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Tectonics is the study of the deformation of Earth's lithosphere—both the causes of deformation and its effects. Tectonics focuses primarily on mountain-building, but involves other unrelated activities as well. Since the development of the theory of plate tectonics, tectonics has become an especially active area of research in geology.

Deformation of rocks, known as tectonism or diastrophism, is a product of the release and redistribution of energy from Earth's interior. This energy is provided by a combination of heat, produced by radioactive decay within the earth and by gravity. Uneven heating (or unequal distribution of mass) within the earth's interior creates pressure differences, or gradients. These pressure gradients cause rocks to experience compression, extension, uplift, or subsidence. As a result, rocks deform and shift about on the earth's surface. When deformed, rocks may break, bend, warp, slide, flow (as solids), or even melt. Structures that indicate past episodes of tectonism include faults, folds, and volcanoes. Among activities that suggest active tectonism are earthquakes and volcanic eruptions.

Tectonism is generally divided into two categories of deformation: orogenesis and epeirogenesis. Orogenesis (derived from the Greek words oros, meaning mountain, and genesis, meaning origin), or mountain-building, involves the formation of mountain ranges by folding, faulting, and volcanism. Most mountain ranges form where lithospheric plates converge (that is, along plate margins) and are a product of plate tectonics. Epeirogenesis (epeiros, Greek for mainland) involves vertical displacement (uplift or subsidence) of large regions of the earth's lithosphere, as opposed to the elongated belts of lithosphere involved in orogenesis. There are a number of other important differences between epeirogenesis and orogenesis as well. Folding and faulting is less significant in epeirogenesis and volcanism is not often involved. Epeirogenesis is also not as closely related to plate tectonics and is common throughout all areas of plates, not just at plate margins.

The products of orogenesis are obvious—mountains. Epeirogenesis is usually more subtle. Erosion of material from a region could cause uplift (epeirogenesis) in response to the decrease in crustal mass. Conversely, sediments eroded from one area of the earth's crust may accumulate elsewhere. The weight of these accumulated Rift valleys like this one in Thingvellir, Iceland, are formed when an area of the earth's crust is warped upward by pressure from magma below and cracked into sections. The sections sink back lower than before as the adjacent areas of intact crust are drawn away from the fractures. JLM Visuals. Reproduced by permission. sediments could cause subsidence (epeirogenesis) of the crust. The vertical movements in these examples are driven by gravity and result from changes in crustal mass within an area. Such movements are known as isostatic adjustments; they are among the most common, but subtle, epeirogenic activities.

Prior to the development of the plate tectonic theory in the 1960s, many theories had been proposed that attempted to explain regional tectonic activity, especially mountain building. None of these theories could adequately account for the range of activities and structures observed. One of the main reasons plate tectonic theory was so quickly and widely accepted is that it provides a workable explanation for a wide variety of tectonic processes and products.

In fact, plate tectonics has been called the "unifying theory of geology" because it explains and relates so many different aspects of Earth's geology. Many Earth processes, which previously were not even believed to be connected, are now known to be closely interrelated. As a result, tectonic research is now highly interdisciplinary. Thorough interpretation of the causes and effects of a single episode of deformation often requires input from a large number of geologic subdisciplines (structural geology, sedimentology, geomorphology, geochemistry, etc.) And to further complicate matters, most areas of the earth have been involved in multiple tectonic episodes.

See also Earthquake; Volcano.

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