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Geology 103 Review Questions
Terms in this set (89)
Describe the general steps used by geologists and other scientists in their attempt to solve particular problems or explain natural phenomena.
Scientists follow a procedure called the scientific method, which has four steps:
1. develop a question about an observation in nature;
2. formulate a hypothesis or plausible explanation to answer the question;
3. test the hypothesis by seeing if it fits the data; and
4. accept, reject, or modify the hypothesis. If scientists modify the hypothesis, they test it again to see if it better explains the phenomenon in question.
Discuss the principles that Steno, Lyell, and Smith formulated for the development of the geologic time scale.
Steno established superposition of strata, original horizontality, and lateral continuity. Lyell popularized uniformitarianism by showing how it applied to the real world. He did this in his famous book Principles of Geology, the founding document of the geological sciences.William Smith demonstrated equivalency of strata by noting consistencies in their stratigraphic position relative to overlying and underlying layers, and by the unique assemblage of fossils they bore. He is best remembered for recognizing that strata were often characterized by particular fossils and that there was a general progression toward more modern-looking fossils in successively younger strata (faunal succession).
Explain the difference between a geochronologic term, sometimes called a time term, and a chronostratigraphic term, sometimes called a time-rock term.
A geochronologic unit is an abstract unit because it is a unit of time. The basic geochronologic unit is the period (Cambrian, Ordovician, and so on). Other geochronologic units include eons, eras, epochs, and ages. Consider the Devonian period, a 56-million-year-long interval that began 418 million years ago and ended 362 million years ago. Just like the year 1999, this interval of time has come and gone, never to be revisited. So how do we know anything about the Devonian period if it happened over 362 million years ago? We know about the Devonian because during that time, geological and biological processes, such as sedimentation, plate tectonics, and organic evolution, were operating and leaving a record of their operation in Earth's rocky crust. The rock strata formed by these processes during this 56-million-year interval comprise the chronostratigraphic unit known as the Devonian System. Hence, a geochronological unit is an increment of pure time, whereas a chronostratigraphic unit is a body of rock (a tangible, nonabstract entity) deposited or otherwise formed during a particular time interval.
What is meant by uniformitarianism? Cite an example of a process occurring on the Earth today that did not occur in the geologic past.
The essence of uniformitarianism is embodied in the phrase, "The present is the key to the past." In other words, we can apply our knowledge of modern geological and biological processes (and their products) to the rock and fossil record. Under uniformitarianism, we can assume that 400-million-year-old basalt flows were the product of volcanic eruptions just like those basalts forming on Hawaii today. Without uniformitarianism we have no intellectual basis for studying past events.
Why is the concept of half-life necessary? Why not use whole life?
Half-life is the time required for half of a quantity of radioactive atoms to have decayed into daughter elements. Because only half of the remaining radioactive atoms decay during the following half-life, the decay process goes on indefinitely making "whole life" impossible to determine. The half-lives of uranium 238, potassium 40, and carbon 14 are 4,500 million, 1,300 million, and 5,700 years, respectively.
How do isotopes of a given element differ from one another in regard to number of protons and neutrons in the nucleus?
Each of the known elements is defined by the number of protons in the nucleus of the atom. For example, each atom in the universe that possesses two protons in the nucleus is, by definition, helium. All atoms in the universe with six protons are carbon. For a given element, the number of protons is in the nucleus is fixed. The number of neutrons may, however, vary within the nucleus of atoms. Isotopes are atoms of a given element that vary in the number of neutrons. For example, some chlorine atoms have eighteen neutrons, while others have twenty. As long there are seventeen protons, these atoms are still chlorine (table 1-2 on page 20 of your text).
What would be the effect on the isotopic age of a zircon crystal being dated by the potassium-argon method if a small amount of argon-40 escaped from the crystal?
If argon 40 escaped from the zircon crystal, it would change the ratio of parent to daughter material in favor of the parent. Since daughter material increases with time, the decrease in daughter because of leakage would yield an age that was too young.
How do fission tracks originate? What geologic events might destroy fission tracks in a mineral crystal?
Fission tracks are small tunnels, or bullet holes, that are produced in certain uraniumbearing crystals when high-energy particles are fired off of the uranium nucleus during spontaneous fission. If a track-bearing crystal is heated to elevated temperatures, the tracks often heal, leaving no trace.
What is a mineral? What characteristics of a true mineral such as quartz or feldspar would not be present in a piece of glass?
It is a naturally occurring, inorganic solid with a specific chemical composition and atomic arrangement. In other words, a mineral is an orderly arrangement of atoms. In a glass, the atoms are not ordered.
What are the eight most abundant elements found in rocks and minerals?
They are, in order of decreasing abundance, oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium (table 2-1 in your text).
Why are silicate minerals important in geology? Which silicates might one expect to find in granite? Which silicates occur in sedimentary rocks?
Silicate minerals comprise over 90 percent of Earth's crust. As a result, they are called the rock-forming minerals. Granite is comprised of potassium feldspar, quartz, and muscovite mica, all of which are silicate minerals. The common silicates in sedimentary rocks are quartz, mica, and clay.
Which igneous rock best approximates the composition of the continental crust? The oceanic crust?
The igneous rock that best approximates the composition of continental crust is granite. The igneous rock that best approximates the composition of oceanic crust is basalt.
List the clastic sedimentary rocks in order of increasingly finer grain size.
Conglomerate, sandstone, siltstone, shale or claystone.
List the foliated metamorphic rocks in order of increasingly coarser foliation.
Slate, phyllite, schist, and gneiss.
What features in a sedimentary rock might indicate it was deposited in each of the following environments of deposition? a. Shallow marine environment b. Deep marine, continental rise environment c. Transitional, deltaic environment d. Continental, desert environment
a. Think of the prevailing processes and the resulting sedimentary features. A shallow marine setting is well-oxygenated and subject to wave and storm energy. The abundance of light would result in an active plant and animal community. If this seafloor were adjacent to a delta or other source of silicate sand and gravel, the resulting deposits would be light-colored (tan or greenish-gray), moderately well-sorted, cross-bedded sandstone with abundant trace fossils. If the seafloor was protected from influx of clay and sand, the dominant sediment source of would be the remains of plants and animals living on the seafloor. As a result, a light gray, fossilrich limestone would be deposited.
b. Deep marine rocks of the continental rise will reflect deposition by turbidity currents in an oxygen-starved setting. Graded bedding and dark coloration would be the two biggest clues to this setting.
c. Deltas are characterized by river and wave energy and an influx of sand, gravel, and mud. Swamps often form on the top of the delta plain. Interbedded sandstones, coals, and clays would result.
d. Wind is the dominant means of sediment transport in a desert. Desert sediments are readily distinguished by the high degree of sorting. Coarse sand and gravel cannot be moved, and silt and clay are blown into the atmosphere, leaving behind the fine- to medium-sized quartz particles. These grains are bounced along the ground through a process known as saltation. As the grains bounce, the corners are knocked off, leaving the grains rounded. The surface is scratched and pitted giving a frosted appearance under the microscope. Wind shapes the sand into large dunes with internal cross-beds that might be tens of feet high. Hence, sedimentary rocks deposited under desert conditions will be well-rounded, quartzose, well-sorted, light colored (tan to reddish-tan), and crossbedded.
Why are sandstones and siltstones of desert environments rarely black or gray in color?
Black and dark gray coloration in sedimentary rocks results from the presence of compounds rich in organic carbon and iron. Hence, black sediments represent deposition in an environment where
1. organisms were abundant and
2. where oxygen levels are low enough to prevent destruction of the organic matter by oxidation or bacterial action.
The desert environment lacks both the many organisms and the anoxic conditions necessary to preserve organic matter.
How does matrix in a rock differ from cement? What are the most common kinds of cements found in sandstones? Which of these is most durable?
Matrix is fine-grained material deposited with sand grains, pebbles, or boulders at the time of sediment deposition. Cement is a post-depositional component of a sedimentary rock. As groundwater percolates through tiny pores in the unconsolidated sediment, tiny crystals precipitate on the walls of the pores much like stalactites and stalagmites form on the walls of a cave. These crystals (cement) eventually fill up the porosity and glue the particles together. Common cements in sandstones are silica and calcite. Of these, the former is much more durable.
What differences in texture and composition serve to distinguish between a mature and an immature sandstone?
An immature sandstone is characterized by poor sorting, angular grains, a high percentage of clay and silt in the matrix, and a mineral composition that includes unstable minerals, such as feldspar and ferromagnesian minerals. A mature sandstone is comprised chiefly of quartz grains that are wellsorted and well-rounded.
In a columnar section of sedimentary rocks, a limestone is overlain by a shale, which in turn is overlain by sandstone. What might this coarsening upward sequence indicate with regard to the advance or retreat of a shoreline?
The limestone represents deposition furthest from shore, the shale represents deposition closer to shore, and the sandstone represents deposition at the shoreline. The vertical succession reflects an upward transition from offshore limestone to nearshore sandstone; thus, the water was becoming increasingly shallow as the rock column was deposited. This would reflect the retreat of the shoreline towards the ocean (regression).
What conditions in the Bahama Banks carbonate platform result in the high production of calcium carbonate sediment?
The main condition required for the development of a Bahama-like carbonate platform is clear, warm, shallow seawater. Carbonate production is so prolific in the Bahamas because there are no rivers dumping clay and sand into the region. As a result, the waters are quite clear. The top of the carbonate platform is covered with nutrient-rich water that is no deeper that thirty or forty feet, well within the photic zone. Since it is located at low latitudes, the surface water is warm. The warm, clear water promotes production of marine plants and algae, which in turn support a prolific community of carbonatesecreting invertebrates that, upon death, donate their shells to the growing layer of seafloor sediment.
What factors determine whether or not a particular fossil will be valuable as an indicator of the age and correlation of a stratum?
The most valuable index fossils (indicators of age) are those that had a short geological range (existed only a short time) and a widespread geographical distribution. If they are easily identified and abundant, they are even more valuable.
Fossil A occurs in rocks of Cambrian and Ordovician age. Fossil B occurs in rocks that range in age from early Ordovician through Permian. Fossil C is found in Mississippian through Permian strata.
a. What is the maximum possible range of age for a stratum containing only fossil B?
b. What is the maximum possible range for a stratum containing both A and B?
c. Which is the better guide fossil, A or C?
a. Silurian and Devonian
c. This is difficult to evaluate given the scanty data. A would be a better guide fossil than C for Cambrian and Ordovician rocks and vice versa.
A chronostratigraphic (time-rock) unit contains a different fossil assemblage at one locality than at another located 200 km away. Suggest a possible cause for the dissimilarity.
Given that the two fossil assemblages are time equivalent, the difference must be because of ecological differences. A shallow marine location would have different fossils than a time-equivalent, deep-water locality two hundred kilometers away.
In drilling for oil, geologists recover Devonian conodonts in a stratum known to be Permian in age. Explain how this may have occurred.
Conodonts are made of the durable mineral apatite. The conodont in question was eroded from a Devonian layer (along with quartz sand grains), was transported by wind and water, and was then deposited into a layer of sediment during the Permian. This is known as a reworked fossil.
Using fossils for age correlation is dependent on a priori knowledge of their age ranges. How has this knowledge been obtained?
This knowledge has been obtained through carefully observing fossil occurrences in rock layers during the last 200 years. For example, dinosaur bones have been found in Mesozioc rocks, but to date, no dinosaurs have been found in Paleozoic rocks. Similarly conodonts have been found in late-Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian, and Triassic strata. They have not been found in Jurassic or younger formations, in spite of efforts to find them there.
What are the differences between the following? a. Mitosis and meiosis b. Haploid and diploid c. Gymnosperm and angiosperm d. Stratophenetic and cladistic phylogeny e. Domain and kingdom
a. Both mitosis and meiosis are methods of cell division. Mitotic division produces daughter cells in which each nucleus contains exactly the same complement of chromosomes as the parent cell. Meiotic division involves two successive cell divisions, resulting in daughter cells having one-half the number of chromosomes as the original cell.
b. Haploid cells contain only one set of chromosomes; diploid cells contain two sets of paired chromosomes.
c. These are the flowerless and flowering plants, respectively. Gymnosperms, such as cycads and conifers, evolved early and are still present. Angiosperms are more advanced plants in which the seeds are protected by an ovary. Gymnosperms made their appearance in the early Paleozoic. Angiosperms did not evolve until the late Mesozoic.
d. Stratophenic phylogeny is the traditional method of reconstructing ancestordescendant relationships. Under this model, organisms are arranged in an ancestor-descendant tree chiefly based on geological age, with older forms near the bottom and younger forms at the tips of the branches. Using cladistics, family trees are based upon the state and presence or absence of structural characteristics such as limbs, appendages, and so forth.
e. Domains and kingdoms represent major taxonomic subdivisions of the Linnean classification scheme. Everyone is familiar with the kingdoms Plantae and Animalia. With the invention of the microscope, it became clear that some living things were neither plant nor animal; consequently, the kingdoms of Protoctista, Fungi, and Monera were added. Recently, the term domain has been added to encompass two or more kingdoms. The three domains currently recognized are Archaea, Bacteria, and Eukarya. The domain called Eukarya is divided into four kingdoms known as Protoctista, Plantae, Animalia, and Fungi.
What is meant by the term adaptation? Cite an example of adaptive radiation.
An adaptation is a modification of an organism that enables it to better live in its current surroundings or to compete in a new environment. Adaptive radiations occur when several organisms adapt simultaneously. These typically occur on the heels of mass extinctions. Examples include the Cambrian explosion, the rise of reptiles to dominance during the Mesozoic, and the early-Cenozoic rise of mammals following the demise of dinosaurs at the end of the Mesozoic.
What are peripheral isolates? Why do new species commonly arise from peripheral isolates?
A peripheral isolate is a population that lives on the margins of the species ecological range and is somehow kept in reproductive isolation from the main species population. The peripheral isolate will adapt in response to the less than optimum environmental conditions. Adaptations will be retained and enhanced because of the reproductive isolation. If isolated long enough, the peripheral population may become significantly different from the parent population, giving rise to a new species.
What are the contributions of Darwin and Mendel to our modern concept of organic evolution?
Darwin mustered a large body of circumstantial evidence suggesting that life evolved in response to its environment through natural selection. He also outlined a methodology for testing evolutionary theory. He was unable to tell us how favorable traits were passed on from one generation to the next. Gregor Mendel started us on the path towards modern genetics with his work on peas and other organisms. Modern genetics has shown how adaptations can be transferred to successive generations.
Distinguish between the concepts of phyletic gradualism and punctuated equilibrium. How did Charles Darwin account for the rapid or abrupt appearance of new species? Which, then, would be a more appropriate geologic section to study for proof of punctuated equilibrium - a continuous set of cores from the floor of the ocean, or a section on the continent where there has been repeated episodes of uplift and erosion throughout geologic time?
Under phyletic gradualism, evolution occurs progressively, evenly, and slowly. If the fossil record were complete, we would see one species develop into another species through several intermediate steps. According to phyletic gradualism, this transition affects the entire species population. Gaps, or missing links between similar groups are attributed to missing time or imperfect preservation of the fossil record. By contrast, punctuated equilibrium suggests that evolution proceeds fitfully, with long periods of stasis interrupted by brief periods of rapid change. Initially speciation affects only those organisms at the edge of the environmental range of the species and not the species population as a whole. Once the speciation event has occurred, the species remains unchanged until the next period of rapid change. One of the best indicators that validates punctuated equilibrium is the fossil record itself. In sequences of layered, fossil-rich sedimentary rocks, new species often appear suddenly in a specific bed, remain relatively unchanged in higher strata, and then disappear.
If one were able to drill a well from the North Pole to the center of the Earth, what internal zones would be penetrated?
Crust, mantle, outer core, inner core.
What are the three major categories of seismic waves? Describe their characteristis.
Primary (P), secondary (S), and surface waves. P waves are compressional waves that travel at relatively high velocities through solids and liquids. S waves are relatively slower transverse waves that only travel through solids. Both P and S waves travel through the interior of earth. Surface waves are large-motion waves that travel through the outer crust like ripples on a pond.
What is a seismic discontinuity? Where are the Gutenberg and Mohorovicic discontinuities located?
A seismic discontinuity is reflected by a sudden decrease or increase in seismic wave velocity. The Mohorovicic discontinuity is the sudden increase in the velocity of P and S waves at the base of the crust. It reflects a sudden density increase at the top of the mantle. The Gutenberg discontinuity is the rapid deceleration of P waves and the halting of S waves at the top of the liquid core.
How do anticlines (and domes) differ from synclines (and basins) with regard to the age relations of rocks exposed across the erosionally truncated surfaces of these structures?
Rocks become older as you approach the center of an anticline but become younger as you approach the center of a syncline or basin.
What are the principal categories of faults? What kinds of faults might one find in regions subjected to great compressional forces? What kinds of faults result primarily from tension in the Earth's crust?
Normal, reverse, transform. Reverse faults predominate in regions subjected to compressional stresses. Normal faults are formed in response to tensional stresses.
Compile a list of items that Alfred Wegener might have used to convince a skeptic of the validity of his theory of continental drift.
List of evidence in support of continental drift:
a. apparent fit in outline of continents bordering the Atlantic Ocean
b. distribution of Late Paleozoic tillites
c. distribution of Permian plants (Glossopteris) and animals (Mesosaurus)
d. similarity in vertical stratigraphic sequence (suggesting nearly identical history) of Carboniferous-Cretaceous rocks in Africa, South America, and India
According to plate tectonics, how did the Himalaya Mountains form? The San Andreas fault? The Dead Sea and Red Sea?
Each of these reflects a different type of plate margin interaction. The Himalayas are the result of India colliding with Asia (convergent plate margin). The San Andreas fault is a transform fault (passive margin) between the North American plate, which is moving south, and the Pacific plate, which is moving north. Finally, the Dead Sea and Red Sea occupy depressions resulting from the early stages of rifting (divergent plate margin).
According to plate tectonics, where is new material added to the sea floor, and where is older material consumed?
New oceanic crust is generated at spreading centers (midocean ridges) and destroyed at subduction zones.
What is remanent magnetism? What is its origin? How is it used in finding ancient pole positions? How has remanent magnetism helped validate the concept of plate tectonics?
Remanent magnetism is magnetic information frozen into iron-bearing igneous or sedimentary rocks as they form. With igneous rocks, as lava begins to cool, magnetic ironoxide minerals form and align their polarity with Earth's magnetic field. The alignment is retained in the rocks unless they are heated beyond the Curie Point (the temperature at which the minerals are no longer magnetic). Iron-rich grains settling through quiet water may align themselves with Earth's magnetic field. If undisturbed by burrowing animals, the resulting layers of sediment will retain a magnetic signature. The position of the modern north magnetic pole is indicated by observing a compass needle. As the "compass needles" of ancient rocks were analyzed, they suggested that the magnetic north pole was not always in its present geographic position relative to where the paleomagnetic data had been gathered. This suggests that either the magnetic pole has moved through time or that the continent where the paleomagnetic data was collected had moved. Paleomagnetic data from all the continents indicates that the magnetic pole has not changed and that the continents have drifted, changing their position relative to magnetic north.
What is the source of the Sun's heat? Given the amount of solar radiation intercepted by the Earth, why is the Earth's surface not hotter than it is?
The source of the sun's heat is nuclear fusion. In the core of the sun, hydrogen nuclei are fused together to produce helium. This fusion of nuclei into larger nuclei released tremendous amounts of energy. Three factors play a role in maintaining a surface temperature range of -50 to +60 degrees Celsius. First, only half of Earth receives radiation at any one time, but heat is returned to space over the entire surface. Second, much of the incoming solar radiation is reflected back to space by the outer atmosphere. Finally, another part of the incoming radiation is absorbed by the atmosphere and radiated back to space.
What evidence indicates that the universe is expanding?
The spectral signatures of galaxies are red shifted.
Distinguish between the terms Precambrian shield, craton, and platform.
All continents are comprised of three parts: shield, platform, and mobile mountain belt. The shield is an extensive area of continent where Precambrian igneous and metamorphic rocks are exposed and have been eroded to a nearly flat surface close to sea level. The platform is similar to the shield except the Precambrian rocks have been covered with layers of sedimentary rock. Together, the platform and shield constitute the craton (stable interior portion of a continent).
What geologic evidence suggests that free oxygen was beginning to accumulate in the Earth's atmosphere about 3 billion years ago?
Banded iron formations are important as a source of iron and as an indicator of atmospheric evolution. Banded iron formations are special sedimentary rocks comprised of alternating layers of iron-rich (red) and ironpoor (gray) layers (figure 7-4 on page 239 of your text). Most rocks of this type are older than 2 billion years. They account for most of the iron ore that is mined in the world today. The fact that the iron in banded iron formation was weakly to strongly oxidized indicates that oxygen was beginning to accumulate in the environment at that time.
Discuss the role of symbiosis in the evolution of eukaryotes. What organelles may have originated by symbiosis?
The endosymbiotic theory for the origin of eukaryotes suggests that the nucleus and well-ordered genetic material in a eukaryotic cell may have arisen through ingestion, not digestion, of one prokaryotic cell by another. Through time the ingested cell was modified into the chloroplast and mitochondria of the typical eukaryote (see pages 227-28 of your text).
How do eukaryotes differe from prokaryotes? When do eukaryotes appear in the fossil record?
Eukaryotic organisms are those with a well-defined nuclear wall, definite chromosomes, and a capacity for sexual reproduction. By contrast, prokaryotic organisms possess cells with no definite nucleus and lack membrane-bounded organelles. They are capable of only asexual reproduction and are typically smaller. The earliest appeared about 1.6 to 1.4 billion years ago.
What are metazoans? What is the earliest known occurrence of abundant metazoans? With regard to their general appearnce, what are the three major groups of Ediacaran metazoans?
Metazoans are multicellular animals. The earliest known occurrence is in the Rawnsley Quartzite (late Proterozoic) of Australia. The fauna, called the Ediacaran Fauna, is comprised of the impressions of worm-like; discoidal, jellyfish-like; and frond-shaped animal groups.
When did continental glaciation occur during the Proterozoic? What is the evidence that such glaciation occurred? Why is it unlikely that continental glaciers would have formed during the Archean?
In the region north of Lake Huron, there exist conglomerates and laminated mudstones assigned to the Gowganda Formation. The unsorted conglomerates and varved sands and shales are indicative of glacial conditions. The period of glaciation occurred sometime between 2.6 and 2.1 billion years ago. The higher thermal regimes of the Archean would have made sufficient ice accumulation to form glaciers improbable.
Stromatolites were exceptionally widespread during the Proterozoic but became relatively sparse therafter. What other organisms may have contributed to the post-Proterozoic decline of stromatolites?
Stromatolites are cabbage-shaped, laminar, organosedimentary structures formed by the trapping of sedimentary particles and precipitation of calcium carbonate in response to the metabolic activities and growth of matlike colonies of cyanobacteria and other prokaryotes. The evolution of grazers, such as snails, would have cut back the number of stromatolites and limited them to nasty environments where grazers could not live.
What geologic evidence indicates that the transconitnental arch was above sea level during most of the early Paleozoic?
Early Paleozoic strata are absent or thin over this area, suggesting that the area was above sea level and was actively being eroded or not receiving sediment during this time.
What is a clastic wedge? Are clastic wedges primarily marine or nonmarine? Under what circumstances do they develop? What clastic wedge is associated with the Taconic orogeny?
A clastic wedge is an extensive accumulation of chiefly clastic sediment deposited adjacent to uplifted areas. Sediments in the wedge become finer and thinner away from the upland source area. Sediments in the clastic wedge are typically nonmarine. These wedges typically form in conjunction with a major mountain-building event. The Queenston clastic wedge developed during the middle to late Ordovician in the eastern United States. This wedge is a by-product of the Taconic orogeny.
Why are the unconformity that separates the Sauk from the Tippecanoe not the same age at widely separated locations?
They can be traced across the entire continent of North America. Tectonic uplift would have affected only a portion of the continent, not the whole continent.
How does the history of the Cordilleran region differ from that of the Appalachian region during the early Paleozoic?
During the early Paleozoic the Cordilleran region (western margin of North America) was tectonically quiet and was the site of a shallow carbonate platform. By contrast, the Appalachian region (eastern North America) was deformed by the Taconic orogeny.
Describe the relation between the orogenies that produced the Appalachian and Ouachita mountains and the movement and position of tectonic plates.
The Appalachian orogeny affected the Eastern Coast of the United States, while the Ouachita orogeny deformed the Gulf Coast region of the United States. These mountain-building episodes reflect the collision of North America with Gondwanaland—part of the process that led to the formation of Pangaea.
Why is it logical to divide the late Paleozoic into two cratonic sequences? What are the names and durations of those sequences?
It is logical to divide the late Paleozoic into two North American cratonic sequences because the rocks comprising the Kaskaskia and Absaroka sequences are separated by a major unconformity. The shallow cratonic seas withdrew for a period of time, marking a logical break between the two intervals of widespread cratonic seas and more or less continuous deposition.
What is a cyclothem? Prepare a list and discuss each of the eustatic and tectonic conditions that might account for the cyclicity for which cyclothems are named.
A cyclothem is a vertical succession of sedimentary rock layers deposited in response to a rise and fall in sea level. Rocks in the lower part of the succession reflect gradually deepening conditions as sea level rises. Rocks in the upper part reflect gradual shallowing as sea level falls. The repeated rise and fall of sea level necessary to produce a vertical sequence of cyclothems is best attributed to a period of late Paleozoic glaciation. During glacial intervals, a relatively larger portion of Earth's water budget would be locked up in glacial ice, and sea level would drop. During interglacial intervals, ice would melt and return to the ocean, causing a rise in sea level.
Describe the environmental conditions under which the Chattanooga Shale was deposited. What problems are associated with hypotheses for the origin of this extensive blanket of dark shales?
The Chattanooga Shale is part of the black shroud that blanketed most of the North American craton with anoxic black shale during the late Devonian. These shales indicate an episode of anoxia and stagnation of seawater. Why this occurred is not well understood. By the Mississippian, the anoxia had dissipated and limestone production returned to the craton. Orogenic Event Location Age
a. Taconic New England-
b. Caledonian Norway,
What group of algae is considered the probable ancestors of the first land plants (tracheophytes)? For what reasons is this group considered ancestral to land plants?
Chlorophytes. A relationship between chlorophytes and land plants is indicated by the presence of the same kind of green pigment in both groups and by the fact that both produce the same kind of carbohydrate during photosynthesis.
Why was the evolution of a vascular system critical to the invasion of the lands by plants? What were the first plants to make the transition, and what were their characteristics?
Vascular plants have tubes and vessels that convey fluids from one part of the plant to another. This is important in land plants because water is not always present above ground. The plant must be able to derive water and nutrients from the soil and pass it to the rest of the plant. By the same token, the part of the plant underground must benefit from the photosynthesis carried out by the part of the plant above ground.
Describe the Burgess Shale animal named Pikaia. What is the evolutionary significance of this fossil?
Pikaia is significant because it has features characteristic of chordates and may be our earliest known ancestor.
To what geologic systems would rocks containing the following fossils be assigned a. fusulinids b. archaeocyathids c. Archimedes
a. Pennsylvanian or Permian System
b. Cambrian System
How do the sutures of nautiloids differ from those of ammonoid goniatites?
The sutures of nautiloids are straight or slightly undulating. The goniatites exhibit angular and zigzag sutures.
What are conodont elements? What other animal group has hard tissue (tooth or bone) of the same composition as conodont elements?
Conodonts are an extinct phylum of tiny, eellike marine creatures that possessed mineralized toothlike elements of phosphatic composition. They populated shallow oceans of the world from the late Cambrian to the late Triassic. The composition of the toothlike elements is similar to bones and teeth of modern vertebrates.
Discuss those characteristics of therapsid reptiles that indicate that they were on the main line of evolution toward mammals.
Three mammalian traits displayed by the theraspids include 1) fewer bones in the skull than generally found in reptile skulls, 2) mammal-like enlargement of the lower jaw bone, and 3) distinct differentiation of teeth into incisors, canines, and cheek teeth.
What geologic event preceds the separation of a continent into two or more parts? Cite examples from Mesozoic geologic history.
The separation of a landmass is called rifting. The area undergoing rifting is uplifted or domed as magma wells up into the lower part of the lithosphere. Tensional forces then lead to the development of elongate faultedbounded troughs. The downfaulted troughs fill with nonmarine arkosic sandstones and shales. Basaltic lavas also well up along the fractures and pour out into the valleys. The East Coast of the United States underwent rifting as Pangaea began to break up in the Triassic. This is evidenced by the presence of Triassic arkoses and basalts in elongate fault-bounded troughs that run parallel to the East Coast from Nova Scotia to North Carolina.
Describe the general structural and environmental conditions associated with the deposition of the Newark Group. Why did normal faulting predominate rather than reverse faulting? Did older structures influence the location of Triassic faults? Explain.
The Newark Group is a succession of clastic sedimentary rocks and basalts that formed in fault-bounded troughs that parallel the eastern margin of the United States. The faults that created these troughs were normal faults that developed in response to the opening of the Atlantic Ocean during the Triassic.
Discuss the characteristics of structures produced by the Sevier orogeny.
The Sevier orogeny is best characterized as an episode of thrust faulting developed in response to westward migration of North America over the Pacific plate.
What is the total duration in years of the Mesozoic Era? Which period of the Mesozoic was the longest?
The Mesozoic encompasses approximately 160 million years. The Cretaceous period is the longest, about 75 million years. Mesozoic epicontinental seas were most extensive during the Cretaceous and most limited during the Triassic.
What was the probable source area for the sediments of the Morrison Formation of the Rocky Mountain region? What evidence indicates that the Morrison Formation was a continental rather than a marine formation?
The source area for sediments of the Morrison Formation was the Cordilleran highlands, which extended in a north-south direction from Montana through central Utah and south into Arizona and Mexico. The sedimentary structures and rock types comprising the Morrison formation suggest deposition on flood plains, river beds, and freshwater lakes. The Morrison Formation also contains abundant remains of large dinosaurs as well as other land-dwelling organisms.
During what geologic period of the Mesozoic is chalk particularly abundant? What group of organisms provides skeletal remains for chalk deposition? Why are chalk formations rare in Paleozoic rocks?
Coccoliths are marine, golden-brown algae that secrete a spherical external shell comprised of tiny oval plates of calcium carbonate. Chalk is a fine-grained limestone comprised chiefly of coccolith plates. These were particularly abundant during the Cretaceous. In fact, the word creta means "chalk."
What are diatoms? How do they differ in composition and morphology from coccolithophorids? How might the distribution of diatoms and coccolithiophorids be related to the acidity or alkalinity of ocean water?
Diatoms are a type of marine algae that secrete a tiny shell out of silica. The shell is called the frustrule and is usually composed of an upper and lower part that fit together like a lid on a box. Coccoliths are plateshaped and made of calcite instead of silica.
How did the terrestrial plant flora of the Jurassic differ from that of the Cretaceous? What environmental conditions might have driven the change in floras?
During the early Cretaceous the first angiosperms evolved. Angiosperms differ from other plants in that they have flowers and enclosed seeds.
What are ammonoid cephalopods? What attributes of ammonoids result in their having special value as guide or index fossils?
Ammonoids are a group of cephalopod mollusks that had a coiled external shell. The suture patterns of Mesozoic ammonoids were quite complex compared to those of nautiloids. Mesozoic ammonoids are excellent guide fossils because they evolved rapidly and adapted to many marine environments worldwide.
What are foraminifers? Why are the of particular value to petroleum geologists involved in the correlation of subsurface strata?
Foraminifers are small, single-celled marine animals that have a chambered internal skeleton comprised most commonly of calcium carbonate. They evolved rapidly and are abundant in marine rocks and sediments. They are particularly useful in subsurface correlation because they are small enough to be numerous in cores and rock chips taken from wells during drilling.
Discuss the differences between the marine invertebrate faunas of the Mesozoic and those of the Paleozoic. What Paleozoic groups are not seein the Mesozoic?
During the Paleozoic, brachiopods, tabulate and rugose corals, and stalked echinoderms, such as crinoids, were dominant. After the mass extinction at the end of the Permian, brachiopods and crinoids dwindled in number, and the tabulate and rugose corals became extinct. The Mesozoic seas, as now, were dominated by mollusks (clams and snails). Mesozoic and modern corals belong to a group known as the scleractinians. Unstalked echinoderms, such as sea urchins and star fish, are more common than crinoids. Brachiopods are rare in the modern oceans.
What attributes of Cretaceous mammals may have contributed to their survival during the biologic crisis at the end of the Mesozoic?
The mammals had developed improved nervous, circulatory, and reproductive anatomies that allowed them to thrive in cold as well as warm climates.
Discuss the differences between world geography at the end of the Permian as compared to the end of the Cretaceous.
At the end of the Permian all of the continents had been sutured to form Pangaea. By Cretaceous time Pangaea had begun to rift, giving rise to today's continents. The Atlantic Ocean had formed but was not nearly as wide as it is today.
What two classes of vertebrates appear for the first time during the Mesozoic Era?
Mammals and birds.
Cite two examples of revolutionary convergence among animals living during the Mesozoic Era. Formulate single-sentence descriptions of each of the following Mesozoic animals or animal groups.
Evolutionary convergence occurs when two unrelated organisms adopt the same characteristics or behaviors in response to adaptive pressure. For example, birds and bats are unrelated but both have wings and both fly. They converged upon flight from totally different evolutionary beginnings and in the process developed superficially similar appendages (wings).
What is evidence at the boundary between the Cretaceous and Tertiary systems at many localities favors the bolide impact hypothesis for the extinction of the dinosaurs and may other animal groups? What arguments can be advanced against this popular hypothesis?
Evidence for an asteroid impact at the end of the Cretaceous is derived largely from a study of a thin clay layer that marks the Cretaceous-Tertiary boundary at many localities around the globe. This clay layer is enriched in Iridium (a common element common in asteroids), shocked quartz (common product of impacts), and spherules (tiny droplets of glass produced by impact-induced melting of rock). Finally, the boundary clays are rich in soot, which may have been produced by widespread fires caused by the impact. The fossil record shows that dinosaurs were declining in diversity and numbers before the end of the Cretaceous. Hence, an impact cannot explain all aspects of the demise of the dinosaurs. An impact may have been the exclamation point on an otherwise slow (several-million-year) decline.
Describe the manner in which each of the following major physiographic features developed: a. Mountains of the Basin and Range b. Great Plains c. Columbia and Snake River Plateau d. Red Sea e. Teton Range f. Cascade Range
a. The mountains of the Basin and Range are horst blocks separated by down faulted valleys. The faults are predominantly normal faults (those produced by tensional stresses) suggesting that this area of western Utah and Nevada has been stretched and attenuated.
b. The Great Plains are a relatively featureless upland surface that slopes eastward away from the axis of the Rocky Mountains. This province was formed during earlier Tertiary time as a thick apron of sediment was deposited by rivers flowing eastward from the eroding Rockies.
c. The Columbia and Snake River Plateaus are underlain by thick accumulations of basaltic lavas. This area was characterized by intense volcanism during earlier Tertiary times. This activity has now shifted to northwestern Wyoming (Yellowstone National Park).
d. The Red Sea is a relatively young spreading center formed as the Arabian Peninsula split from Africa 30 million years ago.
e. The Teton Range has some of the most spectacular relief in the Rocky Mountains because of block faulting on the east side of the range.
f. The dominant element of the Cascade Range is the large volcanic peaks such as Mount St. Helens, Mount Rainier, Mount Shasta, and so on. Each of these is a composite volcano produced by subduction offshore of Oregon and Washington.
What Eurasian mountain ranges resulted from the compression and upheaval of large areas of the Tethys Seaway?
What epochs of the Cenozoic Era are included in the Tertiary Period, the Quaternary Period, the Paleogene Period, and the Neogene Period?
The Tertiary period is comprised of the Paleocene, Eocene, Oligocene, Miocene, and Pliocene epochs. The Paleogene period, a recently proposed period, is comprised of the Paleocene, Eocene, and Oligocene epochs.
What is the economic important of the Green River Formation? The Fort Union Formation?
The Fort Union Formation contains great amounts of low sulfur coal. The Eocene-age Green River contains much oil shale, a fine-grained clastic rock with hydrocarbons finely disseminated throughout. In case of an oil shortage or if oil prices increase, large quantities of oil could be extracted from these deposits. At today's prices, however, it is too costly to extract these hydrocarbons.
What conditions during the Pleistocene favored the formation of extensive loess deposits? Where did this sediment come from? What accounts for the observation that the grains in loess are narrowly restricted in size, being mostly 1/16 or 1/32 mm in diameter?
Loess is extremely fine-grained sediment (rock flour) produced by the grinding action of glaciers. This type of sediment was not produced during the pre-Pleistocene epochs because there were no glaciers. Loess is deposited by the action of wind, which cannot carry much clay and sand-size sediment.
Early arboreal primates such as Notharctus had close-set eyes and grasping hands. Other than facilitating movement among tree branches, what function might these attributes have provided?
In addition to facilitating movement from tree to tree, it is likely that close-set eyes and grasping extremities permitted early insectivorous primates to accurately judge the distance to their prey without moving their heads. As a primate secured its position with its feet, its hands were free to catch prey.
During what geologic period do we find the earliest known remains of primates? When does the first member of the genus Homo appear?
The earliest know fossil primates, the Prosimians, come from rocks of the Paleocene.
Why is it easier to find fossils and to date them in the eastern African fossil sites than in many other parts of the world?
Since sedimentary rocks are difficult to date radiometrically, it is advantageous if volcanic ashes, which are susceptible to radiometric dating, are interbedded with the fossil-bearing layers.
What distinctly humanlike traits were possessed by Australophithecus? What apelike characteristics were retained?
Like humans, Australopithicus was fully bipedal and walked erect. They differed from humans in that they were well muscled and had slightly longer arms. They were a maximum of four feet tall, had a brain capacity smaller than our own, and had decidedly apelike skulls and facial features.
What features of Cro-Magnon people differentiate them from European Neanderthal people?
Cro-Magnon was taller, had a more vertical brow, less-pronounced muscle markings on the skull, a decided chin projection, and a larger brain capacity.
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