1. ​The layered structure of the earth is due to chemical differentiation plus gravity.

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densest material-center

least dense-surface​

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2. It is also due to its physical property.​

​The weakness of the asthenosphere does not depend on the presence of molten rock, thus, while remaining entirely solid, it is a zone of relatively low strength compared with the overlying colder lithosphere and it flows (plastically) like a viscous fluid.

Melting?-only partial

Partial melting results to molten rock​

Rather, as the temperatures and pressures rise, rock will begin to partially melt when individual minerals exceed their melting temperatures.

Molten Rock

As rock is a heterogeneous mixture of various minerals, there is no single melting point for the rock as a whole.​

​Rock

" It is only where these ‘normal’ conditions change – mainly at plate boundaries that partial melting will occur, which is why volcanoes are confined to specific zones. T " 

​Spreading Centers

Subduction Zone

Hotspots​