Deep Time 2 - Chapter 10: Part 2 Deep Time: How Old is Old?...

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Deep Time: How Old is Old? Chapter 10: Part 2
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Physical Principles for Defining Relative Age 1. Principle of Uniformitarianism The present is the key to the past. Physical processes we observe today also operated in the past, at roughly comparable rates
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2. Principle of Superposition The layer at the bottom of a sequence is the oldest, the layer at the top is the youngest. Physical Principles for Defining Relative Age
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Law of Superposition
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3. Principle of Original Horizontality Surfaces on which sediments accumulate are fairly horizontal, such as a floodplain or lake bed If seds were deposited on a steep slope, they would likely slide downslope before lithification and would not be preserved as sedimentary layers Layers of sediment when originally deposited are fairly horizontal With this in mind, when folds or tilted beds are observed, these are consequences of deformation that occur after deposition Physical Principles for Defining Relative Age
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4. Principle of Original Continuity Sediments generally accumulate in continuous sheets If a sedimentary layer has been cut by a canyon, one can assume that the layer once spanned the canyon and was later eroded by the river that formed the canyon Physical Principles for Defining Relative Age
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5. Principle of Cross-Cutting Relations If one geologic feature cuts across another, the feature that has been cut is older If an igneous dike cuts across a sequence of sedimentary beds, the beds must be older than the dike If a fault cuts across and displaces layers of sedimentary rock, then the fault must be younger than the layers Physical Principles for Defining Relative Age
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6. 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, the sedimentary layer must be younger The fragments, or xenoliths, in an igneous body and the pebbles in the sedimentary layer are inclusions , pieces of one material incorporated in another. The rock containing the inclusion must be younger than the inclusion. Physical Principles for Defining Relative Age
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7. Principle of Baked Contacts An igneous intrusion metamorphoses surrounding rock, or bakes the surrounding rock The rock that has been “baked” must be older than the intrusion Physical Principles for Defining Relative Age
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Beds 1-7 deposited first, intrusion of sill next, folding, intrusion of granite, faulting, intrusion of dike, erosion to yield present land surface.
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Principle of Fossil Succession
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Gaps in the Record Unconformity : a surface representing a period of nondeposition and possibly erosion Forms when the land surface does not receive and accumulate sediment Concept first identified by James Hutton Because of unconformities, rock record preserved in rock layers in incomplete No single location on Earth contains a complete record of Earth history
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This note was uploaded on 04/01/2012 for the course GEOLOGY 1122 taught by Professor Sethrose during the Fall '10 term at Georgia State University, Atlanta.

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Deep Time 2 - Chapter 10: Part 2 Deep Time: How Old is Old?...

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