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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.
How magma chambers form. The decrease in pressure allows rocks in this part of the Earth to melt and collect in magma chambers where they erupt as pillow basalt on the ocean floors. Boundaries of the lithosphere. The boundary with the lithosphere lies between 100 and 200 kilometers deep. Scientists do not know for certain the depth of the lower boundary but it is approximately 700 kilometers deep. Earthquake waves in this part of the Earth travel much slower than they do in the lithosphere because of the density of the rocks.