Igneous rocks form when molten rock cools and solidifies (crystallizes). Molten rock trapped underground is called magma. Molten rock erupted at the Earth's surface is referred to as lava. Approximately 99% of most igneous rocks are composed of only eight elements: oxygen, silicon, aluminum, iron,calcium,sodium, potassium, and magnesium. These elements occur in the crystalline structures of the common rock-forming silicate minerals: feldspars, olivines, pyroxenes, amphiboles,quartz and mica. These six minerals make up over 95% of the common igneous rocks.

Igneous rocks are identified by their mineral content and by their texture. The texture refers to the size, shape and boundary relationships of adjacent mineral crystals in a rock mass. In igneous rocks, texture is dependent upon the composition of the parent magma and the rate of cooling. Magmas deep within the Earth's crust cool slowly, allowing crystals to grow to larger, easily visible sizes. This rock texture is known as phaneritic. A lava that is extruded on the Earth's surface cools rapidly, allowing very little time for crystals to grow. Crystals in this environment are typically so small that they cannot be seen without the aid of a microscope. The resultant rock appears to have little to no crystal structure and is said to be aphanitic.