An increase in volume due to heat is called

Thermal expansion.

Explanation:

Thermal expansion is the tendency of matter to change its shape, area, and volume in response to a change in temperature.  Hope this helps! :)

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answer: the metamorphic rocks are formed when they are exposed to high pressure and high temperatures. this happens when one of the earth's plates comes in contact with the other plate by traveling in opposite directions, straight into each other or slipping under one another. the friction that is caused by this activity results in   melting of rocks and when it cools they are known as metamorphic rocks. this process needs a controlled amount of pressure and temperature otherwise it may turn into magma.