Superposition is a relative age dating principle which states

13-Jul-2016 00:48 by 8 Comments

Superposition is a relative age dating principle which states

Uniformitarianism: The principle of uniformitarianism states that physical processes we observe operating today also operated in the past, at roughly comparable rates, so the present is the key to the past.Original horizontality: The principle of original horizontality states that layers of sediment, when first deposited, are fairly horizontal because sediments accumulate on surfaces of low relief (such as floodplains or the sea floor) in a gravitational field.

Thus, the layer at the bottom of a sequence is the oldest, and the layer at the top is the youngest.Cross cutting relations: The principle of cross-cutting relations states that if one geologic feature cuts across another, the feature that has been cut is older.For example, if an igneous dike cuts across a sequence of sedimentary beds, the beds must be older than the dike.For example, a conglomerate containing pebbles of basalt is younger than the basalt, and a sill containing fragments of sandstone must be younger than the sandstone.Geologists apply geologic principles to determine the relative ages of rocks, structures, and other geologic features at a given location.If sediments were deposited on a steep slope, they would likely slide downslope before they could be buried and lithified.

With this principle in mind, geologists conclude that examples of folds and tilted beds represent the consequences of deformation after deposition.Investors decided to construct a network of canals to transport coal and iron, and hired an engineer named William Smith (1769–1839) to survey some of the excavations.Canal digging provided fresh exposures of bedrock, which previously had been covered by vegetation.Thus, once a fossil species disappears at a horizon in a sequence of strata, it never reappears higher in the sequence or, put another way, extinction is forever.Smith’s observation has been repeated at millions of locations around the world, and has been codified as the principle of fossil succession.The succession of events in order of relative age that have produced the rock, structure, and landscape of a region is called the geologic history of the region.

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