Review Questions and Answers; Introduction to Geology


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1. Geology is traditionally divided into two broad areas. Name and describe these two subdivisions.

These are physical and historical geology, often taught as separate, introductory courses in a one-year sequence. Physical geology deals with the materials (minerals, rocks, water, etc.) that comprise Earth; with processes of rock formation and decomposition; with how surface morphology is altered by the various agents of erosion; and with how rocks deform, lands are uplifted or lowered, continents moved, and ocean basins opened and closed through tectonic forces and lithospheric plate movements.

Historical geology places origins of rock masses, integrated effects of geologic processes, interpretations of ancient environments and life forms, and past tectonic movements into the chronological framework of the geologic time scale. Thus geology is an historical science; passage of time and evolutionary concepts are vitally important.

2. Briefly describe Aristotle's influence on the science of geology.

Aristotle was the foremost of the ancient Greek, natural philosophers who asked questions about nature and natural phenomena and offered explanations based mainly on intuitive feelings, logical deductions predicated on baseless assumptions, and elitist sophistry. His ideas dominated natural science thinking in Europe during the Middle Ages. During this same time, mathematics, astronomy, geographical knowledge, metallurgy, and other early technological skills were preserved in Islamic centers of the Middle East and North Africa. For example, Earth's diameter, as determined by Eratosthenes, third century, B. C., was much closer to the true value than the one that Columbus used to plan for his historic 1492 voyage to find the East Indies by sailing to the west.

Modern science was born in the seventeenth and eighteenth centuries when the mental discipline and logic rules of natural philosophy were fused with the crude, empirical materials science and information technologies that had been progressing slowly for hundreds of years. Geology was somewhat slower than other sciences to emerge in its modern form because, prior to the late eighteenth century, theological doctrines dominated learned discourse and thinking about Earth and its geological history. To this day, a few latter-day creationists, diluvialists, and anti-evolutionists eagerly proselytize and defend these same naive beliefs that were abandoned by geologists over 150 years ago.

3. How did the proponents of catastrophism perceive the age of Earth?

They believed Earth to be a very young planet. Accepting such a brief geologic history forced them to explain Earth's evolution in terms of many, rapid, short-term, catastrophic events. Stupendous natural features like the Grand Canyon, mountain ranges, the polar ice caps, the oceans, etc. had to develop quickly. Integrated effects of slow movements, or of slowly operating processes, were viewed as having had little importance in Earth's geologic history and evolution. Latter-day creationists face the same problems with excessive geologic time compression as the eighteenth century catastrophists. All rocks, geologic features, and life forms, extinct and living, had to have existed simultaneously or developed at breathtaking speed, and well-studied scientific processes such as radioactivity and molecular genetics have to be turned inside out or denied completely. On an intellectual basis, the prolonged, uniformitarian view of Earth's origin and geological history is much easier to accept and defend than the short time scale of the creationists.

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4. Describe the doctrine of uniformitarianism. How did the advocates of this idea view the age of Earth?

Uniformitarianism basically says that rational observations and analyses of modern geologic processes and events give an accurate representation of geologic workings in the past. For example, seemingly inconsequential and barely recognizable stream erosion can cut a Grand Canyon, given enough time. Lateral movements of a centimeter per year can build oceans and move continents hundreds of miles, given enough time. In addition to the slow day-to-day processes, occasional, large-scale, powerful events (volcanic eruptions, earthquakes, meteorite impacts, etc.) occur as part of the very long, evolutionary history of Earth. Acceptance of the uniformitarian concept logically forces one to accept a very old age for Earth and a very long geologic time scale.

Ask your students what they think and believe about the very long span of geologic time that we, as geologists, accept as scientifically proven. Don't be surprised or offended if some students reject our old-planet theory and opt for a short-lived, divinely-configured Earth. Most of us do live in free countries; we can believe what we want to believe, even if out beliefs are irrational and directly conflict with well-documented scientific evidence.

5. Briefly describe the contributions of Hutton, Playfair, and Lyell.

James Hutton was a Scottish geologist (late 1700s) who led the intellectual fight in behalf of uniformitarianism and a long geologic time scale. He recognized that the igneous rocks originated by cooling and crystallization of molten rock (magma), not by precipitation from seawater as believed by some geologists of his time. In his major work, The Theory of the Earth, uniformitarianism was, for the first time, clearly formulated as a fundamental concept in geologic thinking.

John Playfair was a friend and colleague of Hutton. Playfair's book, Illustrations of the Huttonian Theory (published in 1802), alerted natural science scholars of that time to the importance of Hutton's concepts and ideas. His book was written in a lively, reader-friendly mode, in contrast to the laborious, intricately complex style of Hutton's writings.

Charles Lyell (mid 1800s) was an influential English scientist and teacher. His book, Principles of Geology (in numerous editions), was the most widely used and most influential text of that time. Lyell was a strong advocate of the Huttonian or uniformitarian view of Earth history and of a very long geologic time scale.

6. How old is Earth currently thought to be?

The currently accepted age of 4.5 to 4.6 billion years is based on meticulous experimental measurements of lead isotopes on meteoritic and terrestrial samples. The basic assumptions and results are supported by rubidium-strontium isotopic age determinations on meteorite samples. This age gives the time passed since originally dispersed, chemical constituents of the solar system were assembled into meteorites, asteroids, planetary satellites, and planets. The oldest rocks yet dated formed about 4 billion years ago. Because Earth is a dynamic planet, most rocks we see formed much later during Earth's history and thus are much younger than the age of the Earth.

7. The geological time scale was established without the aid of radiometric dating. What principles were used to develop the time scale?

In a series of horizontal, stratified rocks, younger strata lie above older strata. This is known as the law of superposition and assumes that all sedimentary strata were originally deposited as horizontal layers. Fossils (remains of ancient living organisms) changed through geologic time so that specific fossils or assemblages of fossils are found only in strata of specific ages and are unique indicators of geologic age; this concept is called the principle of faunal succession. Relative ages of contacting igneous and sedimentary rocks can be determined by recognizing cross-cutting relationships and erosional unconformities. These concepts and relationships enable geologists to identify and correlate rocks of similar ages anywhere on Earth and to place these rocks in their proper, chronological order and position in the geologic time scale.

8. Why are the Archean and Proterozoic eons not divided into as many subdivisions as the Phanerozoic eon?

The Phanerozoic time scale and its divisions are based mainly on similarities and differences in fossil assemblages. The first multicellular organisms occur in rocks of latest Proterozoic age. Earlier life forms were very small, single-celled organisms such as algae and bacteria. Their fossils are very small, difficult to study and hard to find. Fossils of Earth's earliest Archean organisms are very scarce. Rich and diverse faunas and floras characterized the later stages of Earth history; thus a wealth of fossil and geologic information is available for constructing the Phanerozoic time scale, but very little fossil information is available to subdivide the Archean and Proterozoic eons.

The Ediacara biota (Narbonne, G. M., GSA Today, v. 8, no. 2, 1998) have attracted much interest in recent years and may eventually be recognized as the basis for defining the Vendian and Cryogenian Periods of the Neoproterozoic Era. The soft-bodied, highly diverse, novel to bizarre, Ediacara organisms evidently represent the first reasonably complex metazoans to appear on Earth. Their fossilized impressions, now recognized on all continents except South America, are restricted to rocks that formed during the 20 million years or so preceding deposition of basal Cambrian sections marked by the appearance of abundant invertebrates with hard parts. The time interval dominated by the Ediacara biota would constitute the Vendian Period; the earlier portion of the late Proterozoic (Neoproterozoic Era) marked by mainly prokaryotic algae would be called the Cryogenian Period.

9. How is a scientific hypothesis different from a scientific theory?

An hypothesis is a specific idea or explanation the validity of which can be tested by observations and experimental studies. It may be one of many, different, competing ideas or statements purporting to explain some scientific phenomenon. Depending on the outcomes of the observations and experiments, an hypothesis can be accepted or rejected. Hypotheses usually are directed to specific, scientific questions and issues. A theory is a useful, currently accepted, unifying body of concepts and principles in a science. A theory helps to explain what otherwise might be perceived as disjointed and unrelated observations and phenomena. A theory is based on far more observations and experiments than an hypothesis and applies to a broader range of scientific phenomena.

However, even a theory can be shaken or brought down by new observations, experiments, and interpretations of existing data. Consider the now-discarded, static continent theory, deeply entrenched in English and American geologic thought for the first half of the twentieth century. With widespread acceptance of the plate tectonic theory, the static continent idea quietly and without a fitting eulogy slipped into intellectual oblivion.

10. List and briefly describe the four "spheres" that constitute our environment.

These are the four, major spheres of our living environment (Fig. 1.12):

1) atmosphere - the gaseous envelope surrounding our planet

2) hydrosphere - those environments (oceans, rivers, lakes, ice, groundwater and water vapor in the atmosphere) involved in the hydrologic cycle

3) biosphere - the diverse, surficial and near-surface environments that include all living organisms and their habitats

4) solid earth - the soils, regolith, and crustal bedrock layers of Earth; it hosts most of the hydrosphere, forms the inorganic substrate for the biosphere, and interacts extensively with the atmosphere

11. The present shoreline is not the boundary between the continents and the ocean basins. Explain.

The continental shelves are geologically parts of the continents that are for the most part submerged because of today's high, post-glacial sea levels. The mid-depth point on the continental slope and the base of the continental slope, where continental rocks give way to sediments and basalts of the ocean floor, represent the edge of a continent much more accurately than does the present-day shoreline.

12. Briefly describe the events that are thought to have led to the formation of the solar system.

Between 5 and 6 billion years ago, a very dense, collapsed star exploded and dispersed matter throughout the volume of the present-day solar system. This matter formed a disk-shaped cloud that began to rotate and contract gravitationally. A protosun grew from a concentration of matter at the center of the disk; eventually it grew large enough to support thermonuclear reactions and the Sun was born. Eddylike, swirling movements in the rotating disk produced local concentrations of the dispersed matter and promoted growth of small planetoids. Continued growth led to larger gravitational forces and even more collisions and impacts until most of the dispersed matter was collected into a few, remaining, massive protoplanets. The ones nearer the Sun had accumulated large percentages of rock-forming and metallic elements such as silicon and iron. Through internal processes such as gravitational compression, heating from radioactivity, melting, and degassing, they evolved into the more dense, rocky, inner planets (Mercury, Venus, Earth, and Mars). The outer, Jovian planets have much lower bulk densities and consist mainly of frozen and condensed gases. These planets have comparatively small rocky cores surrounded by massive quantities of compressed, frozen and liquid substances such as hydrogen, methane, ammonia, carbon dioxide, and water. Jupiter, the largest planet, is evidently not quite massive enough to support thermonuclear fusion of hydrogen; thus it never became a "second" sun.

Carbonados are pebble to boulder-size masses of microcrystalline carbon with the diamond structure found only in regolith of Earth's oldest shield areas. These unusual objects may represent carbon that crystallized directly from dispersed primitive matter of the solar system and fell to Earth on the earliest-formed parts of the continental shields.

13. List and briefly describe Earth's compositional divisions.

These include, from the surface inward, the crust, mantle, outer core, and inner core. The crust is the most felsic of these, being enriched in silica, alkalis, calcium, and aluminum over mantle rocks that are dominated by magnesium silicate phases. Upper mantle minerals include mostly Mg-rich olivine and pyroxenes; more dense phases with similar chemical compositions are thought to comprise most of the deeper mantle. The outer core is liquid. Consideration of its density and electrical properties suggest the liquid is dominantly iron with smaller quantities of nickel, sulfur, and other elements. The inner core is a crystalline solid; it is thought to be mainly iron alloyed with nickel and a small percentage of other elements.

14. Contrast the asthenosphere and the lithosphere.

These terms describe the outer two layers or "shells" of Earth, the lithosphere being the surface (outermost) shell and the asthenosphere being the shell directly under the lithosphere. The two shells differ significantly in their mechanical responses to stress. Lithospheric rocks under stress fail (deform) by brittle fracturing (faulting). In contrast, deeper rocks of the asthenosphere deform by ductile flowage, in which the rock gradually changes shape and form without ever being physically cracked or broken. Ductile flowage is enhanced by high temperatures; brittle fracturing is typical of "cold" rocks.

Earth's moving tectonic plates comprise the lithosphere; slow flowage movements in the asthenosphere drive the plate movements.

15. With which type of plate boundary is each of the following associated: subduction zone, San Andreas fault, seafloor spreading, and Mount St. Helens?

Subduction zone describes a convergent plate boundary where the more dense plate, usually an old, oceanic plate, is sinking beneath a less dense (more buoyant) plate. A deep, linear, oceanic trench marks the surficial expression of the sinking plate

The San Andreas fault is a deep, vertical fault that forms a transform plate boundary separating two lithospheric plates moving horizontally in opposite directions. The sliver of California and Baja California on the west side of the fault is part of the Pacific Plate and is moving northwest with respect to rocks of the North American Plate east of the fault.

Seafloor spreading occurs at a divergent boundary (a mid-ocean ridge or continental rift); new, basaltic, seafloor crust forms at the trailing edges of the plates diverging away from a mid-ocean ridge.

Mount St. Helens (Washington) is a very young stratovolcano in the Cascade Range. This volcano and others of the Cascade Range are situated above a subduction zone in which a small oceanic plate (the Juan de Fuca) is sinking beneath the western margin of the North American plate.

16. Using the rock cycle, explain the statement "one rock is the raw material for another".

Sedimentary rocks are composed of constituents derived from the disintegration and decomposition of other rocks (igneous, metamorphic, or sedimentary). Metamorphic rocks were once igneous, sedimentary, or metamorphic rocks that have since changed in texture and/or mineral composition in response to elevated temperatures, or elevated temperatures and pressures (deep burial). Igneous rocks form by cooling and crystallization of magmas; magmas form by melting of other igneous, sedimentary, or metamorphic rocks.

Thus any kind of rock can function as "source material" (raw material) for any one of the three, major, rock groups.