Question

Why is asthenosphere in semi molten state ??

Answer 1

The temperature and pressure of theasthenosphere are so high that rocks soften and partly melt, becoming semi-molten. The asthenosphere is much more ductile than either the lithosphere or lower mantle. ... Of course, tectonic plates are not really floating, because the asthenosphere is not liquid.

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Answer 2

The asthenosphere is the denser, weaker layer beneath the lithospheric mantle. It lies between about 100 kilometers and 410 kilometers beneath Earth’s surface. The temperature and pressure of the asthenosphere are so high that rocks soften and partly melt, becoming semi-molten.

The asthenosphere is much more ductile than either the lithosphere or lower mantle. Ductility measures a solid material’s ability to deform or stretch under stress. The asthenosphere is generally more viscous than the lithosphere, and the Lithosphere-Asthenosphere Boundary (LAB) is the point where geologists and rheologists—scientists who study the flow of matter—mark the difference in ductility between the two layers of the upper mantle.

The very slow motion of lithospheric plates “floating” on the asthenosphere is the cause of plate tectonics, a process associated with continental drift, earthquakes, the formation of mountains, and volcanoes. In fact, the lava that erupts from volcanic fissures is actually the asthenosphere itself, melted into magma.

Of course, tectonic plates are not really floating, because the asthenosphere is not liquid. Tectonic plates are only unstable at their boundaries and hot spots.