What is the cause of rock material deep down in the mantle melting into liquid rock?

The cause of rock material deep down in the mantle melting into liquid rock is primarily due to the increase in temperature and the decrease in pressure with depth. The mantle is composed of solid rock, but as one goes deeper into the mantle, the temperature increases significantly. This increase in temperature causes the rocks to reach their melting point.

Additionally, the pressure in the mantle decreases with depth. The weight of the overlying rocks and the pressure exerted on the mantle decreases as one moves deeper. This reduction in pressure further facilitates the melting of the rock material.

The combination of high temperatures and reduced pressure causes the solid rock material in the mantle to undergo partial melting, forming a partially molten or liquid rock called magma. This magma can then rise towards the Earth's surface through cracks and fractures, leading to volcanic activity and the formation of igneous rocks.