For those of you who are unfamiliar with geology or some of the terms used throughout this website, here are some definitions that may be helpful in understanding the material. When magma reaches Earth's surface, much of the dissolved gas is released. Magma reaching the surface in liquid form is called lava . The liquid form of a given material is usually less dense than its solid form.
Molten magma is less dense than the surrounding material from which it has melted and so it rises.
It may erupt at the surface from a volcano as lava and cool to form an extrusive igneous rock. Alternatively, it may crystallize deep in the crust below the surface as an intrusion formed from intrusive igneous rock. This texture forms when the rock cools slowly. This texture forms when the rock cools rapidly. If cooling is very rapid, a glassy texure may result.