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Magma in Earth's Mantle Forms Deeper Than Once Thought. Study simulating pressures in mantle beneath the ocean floor shows that rocks can melt at depths up to 250 kilometers. When magma reaches Earth's surface and erupts from a volcano, it becomes lava.
As for your question, the above should answer that, if a volcano does not erupt and thus empty a magma chamber for what-ever reason, the magma will crystallise and can form a granite (or other intrusive rocks, dependent on the chemical composition of the magma). Due to convergent plates putting more pressure on the mantle, magma will migrate upwards.