The asthenosphere is a highly viscous, mechanically weak and ductile region of the Earth's mantle that lies below the lithosphere.
In-Depth Explanation
The asthenosphere is a crucial layer in understanding plate tectonics and the movement of continents. Key aspects of its meaning in geography include:
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Location: It resides below the lithosphere (the rigid outer layer composed of the crust and uppermost mantle) and above the mesosphere. The depth varies, generally starting around 80-200 km beneath the surface and extending to around 700 km.
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Composition and Behavior: The asthenosphere is primarily composed of solid rock but behaves like a highly viscous fluid over geological timescales. This allows the lithosphere to move and "float" on top of it. The 'weakness' arises from a combination of high temperature and pressure conditions causing partial melting of some of the material. This partial melting (a small percentage) decreases its rigidity.
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Role in Plate Tectonics: The asthenosphere is the zone where convection currents within the mantle drive the movement of tectonic plates. Heat from the Earth's core causes the mantle material to slowly circulate, pushing and pulling the overlying lithospheric plates.
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Relationship to Earthquakes and Volcanoes: While earthquakes primarily occur within the brittle lithosphere, the asthenosphere's behavior significantly influences plate movements, indirectly impacting the frequency and location of seismic activity. Furthermore, the partially molten material can contribute to volcanism as it can be the source of magma that rises to the surface.
Summary
In essence, the asthenosphere is the deformable layer of the Earth's upper mantle that facilitates plate tectonic processes. Its "weak" nature, due to heat and pressure, allows for the slow, continuous movement of the lithosphere above, shaping the Earth's surface and contributing to phenomena like earthquakes and volcanoes.
