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08 geologic time

08 geologic time - Earth Sciences 1022a Lecture Brief 9 Oct...

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Earth Sciences 1022a Lecture Brief 9 Oct 09 GEOLOGIC TIME Relative Time : tells us whether a rock is younger or older than another, based on: superposition - in undeformed, horizontal sedimentary rocks, a bed is older than the one above and younger than the one below; youngest on top, oldest on the bottom original horizontality - layers of sediment tend to be deposited in a horizontal position cross-cutting relations - a fault or a rock must be younger than the rock it cuts inclusions - inclusions within a rock must be older than the rock containing them unconformities – missing rocks (due to nondeposition or erosion) within rock sequences: angular unconformity (flat strata resting on tilted and eroded strata); disconformity (all strata parallel, hard to see time break); nonconformity (commonly flat sedimentary rocks resting on igneous or metamorphic rocks following erosion of the lower rocks) correlation - matching similar rock type, position, especially fossils in the rock record based on
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