Magma, which comes out of volcanoes, hardens into rocks called extrusive rocks, because the material has been extruded, or pushed out, on to the surface. However, much magma never reaches the surface.
Instead, it is intruded or forced into, existing rock strata underground. Enormous bodies of intrusive magma, called laccoliths and batholiths, may bend overlying rock strata upwards into domes. When the magma cools, it forms such intrusive rocks as granite. Eventually, the softer, overlying rocks are erodes away. The erosion resistant granite then becomes exposed, on the surface, forming an intrusive mountain.
The Black Hills of Nevada in the western United States are a perfect example of the way these mountains are formed.
The mighty Half Dome, in the Yosemite Valley in California, is an intrusive mountain. This spectacular peak consists of hard granite. It was formed when magma which cooled and solidified underground.