stratigraphy and what you need to know about it

stratigraphy is an aspect of geology that deals with correlation of different rock strata,

is the study of the history of the earth's crust, particularly its stratified (layered) rocks. Stratigraphy is concerned with determining age relationships of rocks as well as their distribution in space and time. Rocks may be studied in an outcrop but commonly are studied from drilled cores (samples that have been collected by drilling into the earth). Most of the earth's surface is covered with sediment or layered rocks that record much of geologic history; this is what makes stratigraphy important. It is also important for many economic and environmental reasons. A large portion of the world's fossil fuels, such as oil, gas, and coal, are found in stratified rocks, and much of the world's groundwater is stored in sediments or stratified rocks.

Stratigraphy may be subdivided into a number of fields. Biostratigraphy is the use of fossils for age determination and correlation of rock layers; magnetostratigraphy is the use of magnetic properties in rocks for similar purposes. Newer fields in stratigraphy include chemostratigraphy, seismic stratigraphy, and sequence stratigraphy. Chemostratigraphy uses chemical properties of strata for age determination and correlation as well as for recognizing events in the geologic record. For example, oxygen isotopes (forms of oxygen that contain a different number of neutrons in the nuclei of atoms) may provide evidence of an ancient paleoclimate. Carbon isotopes may identify biologic events, such as extinctions. Rare chemical elements may be concentrated in a marker layer (a distinctive layer that can be correlated over long distances). Seismic stratigraphy is the subsurface study of stratified rocks using seismic reflection techniques. This field has revolutionized stratigraphic studies since the late 1970s and is now used extensively both on land and offshore. Seismic stratigraphy is used for economic reasons, such as finding oil, and for scientific studies. An offshoot of seismic stratigraphy is sequence stratigraphy, which helps geologists reconstruct sea level changes throughout time. The rocks used in sequence stratigraphy are bounded by, or surrounded by, surfaces of erosion called unconformities.

reference: microsoft encarta 2009

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