Lab 05. Geologic_Time

# Lab 05. Geologic_Time - Lab 5 GeologicTime The layers of...

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Lab 5 Geologic Time The layers of rock in the Earth’s crust preserve the record of the past changes, and serve as a guide to what may happen in the future. Geologists can read the history of the Earth and determine geologic time from recognizing rock layers, correlating these layers from one rock outcrop to another, and determining the age relationships between these rock layers. In this lab we will learn to apply techniques to determine the relative geologic age of rock layers, and how geologists determine the numerical age of rocks. Part 1: Determining Relative Age The relative age of rocks is based on the relationship of two rock units. The geologist’s first step in determining relative age is to divide the local sequence of rocks into units that can be identified and correlated from one outcrop to the next. These rock layers are called strata. The surfaces between the strata are called contacts . Principles to determine relative age relationships among rock strata Principle of Original Horizontality : Strata are initially deposited only on a relatively horizontal surface and thus have a horizontal top and bottom surface. If the strata are no longer horizontal, they have been tilted or deformed by movement after deposition Principle of Lateral Continuity : Strata extend laterally in all directions until they gradually thin out or reach the edge of a basin. A thick layer of strata does not disappear then reappear suddenly somewhere else in the rock record. Principle of Superposition : Strata are initially deposited so that younger rocks are deposited on top of older rocks. Each successively younger layer is superimposed over the older rocks, so that the oldest rocks are at the bottom of an undeformed pile. Principle of Unconformities : Gaps that occur in the continuous deposition of strata are called unconformities. The layers can represent a hiatus in deposition or a period of erosion when rocks were removed. Most contacts represent conformities meaning that rocks on either side formed in a continuous sequence. An unconformity may be a pause in sediment deposition (disconformity), a period of deformation and erosion (angular unconformity), or the break between a crystalline igneous or metamorphic rock and the surrounding sedimentary rocks (nonconformity).

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Lab 05. Geologic_Time - Lab 5 GeologicTime The layers of...

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