Chapter 29: Geologic Time

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(Note: This reading quiz contains some material from lecture that is not found in the textbook.) Some people think the earth is very old, others that it is very young. The question of the age of the earth is important to geologists because it has implications for understanding how the earth works. If the earth is young, one looks for processes shaping the earth that happen quickly and perhaps only once. This is the view called (catastrophism, uniformitarianism?) . If the earth is old, we look for processes that happen slowly and perhaps repetitively, in cycles. This is the view called (catastrophism, uniformitarianism?) . The modern view is a combination of both, but strongly favors (which?) . Uniformitarianism now is taken to mean "the which govern geologic processes are the same as they have always been (time symmetry)." Principles used to determine relative time: Original horizontality (when sedimentary layers are originally laid down they are horizontal; The Principle of (the youngest feature is on top of the older feature); Cross-cutting relations (younger features cut across older features); Inclusions (included rock must be (younger, older?) than the rock that includes it; succession (fossil organisms appear in the sedimentary layers in an orderly succession of complexity that corresponds to the passage of time.) The layers of sedimentary rock of the earth's crust presumedly get older as you go deeper (superposition). The layers are subdivided into four periods of time called "eras." Eras are further subdivided into "periods," and periods are subdivided into "epochs." These divisions constitute the Geologic . In the layers of each epoch are fossil assemblages which are unique to that epoch. If you find a particular combination of fossils, you can pinpoint which era, period, and epoch the layers containing the fossils belong to. This constitutes the Principle of Faunal Succession. The oldest layers belong to the (Precambrian, Paleozoic, Mesozoic, Cenezoic?) Era. At the bottom of these layers, the evidence indicates that the earth had no life and little or no (oxygen, nitrogen?) in its atmosphere. As one move closer to the top of the Precambrian layers, the first evidence of life appears in the form of fossils of one-celled organisms and (worms, fish, birds?) . The event marking the end of the Precambrian Era is called the Explosion. This event (a rapid spreading of numbers and forms of life worldwide) marks the beginning of the (Paleozoic, Mesozoic, Cenezoic?) Era. The first period in the Paleozoic Era is the Period. An example of a life form that characterizes the Cambrian Period is the (trilobite, tyrannosaurus rex, horse?) . Subsequent periods of the Geologic Column are characterized by the first appearance in the fossil record of different life forms. The last period of the Paleozoic is the Permian Period. In this period, primitive mammal-like (birds, reptiles, dinosaurs?) are found in the rocks. The event marking the end of the Paleozoic Era and beginning of the (Mesozoic, Cenezoic?) Era is the First Great . At this time, as many as (10, 50, 95?) % of the animal species on earth disappear from the fossil record in the rocks and never return! The Mesozoic Era is the Era of the (amphibians, dinosaurs, mammals?) . The Mesozoic Era is also marked by the first appearance of mammals, the first birds and the first flowering plants. The event that marks the end of the Mesozoic Era is the (First, Second, Third?) Great . As many as 50% to 75% of the animal species on the earth disappear, including the (trilobites, saber-toothed tigers, dinosaurs?) . The end of the Mesozoic Era marks the beginning of the Era. Mammals and flowering plants begin to inherit the earth. Absolute Time: But how old is the earth? Several crude estimates, called "hourglass clocks," were made in the 19th century: Cooling of the (estimated by Lord Kelvin); Salinization of the (estimated by John Joly); Rate of sedimentation to form the earth's rock layers. All three estimates agreed crudely that the earth was much older than several tens of (thousands, millions, billions?) of years. Radioactive methods (since 1896) give us the best measurements: Age of the earth? 4.5-4.6 (thousand, million, billion?) years. First evidence of life in the rocks? 3.5 (thousand, million, billion?) years ago (bya). Cambrian explosion? (66, 245, 570?) million years ago (mya). First Great Dying? (66, 245, 570?) million years ago (mya). Second Great Dying? (66, 245, 570?) million years ago? Pleistocene Epoch (series of ice ages, including the Great Ice Age)? spans about 12000 years ago to about 3 million years ago.

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