It is a very hot, very, very thick liquid made mostly of iron, magnesium, aluminum, silicon, and oxygen. The mantle of the earth is very thick. The magma that is near the surface cools slowly, becoming part of the crust. Later erosion removes the crustal rocks above the newly cooled rock, and the now-solid minerals appear on the surface.
Igneous rocks are hard, and often used for building because they are durable.
The Sierra mountain range is made of granite. Not all igneous rocks cool deep in the earth. Sometimes hot rock breaks through to the surface. It spurts out and forms cones, like the one in this picture. The cones are made of cinders and rock, and can be recognized by their distinctive shape.