Geology

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Environmental, Economic, and Engineering Geology

The application of geologic knowledge to practical problems is the focus of the fields of environmental, economic, and engineering geology. Environmental geology involves the protection of human health and safety through understanding geological processes. For example, it is critically important to understand the geology of areas where people propose to store nuclear waste products. The study of geologic hazards, such as earthquakes and volcanic eruptions, can also be considered part of environmental geology. Economic geology is the use of geologic knowledge to find and recover materials that can be used profitably by humans, including fuels, ores, and building materials. Because these products are so diverse, economic geologists must be broadly trained; they commonly specialize in a particular aspect of economic geology, such as petroleum geology or mining geology. Engineering geology is the application of engineering principles to geologic problems. Two fields of engineering that use geology extensively are civil engineering and mining engineering. For example, the stability of a building or bridge requires an understanding of both the foundation material (rocks, soil) and the potential for earthquakes in the area. See also Engineering: Geological and Mining Engineering.

B

Historical Geology

Historical geology focuses on the study of the evolution of earth and its life through time. Historical geology includes many subfields. Stratigraphy and sedimentary geology are fields that investigate layered rocks and the environments in which they are found. Geochronology is the study of determining the age of rocks, while paleontology is the study of fossils. Other fields, such as paleoceanography, paleoseismology, paleoclimatology, and paleomagnetism, apply geologic knowledge of ancient conditions to learn more about the earth. The Greek prefix paleo is used to identify ancient conditions or periods in time, and commonly means “the reconstruction of the past.”

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Stratigraphy

Stratigraphy 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.

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Sedimentology

Sedimentology, or sedimentary geology, is the study of sediments and sedimentary rocks and the determination of their origin. Sedimentary geology is process oriented, focusing on how sediment was deposited. Sedimentologists are geologists who attempt to interpret past environments based on the observed characteristics, called facies, of sedimentary rocks. Facies analysis uses physical, chemical, and biological characteristics to reconstruct ancient environments. Facies analysis helps sedimentologists determine the features of the layers, such as their geometry, or layer shape; porosity, or how many pores the rocks in the layers have; and permeability, or how permeable the layers are to fluids. This type of analysis is important economically for understanding oil and gas reservoirs as well as groundwater supplies.

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Geochronology

The determination of the age of rocks is called geochronology. The fundamental tool of geochronology is radiometric dating (the use of radioactive decay processes as recorded in earth materials to determine the numerical age of rocks). Most radiometric dating techniques are useful in dating igneous and metamorphic rocks and minerals. One type of non-radiometric dating, called strontium isotope dating, measures different forms of the element strontium in sedimentary materials to date the layers. Geologists also have ways to determine the ages of surfaces that have been exposed to the sun and to cosmic rays. These methods are called thermoluminescence dating and cosmogenic isotope dating. Geologists can count the annual layers recorded in tree rings, ice cores, and certain sediments such as those found in lakes, for very precise geochronology. However, this method is only useful for time periods up to tens of thousands of years. Some geoscientists are now using Milankovitch cycles (the record of change in materials caused by variations in the earth's orbit) as a geologic time clock. See also Dating Methods: Radiometric Dating.

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