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There are three main factors that dictate the melting of rocks into magma: temperature, pressure, and rock composition. Most rocks are composed out of at least three or more different substances, and those substances all have differing physical properties, such as melting point. It is also possible to have a minor percentage of the rock to melt, such as 2 or 3 percent, and cause the rock structure to assume a "melt mush", consisting of some solid rock and some melted rock. Pressure would be a main player in determining rock melt, the deeper you go in the crust, the higher the pressure. Pressure could also squeeze out the liquid part, leaving the solid part of the melt mush behind to rise, since it is less dense. Temperature would also increase with depth, so structures deep in the Earths crust would be likely places for rocks to incur such temperature changes. Likely locations would be subduction zones, trenches, rift valleys, and hot spots.
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