What makes igneous rocks?

Answer 1

Ultimately a combination of an increase in temperature or a loss of pressure cause the partial melting of mantle material (usually by decompression), or the partial melting of mantle material near a subducting slab due to the release of volatiles into the surrounding material, lowering it's melting temperature to a point where it becomes liquid.

This lower density material will then be of higher buoyancy and migrate upwards toward Earth's surface where it will either form an igneous rock intrusion (where the molten magma cools and solidifies under the surface), or an extrusive igneous rock where the lava erupts onto Earth's surface and solidifies.