GEO Ch 10 – Geology – Flashcards
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A disconformity is a type of unconformity in which sedimentary rocks overlie either igneous or metamorphic rocks
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False - This is the definition of a nonconformity. A disconformity is a surface between parallel sedimentary beds of significantly different ages. Evidence of stream erosion or existence of a paleosol (ancient soil horizon) may help in recognizing a disconformity
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Which of the following is NOT a method to determine numerical age?
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Cross-cutting relations - Cross-cutting relationships are used for relative, not numerical, dating. Carbon and fission-track dating depend on radioactive decay to give numerical ages; dendrochronology (tree-ring dating) also gives numerical ages
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A hominin (human family) skull was found in a shale layer between two fine-grained igneous rock layers. There were no baked zones on the rocks above the igneous layers. The skull had been 100% fossilized and changed to stone; there was no original skull material left. Which of the following statements is FALSE?
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The skull can be assigned an absolute age range because it is in a layer between 2 sills - The igneous layers are flows, not sills, because they have not baked the sedimentary rock immediately above and below them. Sedimentary rock themselves can't be dated radiometrically. The skull could be 100,000 years old because the genus homo has existed since before then, through now. Because the original solid material of the skull has been mineralogically replaced, trying to age-date it would yield a misleadingly young date
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James Hutton pioneered radiometric age dating
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False - Hutton lived more than a century before the discovery of radioactivity. Arthur Holmes, who first proposed crustal creation and destruction at mid-ocean ridges and deep-sea trenches, was a pioneer in applying the principles of radioactive decay to determining the ages of rock and Earth itself
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Identify the FALSE statement. William Smith's observations..
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Noted that a fossil species could disappear, then reappear in much younger strata - Once a fossil species disappeared, it didn't reappear. Extinction is forever
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Closure temperature is
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The temperature at which a rock system ceases to interchange parent or daughter product with surrounding material - The closure temperature is the maximum temperature at which a rock is a closed system for a particular parent-daughter decay pair.
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The boundary surface between two stratigraphic formations is called a key bed
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False - It's called a contract. A key bed (marker) is a unique bed that helps in correlation of strata
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What are the four steps of radiometric age dating a geologic material?
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Collecting a rock sample; separating the minerals of interest; extracting the isotopes of interest; measuring the concentrations and ratio of the two isotopes - Entire rock samples are generally not used because parent and daughter isotopes are found preferentially or nearly absent in specific minerals. Control amounts of parent isotopes are not mixed into chemically digested mineral samples. The half-life of an isotope is unaffected by pressure or temperature.
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Before the development of isotopic dating methods, the age of the Earth was estimated by
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All possible answers - pg 320
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Identify the FALSE statement. Radiometric dating
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Can be used only if there's some uranium present in the rock - many different radioactive isotopes of many different minerals can be used
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Grains of zircon in Australian sandstones have been dated at 4.57 billion years old, which is the currently accepted age of Earth
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False - Zircon grains are the oldest material found so far on Earth, but they're only 4.4 billion years old. Scientists haven't found and don't expect to find rocks from the first few million years of Earth's existence
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Numerical dating is just a comparison of age; relative dating assigns numbers
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False - the reverse is true (pg 306)
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Which of the following methods of determining the age of materials is a radiometric dating technique?
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Measuring the proportions of certain isotopes with respect to one another - The ratios of daughter-parent isotope pairs can be determined by laboratory testing and used to estimate the time elapsed since a mineral has formed. Magnetostratigraphy analyzes Earth's polar reversals; dendrochronology studies growth rings in trees; analyzing seasonal changes and rhythmic layering travertine, sehlled organisms, and glacial ice does not involve radioactive processes
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How was Lord William Kelvin's estimate of the age of Earth affected by the discovery of radioactive decay?
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Scientists realized his estimated age was too young because he had overestimated how quickly Earth was cooling off - Radioactive decay was discovered to produce heart, and the discovery that decay was occurring throughout Earth, though at low rates, provided another source of heat- one that Kelvin did not know about. This meant that his estimate of Earth's age was too young (pg 323)
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What principle allows tree rings and often permanent snowfields (or glaciers) to be used for age-dating?
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Repeated annual patterns - Both tree growth and snow accumulation varies predictable: each is high in some season and low in another. Because of this, annual growth or deposition is easy to recognize. (pg 322)
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Pebbles within a sedimentary rock and xenoliths within an igneous rock are examples of which principle of relative age-dating?
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Inclusions - One geologic material that is found within another must be older than the one it is found within. This concept is called the principle of inclusions and is used when determining relative ages (pg 307)
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Correlation matches up rock layers across distances on the basis of similar sequences of rock layers and similar fossils in the layers
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True - correlation of rocks or of any other materials is a matching to achieve a more complete picture over a wide area (pg 311)
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If we use the analogy of the entire span of Earth's history being only one year in length and starting Jan 1, approximately when did life adapt to living on land?
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Approximately Nov 1 - The adaptation of life from the oceans to land occurred very recently in Earth's history, approximately 250 million years ago, which is equivalent to about mid-November, if the entire span of Earth's history was one year and began on New Year's Day. (pg 325)
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Eighteenth- and nineteenth-century scientists tried to determine Earth's age by analyzing the thickness of sediments, the rates of geologic processes, and Earth's temperature, but crucial flaws in these techniques gave incorrect ages
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True - Existence of unconformities, transformation of sedimentary rocks into other rock types, and the heat of radioactivity were fatal flaws in these age-determining methods (pg 323)
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A zircon crystal is found to contain 3 times as much Pb-207 ("lead 207") as U-235 ("Uranium 235") crystals form with small amounts of uranium in them but without any lead in them The half-life for the decay of U-235 into Pb-207 is 713 million years. Based on this information, how long ago was the zircon crystal formed?
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1,426 million (2.1426 billion) years - the information above tells you that zircon crystals form with no lead in them but with some uranium in them (as an impurity, substituting for zircon). There are several types of uranium, and the isotope U-235 breaks down into lead-207 at the rate of 50% every 713 million years. After one half-life had gone by, half the U-235 would be gone and turned into Pb-207. After another 713 million years (2 half lives), 3/4 of the original U-235 would have decayed into lead-207. Two half-lives equals 1,426 million years for this decay-pair. (pg 320)
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Which of the following shows four time divisions listed from oldest to youngest?
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Precambrian, Paleozoic, Mesozoic, Cenozoic - The correct order is Precambrian, Paleozoic, Mesozoic, Cenozoic (pg 340)
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Which of the following statements is FALSE? The principle of...
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Inclusion says that rock containing inclusions is older than the inclusions - The principle of inclusions states that the inclusions are older than the surrounding rock (pg 308)
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Identify the FALSE statement
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William Kelvin calculated that Earth was about 4.4 billion years old - William Kelvin, given the title "Lord" in Britain, estimated Earth's age as 20 million years, based on assumed initial temperature and estimated current rate of cooling. Kelvin did not know about radioactive decay, which generates heat, thus making the assumptions of his model and his estimated age inaccurate.
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An area of slightly dipping sedimentary rock layers has large inclusions and is intruded by an igneous dike. Apply the basic principles for determining relative ages, and identify the FALSE statement
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The inclusions are younger than the sedimentary rock they are in - inclusions are always older than the rocks they are found in. They had to exist first in order to be included in sediment or lava and become part of the rock (pg 308)
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It's not possible to say just when an individual radioactive atom will decay, but it is possible to say when half of an existing quantity of radioactive material will be gone
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True - The half-life concept doesn't apply to individual atoms, just to quantities of atoms; it is a statistical truth, not an observation of individual atom behavior
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Which of the following methods may be used to age-date a sedimentary rock
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None of the possible answers - None of the proposed methods is reliable for age-dating a sedimentary rock. This is because most sedimentary rocks consist of clasts of various ages, with younger cements binding them together (pg 323)
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Uniformitarianism
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Is illustrated by scientists' seeing pillow lava form only underwater then theorizing that pillow lava found high in the mountains today did nevertheless form underwater - Features of the Earth are constantly changing; catastrophic events still happen. Because the present earth processes provide a key to understanding the past and to the future and since pillow lava forms underwater today, they must have done so in the past (pg 306)
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In the figure below, what is the layer pointed to by the arrows, and how do you know?
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An angular unconformity - The horizon shown separates folded metamorphosed rock from relatively undisturbed sedimentary rock. It is both an angular unconformity and a nonconformity, but the latter was not among choices
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in 1815 William Smith correlated strata from many locations and plotted it in paper to show the spatial distribution of rock units on Earth's surface. This document was the first modern
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Geologic map - A stratigraphic column is the sequence of strata at a location; a geologic column is a composite stratigraphic column; the geologic time scale is a dated geologic column; a range is the interval of geologic time during which a fossil species lived (pg 314)
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The oldest rocks found so far in our solar system are 4.57 billion-year-old meteorites, leading scientists to conclude that this is the approximate age of Earth
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True - Scientists have come to the conclusion that nearly all the objects in the Solar System developed at roughly the same time
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The isotope samarium-147 decays to neodymium-143 with a half-life of 106 billion years. If an igneous rock was found to have equal amounts of Sm-147 and Nd-143, which of the following statements must be true?
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There must have been some Nd-143 present when Earth formed - Both Nd-143 and Sm-147 were present at the time Earth formed
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The largest subdivisions of time on the geologic column are eras, which are broken down into smaller units called eons, then periods, then epochs
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False - the names of geologic time intervals, in order from largest to smallest, are eon, period, and epoch. (pg 359)
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On the diagram shown, which is false?
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There are equal amounts of parent and daughter material present after the passage of two half-lives - There was no daughter material present at time "zero" (when the mineral was formed) and after one half-life has passed, half the parent would decay into daughter material. After an additional half-life had passed, the proportion of daughter-to-parent would now be 3:1
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Which of the following statements is FALSE?
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An unconformity is a break in the rock record that indicates the area was underwater for millions of years - An unconformity is a break in the rock record indicating erosion and/or nondeposition; being "underwater" is a location and one where deposition is likely
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On the diagram shown, a geologist has used principles of stratigraphy to correlate the geologic material intercepted in each of three vertical boreholes. Which rock type consistently "pinches out" landward, causing the units above and below it to be unconformably against each other at drill hole C?
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3 Half-lives - Pg 320
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Which of the following assumptions that you might use in radiometric age-dating would cause you to underestimate the age of a rock?
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The rock had been a closed system since the time it formed, but it had been metamorphosed enough that some daughter product-but no parent product- escaped - This very tough question hints at some of the real-life challenges that exist with radiometric age-dating. Most of the answer-choices offered to you have occurred, and would cause a scientist to overestimate the age of a rock. Only this choice would cause you to underestimate the age of a rock
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If you equate all Earth history to one calendar year, all recorded human history occupies the week from Christmas to New Year's Eve
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False - Far less; only the last 30 seconds of the year, which would be 0.000001% of the Earth history
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Choose the proper list of names to fit the following 3 descriptions: age of mammals, age of dinosaurs, and longest named geologic unit of time
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Cenozoic, Mesozoic, Precambrian - Mammals were the dominant life form during the Cenozoic, and dinosaurs were dominant during the Mesozoic; the Precambrian represents more than 7/8 of all of Earth's history
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Which of the following principles of relative age-dating is NOT in the figure below?
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Cross-cutting relations - A succession of horizontally layered sedimentary units that contain fossil illustrate superposition, original horizontality, and the principle of inclusions. Cross-cutting relations are not exhibited here because there are no faults or igneous intrusions
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The first half-life period of potassium-40 is 1.3 billion years; the second half-life period of potassium-40 is half of this, and the number of years is cut in half for each succeeding half-life period
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False - The half-life period of any isotope is constant
