Igneous rocks form directly from magma inside the earth's crust (called: plutonic or intrusive) or lava on the earth's surface (volcanic or extrusive).

Light-colored igneous rocks (pumice, rhyolite & granite) are composed of larger amounts of silica and are said to be felsic (feldspar + silica). Such light colored minerals include quartz and orthoclase (potassium) feldspar. Molten rock material with abundant silica is thick, slow moving, and form rocks that have lower densities. Dark colored igneous rocks (basalt, diabase & gabbro) have a higher proportion of ferromagnesian minerals, such as hornblende, augite, and olivine. These rocks are called mafic (magnesium + iron). Molten rock which less silica content is typically thinner, flows more easily, and form rocks that have a higher density.

The size of the mineral grains are determined by the rate of cooling. Those that cool quickly at the earth's surface (extrusive igneous rocks) are glassy (obsidian & pumice) or fine-grained (basalt & rhyolite). Those that form slowly within the earth's crust (intrusive igneous rocks) are coarse-grained (granite & gabbro).

Andesite is an example of a medium-colored igneous rock that is relatively fine-grained, which means that it contains felsic (mostly plagioclase feldspar) & mafic (horneblende, augite & biotite) minerals and cooled at or near the earth's surface

Some igneous rocks may display large crystals that are surrounded by a finer-grained material. Rocks with these two different crystal textures are called porphyry. It is thought that the larger crystals develop earlier in the rocks formation, while the magma is still deep within the earth. Then, as the magma nears the surface, the rest of the material cools, at a more rapid pace, and forms the surrounding fine-grained rock.

• magma -molten rock underground
• lava - molten rock at surface; extrusive (= volcanic); fine-grained or glassy
• plutonic - intrusive (in the earth) igneous rock from magma, coarse-grained, most are felsic (granite)
• volcanic - extrusive (at the surface) igneous rock from lava, fine-grained, most are mafic (basalt)
• felsic (feldspar + silica) - light-colored (including pink/red); high silica content
• mafic (magnesium + iron) - dark-colored; low silica/higher % of ferromagnesian minerals
• porphyry - two distinct textures
• texture - size, shape, and arrangement of mineral crystals (glassy, fine-grained, coarse-grained, porphyritic); provides clues to the location (depth) at which rocks formed
• composition - mineral (chemical) content of the rock; felsic (light color), mafic (dark color), intermediate (medium color)