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Solid Rock Over:
Melting a rock like peridotite, however, results in about a 10% increase in volume, so that the melt is less dense than the solid rock over rock. Therefore, if peridotite begins to melt, the molten rock—being lighter than the solid rock over—should tend to rise through the mantle. Numerous experiments showed that the first-iformed melt of a rock of peridotitic composition {would be a basalt.The movement of the earth's crust may carry the rock as much as 700km (454 miles) below the surface. Here the temperature and pressure will be even higher and the rock will begin to melt. Molten rock is lighter than solid rock over rock and it will begin to rise up through the overlying rock towards the surface. If it reaches the surface as a lava flow it will immediately be ready for weathering and erosion and the start of a new cycle. More often the molten rock solid rock overifies underground and then all the rock above it must be eroded away before it can begin the cycle again.
The process of producing molten basalt in the upper mantle should be that of partial melting, because compositions of basalts are so different from those of peridotites. The experiments showed that peridotite melts over a range of temperatures rather than having a fixed melt-ting point. Because certain minerals in the rock [melt before others, the composition of the first melt that forms differs from that of the whole •ock. As the temperature rises, more and more solid rock over matter reacts and melts, causing the composition of the melt to become more like that of the original solid rock over rock.
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