Igneous rocks are formed by the cooling of molten rock to form a solid. Lava is molten rock that reaches the surface. Magma is molten rock that is still within the earth. Rocks that form deep within the earth are known as intrusive rocks. Rocks that form at the surface are called extrusive rocks.
The process by which rocks change from a liquid to a solid is known as crystallization.
Crystallization begins at a nucleation site. For example, when water crystallizes (freezes), it begins at a part in the water, and ice "grows" around the nucleation site. In order for larger crystals to form, the process must have a lot of time. So, larger crystals are found in intrusive deposits.
Igneous rocks are classified by chemical composition and texture.
Intrusive rocks generally have coarse-grained textures, because they have larger crystals. Extrusive rocks cool quickly and have small or no crystals. When magma partially crystallizes deep in the earth and on the surface, there are large and small crystals. This texture is knownw as porphyritic.
Igneous rocks are divided into four categories based on mineral composition.
Granitic or Felsic composition: 0- 25% dark mineral composition
Andesite or intermediate composition: 25- 45% dark mineral composition
Basaltic or mafic: 45 - 85% dark mineral composition
Ultramafic: 85-100% dark mineral composition
Felsic: Made up of Quartz, K-spar, muscovite. Coarse: Granite. Fine: Rhyolite
Intermediate: Made up of Amphibole. Coarse: Diorite. Fine: Andesite
Mafic: Made of Pyroxene, Plagioclase. Coarse: Gabbro. Fine: Basalt
Ultramafic: Made of Pyroxene, olivine. Coarse: Peridotite