When a part of the earth's upper mantle melts,magma forms.
causes are-
the buoyancy of the magma
the pressure from the ex-solved gases in the magma
the injection of a new batch of magma into an already filled magma chamber 
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The Brainliest Answer!
It is because as the inner part of the earth was too hot,that even the metals would melt into liquid state and the fissures that makes the inner part too hot and the vents may get the magma(the hot liquid present in the core)fill the volcanic mountain and overflows from it

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