Geology

D

Radiometric Dating

Another fundamental goal of geochronology is to determine numerical ages of rocks and to assign numbers to the geologic time scale. The primary tool for this task is radiometric dating, in which the decay of radioactive elements is used to date rocks and minerals. Radiometric dating works best on igneous rocks (rocks that crystallized from molten material). It can also be used to date minerals in metamorphic rocks (rocks that formed when parent rock was submitted to intense heat and pressure and metamorphosed into another type of rock). It is of limited use, however, in sedimentary rocks formed by the compaction of layers of sediment. One of the great triumphs of geochronology is that numbers acquired by radiometric dating matched predictions based on superposition and other means of geologic age determination, confirming the assumption of uniformitarianism. Using dated rocks, geologists have been able to assign numbers to the geologic time scale. See also Dating Methods.

IV

Geologic Spatial Scales

In order to understand geologic processes and to reconstruct the geologic past, geologists work at different spatial, or size, scales—scales that range from microscopic to planetary. In order to work at these spatial scales, they use a number of tools. At the microscopic level, traditional tools include the petrographic microscope, used to identify minerals and examine rock textures. Modern tools for examining rock chemistry and structure include complex scanning electron microscopes, microprobes that can obtain very small geologic or mineralogic samples, and mass spectrometers (instruments that measure the quantity of atoms, or groups of atoms, in a geologic sample). Geologists can also use lasers and particle accelerators for high-precision work, such as in argon-argon radiometric dating, the use of isotopes of the element argon to date geologic samples.

Some geologic features are very large, and geologists must create detailed maps to observe them completely. Geologists use maps to record basic information, to examine trends, and to understand processes and geologic history. For example, a map may record the locations of historical earthquakes, helping to identify faults. Geologic maps can help geologists understand the history of a mountain belt or locate new mineral deposits. On a planetary scale, geologists can map the earth’s surface using data from orbiting satellites. Geologists also make maps reconstructing a view of the earth at some time in the past; such maps are called paleogeographic maps. Geologists who study Mars map the planet’s surface features with the help of images and information from spacecraft probes sent to Mars.

Traditionally, maps have resulted from fieldwork. In the field, geologists locate exposures of rock, or rock outcrops, and features such as faults, folds, or other geologic structures on a base map or aerial photograph. Mapping has improved through the use of remote sensing techniques, such as radar and infrared mapping from aircraft and satellites, and this in turn has helped geologists better understand the earth. Geologists can now determine latitude and longitude positions on the earth by using the global positioning system of satellites (GPS). Map information can now be stored digitally, as in geographic information systems (GIS). Subsurface, or underground, mapping is becoming more common. This technique uses drilled cores and sound waves sent below the ground to map structures such as faults.

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V

Fields of Geology

Geologists have found it useful to divide geology into two main fields: physical geology, which examines the nature of the earth in its present state, and historical geology, which examines the changes the earth has undergone throughout time.

A

Physical Geology

Physical geology can be subdivided into a number of disciplines according to the way geologists study the earth and which physical aspects they study. Fields such as geophysics, geochemistry, mineralogy and petrology, and structural geology apply the sciences of physics and chemistry to study aspects of the earth. Hydrology, geomorphology, and marine geology incorporate the study of water and its effects on weathering into geology, while environmental, economic, and engineering geology apply geologic knowledge and engineering principles to solve practical problems.

A

1

Geophysics

In the field of geophysics, geologists apply the concepts of physics to the study of the earth. Geophysics is such a broad field that scientists sometimes consider it a separate field from geology. The largest subdiscipline in geophysics is seismology, the study of the travel of seismic waves through the earth. Seismic waves are generated naturally by earthquakes, or they can be made artificially by explosions from bombs or air guns. Seismologists study earthquakes and construct models of the earth's interior using seismic techniques. Geophysics also includes the study of the physics of materials such as rocks, minerals, and ice within the fields of petrology, mineralogy, and glaciology. Geophysicists study the behavior of the planet’s oceans, atmosphere, and volcanoes. Specialists called volcanologists study the world’s volcanoes and try to predict eruptions by using seismology and other remote sensing techniques, such as satellite imagery. Monitoring active volcanoes is especially important in highly populated areas.

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