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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.
About. Magma is liquid rock in between the crust and the mantle, formed when part of the lower crust or upper mantle melts. When magma travels up through a volcanoâs vent, bubbles of gas also appear. All the gas builds up pressure, which causes the magma to explode out of the volcano. Many volcanoes are mountains, made up of layers of lava and ash. 2  Many volcanoes have several vents â a main one, and secondary ones that branch off the main vent. 3  The volcanoâs main vent goes all the way down to the layer of magma in between the Earthâs crust and mantle.