1.3.1 Igneous rocks in the landscape
The rocks that erupt from volcanoes are called extrusive igneous rocks, simply because they are formed by the extrusion of magma on to the Earth's surface. Igneous rocks can also form deep underground, and these are called intrusive igneous rocks, because the magmas were intruded into pre-existing rocks and then slowly cooled. The reason that intrusive igneous rocks are now visible at the surface is that over many millions of years erosion has stripped away the overlying rocks. In this way, bodies of igneous rock that were once subterranean pools of magma are revealed at the surface. Some of these intrusions can be up to several kilometres across. In other cases, magma had intruded pre-existing rocks to form long slab-shaped bodies of igneous rock whose longest dimensions are measured in kilometres but whose shortest dimension is no more than a few metres. These intrusions are called dykes in the case of vertical bodies, and sills in the case of horizontal bodies.