Review Ch12 and 13

Review Ch12 and 13 - Chapter 12 13 Geologic Time 1 What is...

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Chapter 12 & 13 – Geologic Time 1. What is Geologic time? The span of time since Earth’s formation What are the 4 main Eons? Hadean, Achaean, Proterozoic, Phanerozoic What is the Precambrian? The interval of geologic time between Earth’s formation about 4.57 Ga and the beginning of the Phanerozoic Eon 542 Ma. What are the main eras of the Phanerozoic Eon? The Phanerozoic Eon is subdivided into eras. In order from oldest to youngest, they are the Paleozoic (“ancient life”), Mesozoic (“middle life”), and Cenozoic (“recent life”) eras. What period and epoch are we living in? Period: Quaternary, Epoch: Holocene 2. What is the difference between relative and numerical dating? Relative dating: using relative positioning of rocks, fossils, and features to prescribe relative ages. Numerical Dating: Numerical dates pinpoint the time in history when an event took place, such as the extinction of the dinosaurs 65 million years ago. Our current understanding of radioactivity allows scientists to use the natural radioactivity of certain elements in rocks to accurately determine their numerical dates. What are the 6 principles of relative age dating? The Principle of uniformitarianism: Physical processes we observe operating today also operated in the past, at roughly comparable rates. The Principle of superposition : In a sequence of sedimentary rock layers, each layer mush be younger than the one below, for a layer must be younger that the one below, for a layer of sediment cannot accumulate unless there is already a substrate on which it can collect. The principle of original horizontality: sediments on earth settle out of a fluid in gravitational field. Typically, the surfaces on which sediments accumulate are fairly horizontal. If sediments were deposited on a steep slope, they would likely slide down slope before lithification, and so would be preserved as sedimentary layers. The principle of original continuity : Sediments generally accumulate in continous sheets. The principle of cross-cutting relations: If one geologic feature cuts across another, the feature that has been cut is older. The principle of inclusions: If an igneous intrusion contains fragments of another rock, the fragments must be older than the intrusion. If a layer of sediment deposited on an igneous layer includes pebbles of the igneous rock, then the sedimentary layer must be younger.
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