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define asthenosphere
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The asthenosphere is the mechanically weak, ductile part of the upper mantle that lies just below the rigid lithosphere. It is typically a low-velocity, low-viscosity layer (often called the seismic low‑velocity zone) composed mainly of peridotitic mantle rocks that are near their melting temperature and contain small amounts of partial melt. Because it can flow on geological timescales, the asthenosphere allows rigid tectonic plates to move and participates in mantle convection.
Typical depth: roughly beneath the lithosphere—commonly beginning around 50–150 km and extending down to the transition zone (≈410 km)—but its thickness and depth vary with tectonic setting and thermal structure.
Typical depth: roughly beneath the lithosphere—commonly beginning around 50–150 km and extending down to the transition zone (≈410 km)—but its thickness and depth vary with tectonic setting and thermal structure.
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