What are the primary sources of heat in the partial melting of pre-existing rocks?

The primary sources of heat in the partial melting of pre-existing rocks are fundamentally tied to the geothermal gradient and the hotter mantle. The geothermal gradient refers to the rate at which temperature increases with depth into the Earth's crust. As one moves deeper into the Earth, temperatures rise significantly due to the heat generated from the decay of radioactive isotopes and the residual heat from the planet's formation. This increasing temperature can reach conditions sufficient to cause partial melting of existing rocks, transforming them into magma.

Additionally, the hotter mantle plays a crucial role. The mantle is composed of solid rock that can flow very slowly over geological time. In certain areas, such as divergent plate boundaries or hotspots, geological processes can bring hotter mantle material closer to the crust. This heat can also induce partial melting in overlying rocks.

Other options, while related to geological processes, do not serve as primary sources of heat for melting pre-existing rocks in the Earth. For instance, solar radiation largely affects surface temperatures but has minimal influence on the deep Earth processes involved in rock melting. Similarly, volcanic activity is a result of underlying heat but is not a primary source itself; it arises from the heat present in the mantle and crust. Lastly, atmospheric pressure and water content can affect the melting points and