Half Dome California, hard igneous rock exposed by erosion
The outer crust of the Earth floats on top of a semi-liquid part of the mantle called the asenthosphere and when it heats up, through reduction in pressure or mantle convection currents, liquid magma can be forced up into the upper continental rock. If the magma breaks out to the surface, volcanoes and lava flows result. If it remains within the earth and cools slowly, allowing the minerals to crystallize then it is termed a pluton or batholith.
Rocks that form from such hot liquid mantle intrusions are called igneous (meaning fire) rocks and tend to form the underlying core of most large mountain ranges. When the upper sedimentary rocks are eroded away, the underlying rocks, usually some form of granite are exposed. Because they are trapped underground and under pressure they cool slowly and the individual mineral grains or crystals can be seen with the naked eye.
Batholiths and Plutons
Huge mantle intrusions can extend hundreds of miles and resist weathering and glacial action better than sedimentary rocks. A batholith will often form at plate tectonic borders and subduction zones where there is already fracturing and active mountain building. As pressure is released by the tectonic and earthquake activities more and more magma flows and melts upward. Offshoots from the main batholith about 10 to 15 miles across are called plutons and generally form where there is a weakness in the crust where they can flow up.
Dykes and Sills
Dykes and sills are plutons, smaller magma intrusions that have exploited specific weaknesses in the overlying rocks. Many large dykes are the remnants of interior channels that fed active volcanoes. When the pressure is finally released the remaining channel is still full of the magma and being underground it cools slowly forming the granite cores of overlying basalt and lava volcano. Smaller dykes frequently can be seen where magma has forced its way along fractures and then cooled. A sill may never reach the surface but in particular exploits the fracture zone between layers of sedimentary rock and both melts in-between the beds of sedimentary rock and pushes them further apart.
A laccolith is a pluton formed in a sedimentary formation where an underlying layer, perhaps of earlier igneous material, is strong enough to form a flat bed. The magma being forced in from below rather than pushing the two apart to form a large sill instead bulges up in a dome shape forcing the weaker upper sedimentary rock to bend and crack. Later when erosion wears away the covering sedimentary rock a classic dome shape is revealed as the core of the formation.