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Are the tectonic plates floating on magma?

Earth’s tectonic plates rest upon the asthenosphere, the upper layer of Earth’s mantle. Over short timescales, the mantle behaves like a solid. If you could reach down and grab a handful of mantle rock it would be just that; solid rock. However, over long geologic timescales the mantle can behave like a thick liquid that slowly flows at about the same rate that fingernails grow.

Magma does exist in the asthenosphere in some places along subduction zones and spreading centers. Water that is brought down by subduction allows rock to melt at lower temperatures. Decreased pressure where the plates are separating also allows mantle rock to melt, but overall, these areas of melt are comparatively small. They are represented by the red blobs against the orange asthenosphere in this cartoon.

The only place in Earth's interior that is entirely liquid is the outer core. This is well-known because of the way it effects seismic waves; it stops S-waves and the velocity of P-waves are sharply reduced.

Learn More: This Dynamic Earth – The Story of Plate Tectonics