continental crust in the west (the North American continent) and by oceanic crust in the east and extends under the Atlantic Ocean as far as the Mid-Atlantic Ridge. Settling of the continental blocks creates a rift valley, such as the present-day East African Rift Valley. In addition, models have indicated that the episodic opening and closing of back-arc basins have been the major factors in mountain-building processes, which have influenced the plate-tectonic evolution of the western Pacific for at least the past 500 million years. Earth s surface layer, 50 to 100 km (30 to 60 miles) thick, is rigid and is composed of a set of large and small plates. At a depth of about 2,900 km (1,800 miles the lower mantle gives way to Earths outer core, which is made up of a liquid rich in iron and nickel. (Whether magmatism the formation of igneous rock from magma initiates the rifting or whether rifting decompresses the mantle and initiates magmatism is a matter of significant debate.) If the diverging plates are capped by continental crust, fractures develop that are invaded by the ascending magma. Tuzo Wilson and American geophysicist. The number of these hotspots is uncertain (estimates range from 20 to 120 but most occur within a plate rather than at a plate boundary.
The fault surfaces are rarely smooth, and pressure may build up when the plates on either side temporarily lock. Although subduction is a long-term process, the uplift that results in mountains tends to occur in discrete episodes and may reflect intervals of stronger plate convergence that squeezes the thermally weakened crust upward. As the dense slab collapses into the asthenosphere, however, it also may roll back oceanward and cause extension in the overlying plate.
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Because the partial melt is basaltic in composition, the new crust is oceanic, and an ocean ridge develops along the site of the former continental rift. Earth scientists are able to accurately reconstruct the positions and movements of plates for the past 150 million to 200 million years because they have the oceanic crust record to provide them with plate speeds and direction of movement. Although most of Earths volcanic activity is concentrated along or adjacent to plate boundaries, there are some important exceptions in which this activity occurs within plates. The crust behind the arc becomes progressively thinner, and the decompression of the underlying mantle causes the crust to melt, initiating seafloor-spreading processes, such as melting and the production of basalt; these processes are similar to those that occur at ocean ridges. Jason Morgan explained such topographic features as the result of hotspots. During the late 20th and early 21st centuries, evidence emerged supporting the notion that subduction zones preferentially initiate along preexisting fractures (such as transform faults ) in the oceanic crust. Data from alternative models suggest that many plumes are not deep-rooted. It has served as a unifying model or paradigm for explaining geologic phenomena that were formerly considered in unrelated fashion. This activity changes the positions of all plates with respect to Earths spin axis and the Equator.
The crustal root undergoes metamorphic reactions that result in a significant increase in density and may cause the root to also founder into the mantle. Transform faults are so named because they are linked to other types of plate boundaries. Divergent margins As plates move apart at a divergent plate boundary, the release of pressure produces partial melting of the underlying mantle. Irrespective of the exact mechanism, the geologic record indicates that the resistance to subduction is overcome eventually. This chaotic mixture is known as an accretionary wedge.
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