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The Test on Geologic Time consists of 18 questions. Correct answers are found through links located at the end of each question.

The answer section is formatted for on-screen use, not printing, and will waste a lot of paper if printed directly.

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Question #1: When the trunk of a tree has been completely replaced by minerals, the tree is

(A). petrified.

There are two processes involved in the making of petrified fossils, and they are not restricted to just wood. The processes involve mineralization, which is the filling of pore spaces with deposits of calcium carbonate, silica, or pyrite, and/or replacement, which is the dissolving of the original material and depositing of new material an ion at a time. Petrified wood is formed by both processes over a long period of time. As it decayed, the original wood was replaced by mineral matter. Over time the "mix" of minerals being deposited changed and the various resulting colors appear to preserve the texture of the wood.














































Question #2: Which of the following is not a fossil?

(C). pot shards of an ancient tribe

A fossil is any evidence of former life, so the term means more than fossilized remains. Evidence can include actual or altered remains of plants and animals such as an insect entombed in amber or a frozen mammoth. It could also be just simple evidence of former life such droppings from bats in a cave. The word "fossil" carries the meaning of any evidence of ancient organisms in the history of life. Pot shards from an ancient tribe are therefore not fossils.














































Question #3: Most fossils are usually found in

(B). recently eroded sedimentary rock.

A fossil must be protected, formed, and then found and studied to reveal its part in the history of life. This means the rocks in which the fossil formed must now somehow make it back to the surface of the earth. This usually involves movement and uplift of the rock, and weathering and erosion of the surrounding rock to release or reveal the fossil. Most fossils are found in recently eroded sedimentary rocks, before weathering and erosion has had the time to destroy them.


















































Question #4: A logical principle used in interpreting the history of rock layers is

(C). superposition.

The principle of superposition is a logical and obvious principle that is applied to sedimentary rocks. Layers of sediments are usually deposited in succession in horizontal layers, which later are compacted and cemented into layers of sedimentary rock. An undisturbed sequence of horizontal layers is thus arranged in chronological order with the oldest layers at the bottom. Each consecutive layer will be younger than the one below it, with the understanding that the layers have not been turned over by deforming forces.




















































Question #5: An unconformity in a sedimentary rock sequence results when

(C). erosion occurs for a long period of time.

As erosion wears away the rock layers at a site, the sediments produced are deposited some place else. Later, the sites of erosion and deposition may shift, and the sediments are deposited on top of the eroded area. When the new sediments later are formed into new sedimentary rocks, there will be a time lapse between the top of the eroded layer and the new layers. A time break in the rock record is called an unconformity. The unconformity is usually shown by a surface within a sedimentary sequence on which there was a lack of sediment deposition, or where active erosion may even have occurred for some period of time. When the rocks are later examined, that time span will not be represented in the record, and if the unconformity is erosional, some of the record once present will have been lost.




















































Question #6: Index fossils serve to identify

(B). specific layers of rocks anywhere.

Distinctive fossils of plant or animal species that were distributed widely over the earth, but lived only a brief time are called index fossils. Index fossils, together with the other principles used in reading rocks, make it possible to compare the ages of rocks exposed in two different locations. This is called age correlation between rock units. Correlations of exposed rock units separated by a few kilometers are easier to do, but correlations have been done with exposed rock units that are separated by an ocean.




















































Question #7: Which of the following is most important in reading the history from layers of sedimentary rocks?

(C). determining the composition of the rock layers

The understanding of geologic processes has been made possible through the development of various means of measuring ages and time spans in geologic systems. An understanding of geologic time leads to an understanding of geologic processes, which then leads to an understanding of the environmental conditions that must have existed in the past. Thus, the mineral composition, texture, and sedimentary structure of rocks are clues about past events, events that make up the history of earth.





















































Question #8: Fossils show that over time some life forms disappeared from the record and new forms appeared. This is described by the logical frame of reference known as the principle of

(A). faunal succession.

The principle of faunal succession recognizes that life forms have changed through time. Old life forms disappear from the fossil record and new ones appear, but the same form is never exactly duplicated independently at two different times in history. This principle implies that the same type of fossil organisms that lived only a brief geologic time should occur only in rocks that are the same age. According to the principle of faunal succession, then, once the basic sequence of fossil forms in the rock record is determined, rocks can be placed in their correct relative chronological position on the basis of the fossils contained in them. The principle also means that if the same type of fossil organism is preserved in two different rocks, the rocks should be the same age. This is logical even if the two rocks have very different compositions and are from places far, far apart


















































Question #9: An index fossil is most useful if it is a fossil of an organism that existed a

(A). brief time over a wide area.

Distinctive fossils of plant or animal species that were distributed widely over the earth, but lived only a brief time are called index fossils. Index fossils, together with the other principles used in reading rocks, make it possible to compare the ages of rocks exposed in two different locations. This age comparison can be made with more confidence if the index fossil is from an organism that lived for only a brief time and more comparisons can be made if it lived over a wide area.
















































Question #10: The earth dating method that is believed to be most accurate is

(B). radioactive decay.

The discovery of the radioactive decay process in the elements of minerals and rocks led to the development of a new, accurate geologic clock. This clock finds the radiometric age of rocks in years by measuring the radioactive decay of unstable elements within the crystals of certain minerals. Since radioactive decay occurs at a constant, known rate, the ratio of the remaining amount of an unstable element to the amount of decay products present can be used to calculate the time that the unstable element has been a part of that crystal. Potassium, uranium, and thorium are radioactive isotopes that are often included in the minerals of rocks, so they are often used as radioactive clocks.














































Question #11: Major blocks of time in the earth's geologic history are based on

(D). appearance and disappearance of particular fossils in the rock record.

The major blocks of time in Earth's geologic history are called eras, and each era is identified by the appearance and disappearance of particular fossils in the sedimentary rock record. There are four main eras, which are: (1) Cenozoic, which refers to the time of recent life. Recent life means that the fossils for this time period are similar to the life found on earth today. (2) Mesozoic, which refers to the time of middle life. Middle life means that some of the fossils for this time period are similar to the life found on earth today, but many are different from anything living today. (3) Paleozoic, which refers to the time of ancient life. Ancient life means that the fossils for this time period are very different from anything living on the earth today. (4) Precambrian, which refers to the time before the time of ancient life.














































Question #12: The least number of fossils are found in rocks formed during which era?

(A). Precambrian

The part of the earth's history with evidence of life makes up about 15 percent of the total record and about 85 percent of the record occurred during the Precambrian era. This does not mean that life appeared suddenly in the Cambrian period. The fossil record is incomplete since it is the hard parts of animals or plants that form fossils, usually after rapid burial. Thus, the soft-bodied life forms that existed during the Precambrian era would make Precambrian fossils exceedingly rare. The Precambrian fossils that have been found are chiefly those of deposits from algae, a few fungi, and the burrow holes of worms. Another problem in finding ancient fossils of soft-bodied life forms is that heat and pressure have altered many of the ancient rocks over time, destroying any fossil evidence that may have been present.

















































Question #13: Most of the earth's geologic history took place during which era?

(B). Precambrian

Geologic history is divided into four main eras. The Precambrian era was first, lasting the first 4 billion years, or about 85 percent of the total 4.6 billion years of geologic time. The Paleozoic lasted about 10 percent of geologic time, the Mesozoic about 4 percent, and the Cenozoic only about 1.5 percent of all geologic time.




















































Question #14: Which of the following is a Mesozoic Period?

(C). Jurassic

The Mesozoic refers to the time of middle life. Middle life means that some of the fossils for this time period are similar to the life found on earth today, but many are different from anything living today. The Mesozoic began with the extinction of many types of plant and animal life of the Paleozoic era and closed with a great dying of land and marine life that resulted in the extinction of many species, including the dinosaurs. The Mesozoic Era is subdivided into the Cretaceous, Jurassic, and Triassic Periods.




















































Question #15: Which era could be called The Age of the Reptiles?

(A). Mesozoic

The dinosaurs first appeared in the Triassic, outnumbering all the other reptiles until the close of the Mesozoic. Fossils of the first birds, the first mammals, the first flowering plants, and the first deciduous trees appeared in the rocks of this era. Like the close of the Paleozoic, the Mesozoic era ended with a great dying of land and marine life that resulted in the extinction of many species, including the dinosaurs. Throughout the Mesozoic era the reptiles dominated life on the earth, so the Mesozoic ear could be called The Age of the Reptiles.




















































Question #16: According to the rock record, fossils of fish first appeared in which period?

(D). Ordovician

Fossils of fish first appear in the Ordovician, then become abundant and diversified by the Devonian. Sharks were common, as was a primitive form of an air-breathing fish. Primitive evergreen and fern-like trees appeared on the land at this time, according to the Devonian fossils.





















































Question #17: The dominant life form of the Cambrian Period was

(B). trilobites

The earliest abundant fossils are found in rocks from the Cambrian period, representing an abundance of oceanic life with over a thousand different species of animals. There is no fossil evidence of life of any kind living on the land during the Cambrian. The dominant life forms of the Cambrian ocean were trilobites and brachiopods. The trilobites, now extinct, made up more than half the life population during the Cambrian.


















































Question #18: Which era is known as The Age of the Mammals?

(D). Cenozoic

As the Cenozoic era opened, the dinosaurs were extinct and the mammals became the dominant life form. The Cenozoic is thus called The Age of the Mammals.