

Changes in spreading rates along mid-ocean ridges alter the magnesium-calcium ratio in the ocean. Shifts in this ratio have caused different kinds of minerals to precipitate at different times during the Phanerozoic, resulting in periods of "aragonite seas" and "calcite seas." Aragonite seas produce high-magnesium calcite and aragonite; calcite seas produce calcite.

(CH.11) Why did oxygen remain at a low concentration in the atmosphere until long after photosynthesizing cyanobacteria were very abundant on Earth?

Chemical sinks, mostly from reduced ions like iron and sulfur and iron compounds, were so abundant that they soaked up nearly all the available oxygen of the Archean atmosphere. For that reason, oxygen concentrations remained low during Archean.

(CH.12) List as many differences as you can between the Archean world and the world as it existed 1 billion years ago.

Cratons, which had just begun to form in the Archean, had grown larger and more well-developed; by the beginning of the Neoproterozoic, continental accretion and suturing had created the supercontinent Rodinia. Stromatolites had become much more widespread and diversified. Atmospheric oxygen, which had been virtually absent in the Archean atmosphere, had increased greatly through the agency of photosynthetic organisms. As oxygen built up in the atmosphere, the concentration of dissolved oxygen in the upper ocean also increased. Eukaryotes more advanced than the bacteria-like eukaryotes seen in the Archean had appeared.

(CH.8) Why do deep-focus earthquakes occur along subduction zones?

Deep-focus earthquakes occur along subduction zones because that is where slabs of lithosphere descend into the asthenosphere. The slab descends because it is cooler and therefore denser than the partially molten asthenosphere, and it produces earthquakes because it occasionally takes a sudden step downward.

(CH.8) What is the geographic extent of the lithospheric plate on which you live?

Depends on the location of the students. For North America, the plate's eastern margin is at the mid-Atlantic ridge and its western margin is comprised of a hybrid boundary. This hybrid boundary starts on the southern end with a sliding (fault) boundary, which passes laterally into a convergent (subduction) boundary. On the southern side of North America, there is another hybrid boundary, which is comprised of both sliding and convergent segments.

(CH.13) The Cambrian and Ordovician periods differed from one another in many ways. Using the Visual Overview on pages 302-303 and what you have learned in this chapter, compare these two periods with respect to sea level, the distribution of landmasses on Earth, and the nature of life in the oceans.

During Cambrian, there was a long global sea-level rise with two relatively minor regressions. These regressions occurred near the end of Early Cambrian and the end of Middle Cambrian. During Cambrian, Gondwanaland was fragmented so that Baltica, Siberia, and Laurentia were separated from Gondwanaland's main mass. The Cambrian fossil record started out with few skeletal fossils, but there was an "explosion" of fossilized shelly life during Tommotian. The trilobites and other animals radiated and diversified vastly starting in middle Early Cambrian. Large predators came on the scene, including Anomalocaris and large nautiloids. There were three mass extinctions among the trilobites during Late Cambrian. During Ordovician, sea level remained high but oscillated in cycles of about 10 million years and Gondwanaland continued to fragment. During Ordovician, the stromatolites declined as animals diversified. Ordovician organisms inhabited the soft sediment and burrowed it extensively. During Middle Ordovician, the earliest jawless fishes appeared and there was a large radiation of many life forms of the sea. The coral-strome reef community developed and land plants took hold and dropped spores. During Late Ordovician, there were two mass extinctions. The first struck at warm-adapted species and the second (the end-Ordovician mass extinction) took out cold-adapted species. The end-Ordovician was the first great mass extinction of life on Earth.

(CH.12) Life underwent extraordinary changes in the course of Proterozoic time. Using the Visual Overview on pages 276-277 and what you have learned in this chapter, describe these changes and explain how some of them may have been related to changes in the chemistry of the atmosphere.

During Paleoproterozoic stromatolites were common and the multicellular algae and simple acritarchs developed. By Mesoproterozoic, the stromatolites were at their peak of development of all time. During Neoproterozoic, more complex acritarchs and other eukaryotes developed. During Late Neoproterozoic, atmospheric oxygen reached a level in the atmosphere that could support animal life and this may have triggered the major radiation that included development of multicellular, soft-bodied life forms (the Ediacaran fauna).

(CH.8) List as many pieces of evidence as you can to support the idea that continents have moved over Earth's surface.

Evidence for continental drift includes the jigsaw-puzzle fit of the continents, similarities of rocks and nonmarine fossils on opposite sides of ocean basins, the presence of the same living species on two bodies of land separated by ocean (for example, Madagascar and India), the rift valleys of Africa, the direction of glacial scouring in eastern South America and southern Australia, paleomagnetism, and seafloor spreading.

(CH.12) What evidence is there that eukaryotic organisms existed 2 billion years ago?

Evidence for the existence of eukaryotic organisms 2 billion years ago includes fossil eukaryotes (algal ribbons) that appear in rocks 2.1 billion years old and younger and fossil acritarchs (early eukaryotes) found in rocks 2 billion years old and younger.

(CH.12) What kinds of geologic evidence suggest that continental glaciers spread widely more than 2 billion years ago?

Evidence of widespread Proterozoic glaciation comes from every modern continent and represents many Proterozoic latitudes, including the equatorial realm. Evidence of Proterozoic glaciation includes well-laminated mudstones composed of varves that formed in the standing water of a lake or ocean in front of glaciers; the presence within mudstones of dropstones (pebbles and cobbles that appear to have fallen from ice that melted as it floated out from a glacial front); and tillites, some of which are faceted or scratched from having slid along at the bases of moving glaciers.

(CH.12) What arguments favor the idea that little atmospheric oxygen existed on Earth until slightly before 2 billion years ago?

Evidence that little atmospheric oxygen existed before about 2 billion years ago includes the following: (1) uranium and iron minerals, which are readily oxidized and dissolved in the presence of oxygen, accumulated in greater concentrations in marine and nonmarine deposits before 2.3 billion years ago but are rare in sandstones younger than about 2.3 billion years; (2) banded iron formations apparently ceased to form about 1.9 billion years ago because the concentration of oxygen increased in the deep ocean; (3) a large amount of organic carbon was apparently buried in marine sediments, and oxygen that would have been used up in the decay of organic matter instead accumulated in the atmosphere.

(CH.13) What evidence is there that plants may have invaded the land before the end of the Ordovician Period?

Evidence that plants might have invaded the land before the end of Ordovician consists of fossilized sheets of cells similar to those that cover the surfaces of modern land plants, as well as structures that resemble the spores released by primitive (non-seed) plants of the modern world.

(CH.11) What features make Earth a more hospitable place than other planets for life as we know it?

Features that make Earth a more hospitable place for life than other planets include its size, which allowed a suitable atmosphere to develop and be retained; and its temperature, which allows most of its free water to exist as a liquid.

(CH.9) What is flysch and where does it form?

Flysch is made up of shales and turbidites that accumulate in deep water within a foreland basin bordering an active mountain system.

(CH.9) What geologic features enable us to recognize ancient continental rifting? What features enable us to recognize ancient subduction zones?

Geologic evidence for ancient continental rifting includes normal faults; mafic dikes and sills; and thick sedimentary sequences within fault block basins that often include lake deposits, coarse terrestrial deposits, and evaporites, followed by oceanic sediments.

(CH.11) What geologic features characterize greenstone belts, and how did greenstone belts form?

Greenstone belts are dark, podlike bodies of rock that exist between bodies of high-grade metamorphic rocks of felsic composition. Igneous rocks of greenstone belts, before they were metamorphosed, were mostly mafic and ultramafic rocks of the kind extruded along volcanic arcs at subduction zones. Greenstone belts were formed when forearc basin sediments, deformed oceanic crust, and arc volcanics along the margins of protocontinents became squeezed between protocontinents during suturing.

(CH.10) How can carbon isotopes in limestones provide evidence of historical atmospheric concentrations of O2? (Hint: Refer to Figures 10-11C and 10-13.)

High values of carbon 13 in limestones indicate that large volumes of carbon accumulated rapidly. By measuring the relative percentage of carbon 13 in limestones of various ages, we can tell when these rapid accumulations occurred. Because an increase in the rate of carbon burial causes oxygen to build up in the atmosphere, we can correlate the rate of carbon burial with the amount of oxygen in the atmosphere.

(CH.10) What are the two possible fates of plant material not eaten by animals?

If it is not eaten by animals, plant material either decays or is buried in sediment.

(CH.13) On which landmasses is the climate likely to have been warmer in Late Cambrian time than it is today? (Hint: Compare Figure 13-17 with a map of the modern world.)

Laurentia, which was positioned near the equator in Late Cambrian time but whose modern continents (Greenland, North America) are now in high latitudes.

(CH.11) Why is it likely that life arose in the vicinity of midocean ridges?

Life probably arose near hot-water vents near mid-ocean ridges because (1) the enormous size of the ridges offered a large range of temperatures, which provided many opportunities for key evolutionary events to take place; (2) organic compounds required for the creation of early life dissolve readily in the warm water; (3) many of these waters are anoxic, which means that chemicals essential to life were not destroyed by free oxygen; (4) the environment offers an abundance of phosphorus, an element required by all organisms; (5) the environment contains metals that all organisms require in trace quantities; (6) these areas are well supplied with clays, which serve as useful substrates for the assembly of large organic molecules; (7) they provide simple organisms with the opportunity to harness a variety of naturally occurring chemical reactions that release energy.

(CH.11) Why did magma rise from the mantle to Earth's surface at a higher rate during Archean time than it does today?

Magma rose from the mantle to the surface faster in Archean times because Earth's radioactive "furnace" was hotter.

(CH.13) Why do some Lower Ordovician rocks in Maine share some trilobite taxa with Great Britain but not with neighboring areas of the United States?

Maine is made up of an exotic terrane that was once part of Avalonia, a fragment of Gondwanaland that moved toward Baltica and includes similar terranes that are now found in Newfoundland and southern Great Britain.

(CH.9) Using the Visual Overview on pages 204-205 and what you have learned in this chapter, trace the history of a continental margin that experiences the following events: it (a) originates by rifting and becomes a passive margin along which sediment accumulates, (b) stalls at a subduction zone, where subduction reverses, (c) grows a mountain chain, (d) becomes a passive margin again when the igneous arc that formed it ceases to function, and (e) eventually loses its root through erosion and isostatic uplift.

Many mountain chains may be examples of the history described in this question. First, rifting of a continent or supercontinent occurred and passive margins spread apart. Then convergence began (intervening ocean closes). Convergence stalled at the subduction zone and subduction reversed causing continental collision. The mountains were thus formed with an igneous arc at the core. The igneous arc ceased to function, and isostatic uplift caused the mountains to be eroded to such an extent that the root of the mountains is largely lost.

(CH.8) Why do mid-ocean ridges form?

Mid-ocean ridges form because that is where new lithosphere is being created. The elevation of the ridge results from the hot, swollen condition of the newly formed crust.

(CH.10) Why does the influence of moist climates on vegetation accelerate weathering?

Moist climates allow forests to occupy the land, and their roots accelerate chemical weathering because they secrete acids and other compounds that break down minerals. Forests also cycle water through the soil repeatedly, which results in the rapid dissolution of soil minerals.

(CH.9) What is molasse? Why does it normally accumulate after flysch?

Molasse is made up of nonmarine and shallow marine sediments that accumulate in front of a mountain system after heavy sedimentation from the mountain has driven deep marine waters from the foreland basin there. It accumulates after the accumulation of flysch because flysch sediments eventually choke the foreland basin, pushing marine waters out and leaving nonmarine sediments in their place.

(CH.8) Why are most volcanoes that have been active in the last few million years positioned in or near the Pacific Ocean?

Most recently active volcanoes are positioned in or near the Pacific Ocean because volcanism is associated with subduction zones, and subduction zones border much of the Pacific Ocean.

(CH.9) How can mountain chains form without continental collision?

Mountains can form along the margin of a continent that is resting against a subduction zone even when that continent does not collide with another continent. These mountains have an igneous core and an associated metamorphic belt and an inland fold and thrust belt. A primary example is the Andes, where the oceanic Nazca Plate is being subducted beneath the continental South American Plate.

(CH.9) Why do mountains have roots?

Mountains have roots to balance the mass of the mountain; the balance is known as isostasy.

(CH.9) Are the rocks that become ophiolites within a mountain chain older or younger than molasse deposits that form along the mountain chain?

Ophiolites within a mountain chain are older than the molasse deposits that form along the mountain chain. Ophiolites are remnants of the seafloor that were pinched up along a continental suture when the mountains began to be uplifted. Molasse deposits occur during the wearing down of the mountains.

(CH.10) In what kinds of marine environments can carbon be buried in large quantities?

Organic carbon can be buried in large quantities in anoxic marine environments. The lack of oxygen means an absence of bacteria that would decompose the organic material.

(CH.10) Why are pelagic carbonates more likely than shallow-water carbonates to melt and return CO2 to the atmosphere?

Pelagic carbonates are easily destroyed by subduction along trenches near their place or origin. Shallow-water carbonates release their carbon dioxide only if they are caught up in mountain building.

(CH.12) Using a world map, locate the modern positions of the various landmasses that make up the supercontinent Nuna shown in Figure 12-22.

Please use a world map to locate the modern positions of the various landmasses that make up the supercontinent Nuna (Figure 12-22).

(CH.11) What reasons are there to believe that Earth was pelted by vast numbers of meteorites early in its history?

Radiometric dating of meteorites found on Earth indicates that most are around 4.6 billion years old. The evidence that Earth was bombarded by meteorites early in its history comes from the craters found on the moon and on other planets, which have not undergone the erosion and weathering that constantly change Earth's surface. Also, the rotation axes of Earth and other planets are tilted; it is believed that the tilting was a result of the impacts of large bodies early in the history of the solar system.

(CH.11) What types of sedimentary rocks were rare in the Archean Eon? What does this suggest about the nature of cratons during Archean time?

Sediments deposited in terrestrial and shallow marine environments are relatively rare in the Archean record. This suggests that the cratons in Archean time were small protocontinents without extensive continental margins.

(CH.10) Draw a diagram depicting the cycle of oxidized carbon that includes limestone, atmospheric CO2, and weathering.

See Figure 10-14.

(CH.10) Draw sketches illustrating how increased burial of carbon reduces the atmospheric reservoir of CO2 and enlarges the atmospheric reservoir of O2.

See Figures 10-5 and 10-6.

(CH.11) What are stromatolites? From what we know of their formation today, why might we expect them to have been present early in Earth's history?

Stromatolites are organic sedimentary structures produced by cyanobacteria. The earliest fossils yet discovered resemble photosynthetic eubacteria of the modern world and may have been cyanobacteria.

(CH.8) How does the Global Positioning System allow geologists to measure the velocity at which a plate is moving in the present world?

The Global Positioning System (GPS) uses artificial Earth-orbiting satellites that serve as know reference points. A radio signal from the satellite to a point on the Earth's surface that is a reference point for plate movement makes positioning of that point on the Earth very precise; over time it can show if that point is moving and if so in what direction. After several years, the rate and direction of motion of that point can be compared to earlier measurements for this purpose. This method confirms earlier estimates of plate movement, which is a yearly average of about 5 cm per year.

(CH.12) Where did the Grenville orogeny occur?

The Grenville orogeny was a step in the accretion of Laurentia, adding a belt that stretched from northern Canada far down into the eastern United States.

(CH.12) How are the basic features of the Wopmay orogen typical of orogenic belts in general?

The Wopmay is an orogenic belt that is in ancient fold-and-thrust mountain ranges that lie along continental margins and have been heavily eroded over time. It has the same basic features as other such orogenic belts, for example, the Appalachians.

(CH.13) What evidence is there that the variety of animals that burrowed in marine sediments increased during early Paleozoic time?

The fabrics of sedimentary rocks from Cambrian through Ordovician show increasing evidence of the effects of organisms that burrow into sediment. These burrows disrupt normal horizontal layering and other sedimentary features and their increasing presence thorough time reflects the diversification and radiation of sediment-burrowing animals.

(CH.12) How were the crustal elements of Gondwanaland assembled?

The large block that separated from Laurentia as the Pacific Ocean formed was eventually to become the eastern segment of Gondwanaland. Between about 700 million and 500 million years ago, numerous small continental blocks were sutured together to form the large body of crust that today constitutes Africa. This Pan-African suturing, together with the separation of Laurentia and Baltica, resulted in the landmass that would become Gondwanaland.

(CH.13) What kinds of organisms formed reefs in Cambrian time? What kinds of organisms performed this role during the Ordovician?

The main reef builders during Cambrian were archaeocyathids; other unknown organisms encrusted the archaeocyathid skeletons and bound them together. During Ordovician, the primary reef builders were tabulate corals and stromatoporoids.

(CH.13) What fossil evidence suggests that distinctive new kinds of predatory animals evolved during Cambrian time?

The presence of several kinds of teeth in the Tommotian fauna indicates that a variety of small predators were present in the Early Cambrian. Evidence for new predatory animals during the Cambrian has also been found in an unusually well preserved Early Cambrian fauna of soft-bodied animals in China. Members of this fauna were rapidly buried by mud, which inhibited the decay of soft tissues.

(CH.8) Suppose that you were to encounter a well-trained geologist who was unfairly imprisoned in 1955 and was deprived of reading materials until he was released last week. He entered prison firmly opposed to the idea that continents have moved great distances across Earth's surface. Given an hour of time, how would you convince this unfortunate geologist that continents have actually moved thousands of kilometers? Use the Visual Overview on pages 184-185 as a guide to develop your argument.

The same arguments used over the history of continental drift and the evolution of plate tectonics would be useful in this instance. Plate tectonics is a unifying concept that helps explain the distribution of well-established fossil occurrences that cannot be explained in any other simple way. Also, continental assemblages like Gondwanaland help explain the distribution of ancient glacial deposits. Pangaea helps us understand the distribution of sediments and fossils formed before its breakup. Plate tectonics explains the rock cycle, as it allows for recycling of materials that are made at mid-ocean ridges and consumed at subduction zones. Plate spreading explains the magnetic signatures of the ocean floors and the thicknesses of sediments on the ocean floors (thicker near the continents and thinner toward the center—the opposite of what would be expected of a static ocean basin). Also, the concept helps explain why ocean floor rocks are so young compared to the continents' rocks. Lastly, plate tectonics explains the distribution of volcanoes, deep-focus earthquakes, and large faults—they are plate boundaries. Using GPS, we can measure plate movement today so we know it is happening.

(CH.13) What is the significance of the Burgess Shale? In what geographic region and environmental setting did it form?

The significance of the Burgess Shale is that it has yielded a spectacular fauna of Middle Cambrian soft-bodied animals. The Burgess Shale is located in the Rocky Mountains of British Columbia and was deposited in deep water at the foot of the steep carbonate shelf bordering western Laurentia. Soft-bodied animals were buried in an anoxic environment and thus avoided bacterial decay.

(CH.10) What can the study of oxygen isotopes tell us about ancient oceans?

The study of oxygen isotopes can tell us about changes in ocean temperature, salinity, and volume of glacial ice.

(CH.8) What are the multiple driving forces of plate movement?

The three driving forces of plate movements are convective motion in the asthenosphere; the ascent of magma at spreading zones; and the descent of cold, dense lithosphere at subduction zones.

(CH.9) Examine a world map or globe to locate mountain chains that are not discussed in this chapter. Then locate those chains on the plate tectonic map of the world (see Figure 8-28). See if you can figure out how the presence of each mountain system might relate to plate tectonic processes. (Some of the answers appear in the chapters that follow.)

There are many mountain ranges to choose from on Earth. The Ural Mountains, which separate Europe from Asia, are like other folded mountains (for example, the Pyrenees) in that they separate what were two ancient continents that are now sutured together because of plate collision.

(CH.11) What evidence is there that certain kinds of bacteria and archaea were present in Archean time?

There is both fossil and chemical (biomarker) evidence. There are fossil stromatolites, which contain fossil cyanobacteria, that are about 3.5 billion years old. Fossil cells from prokaryotes (bacteria or Archaea) are found in rocks as old as 3.5 billion years. In 3.7 billion-year-old rocks, there are isotopically light carbon particles that must have been produced by living organisms, such as bacteria or Archaea. In 3.5 billion-year-old rocks, there is carbon that is so light isotopically that it must have been produced by methane-forming Archaea.

(CH.13) What evidence is there that major extinctions of trilobites occurred very suddenly during the Cambrian?

There is evidence that major sudden extinctions of trilobites occurred during Cambrian. Specifically, each one of the extinctions took place during the deposition of a layer of sediment just a few centimeters thick and thus must have lasted no more than a few thousand years. Above the thin layer that records each extinction lie beds about a meter thick in which a variety of new trilobite genera join species that survived the extinction.

(CH.11) Why might we expect Earth to be nearly the same age as its moon and the material that forms meteorites?

We might expect the Earth to be the same age as the Moon and as meteoritic material because all these bodies originated at around the same time. The age of rocks on the Moon matches that of meteorites found on Earth.

(CH.10) Using the Visual Overview on pages 224-225 and what you have learned in this chapter, (a) summarize how burial of organic carbon, alteration of carbonates at high temperatures, and changes in rates of weathering alter greenhouse warming by Earth's atmosphere, and (b) explain how carbon isotopes are used to assess rates of burial of organic carbon.

When more organic carbon is buried, less carbon returns to the atmosphere and therefore less carbon dioxide enters the atmosphere and greenhouse warming is weaker. When carbonates are altered by high temperature, carbon dioxide is released in volcanic gasses and this puts more carbon dioxide in the atmosphere. This contributes to greenhouse warming but also causes more chemical weathering by carbonic acid on land, thus increasing weathering rates. Increasing weathering rates put more bicarbonate into river waters, which enriches the sea in bicarbonate. Oceanic sediment and biologic uptake of bicarbonate take carbon out of the global system, and this has a feedback effect of removing carbon dioxide from the atmosphere. Because lighter carbon (carbon 12) is preferentially buried when rates of carbon burial increase, the ratio of carbon 13 : carbon 12 in the atmosphere increases and this signal is preserved in sediments and fossil materials formed during these times.