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The uppermost viscous layer of the mantle where pressure and temperature conditions can allow partial melting of the mantle rocks, which forms magmas.
From the Greek "asthenos"=weak and "sphera"=sphere, the asthenosphere is the uppermost layer of the plastic, highly viscous mantle beneath the brittle crust. It extends to depths of 100-200 km.

Because the pressure in this zone is still comparably low while temperature is high, parts of the mantle rocks can be in a molten state and form magmas. This happens especially in areas where pressure is lowered (at rifting zones), or where temperature is increased (at hot spots where mantle rocks rise upwards). A third way to generate magmas in the asthenosphere occurs at subduction zones: here, the addition of fluids extracted from the subducting plate into the mantle lowers the melting point.

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