What forms the lower boundary of the plates?

What forms the lower boundary of the plates?

answer:

>>Earth's tectonic plates may have taken as long as 1 billion years to form, researchers report today in Nature. The plates — interlocking slabs of crust that float on Earth's viscous upper mantle — were created by a process similar to the subduction seen today when one plate dives below another, the report says.>>

>>They are formed when two plates collide, either crumpling up and forming mountains or pushing one of the plates under the other and back into the mantle to melt. Convergent boundaries form strong earthquakes, as well as volcanic mountains or islands, when the sinking oceanic plate melts.>>

>>Along convergent boundaries, subduction, or one plate moving under another, carries the lower one down into the mantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading. In this way, the total surface of the lithosphere remains the same.>>

Explanation:

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