How does magma form and what makes it erupt?

Magma forms from the partial melting of mantle rocks.  These little blobs of melt migrate upward and coalesce into larger volumes that continue to move upward.

As they rise, gas molecules in the magma come out of solution and form bubbles and as the bubbles rise they expand (as outside pressure gets lower).  Eventually the pressure from these bubbles is stronger than the surrounding solid rock and this surrounding rock fractures, allowing the magma to get to the surface.

If you drop some Mento’s into a Diet Coke, the result if very similar…

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