Movements of the crustal plates cause pressure and tensions to build up on rocks, in many cases leading to deformation of the land surface. The general term diastrophism is sometimes applied to the bending, folding, warping and fracturing of the crust. It is important to distinguish between several types of movements and their results.
On a broad scale, earth movements may be divided into two types. Epeirogenic movements are those involving forces acting along a radius from the Earth's centre to the surface, and are characterized by large-scale uplift or submergence of land areas. The movements involved are often so slow and widespread that no obvious folding or fracturing is produced in the rocks. The second type of earth movements are those generated by forces acting at a tangent to the surface of the Earth, as primarily involved in plate tectonics. Where such disturbances have been responsible for the formation of the great fold mountain ranges of the world, they are referred to as orogenic movements. The creation of very complex fold structures, as sometimes involved in orogenesis, is called tectogenesis by some authors.
From the point of view of landform development, the difference between epeirogenic and orogenic crustal movements can be quite striking, and this is illustrated with reference to fold mountains and block mountains later in the chapter. In orogenic movements, structurally identifiable units are usually difficult to recognize, but the results of epeirogenie movements may be clearly defined in the relief.
Rocks vary considerably in their behavior to earth movements. Under surface conditions, rocks are brittle and fracture when subjected to stress and pressure, causing faulting. More deeply buried rocks, subject to higher temperatures and pressure, are relatively plastic and may respond to stress by folding rather than fracturing. The effects of folding and faulting on the disposition of strata are considered later in the chapter.
Earthquakes are the most prominent evidence of present-day earth movements, and are the result of deformation in the crust, which finally ruptures abruptly. Earthquakes are important in landform studies because they can trigger off catastrophic events in erosion processes. These include large-scale landslides and mud flows, as in Peru in 1970. A number of spectacular glacier surges have been observed in Alaska, and some are thought to be associated with earthquakes. Tsunamis are seismic sea waves, which can arrive at coasts with great force, causing considerable damage and shoreline changes, as in recent decades in the Hawaiian islands.
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