The Greek word for fire aptly describes the source of rocks that come from molten magma and lava. Environmental conditions above ground, near the surface, and deep below ground dictate how much time is required for heat to dissipate from this plasma-like, burning liquid. As the magma and lava cool, the molten liquid crystallizes and solidifies into different forms of igneous rock.
Magma originates from deep inside the Earth where extremely hot temperatures abound nearer to the center core. Continental tectonic plates move and shift; they can collide or separate from each other at points known as active plate boundaries. Heat is focused or injected into these points and solid rock melts into a plasma state of molten, liquid rock called magma. Then, the magma moves outward from the core, rising to the surface under pressure from surrounding cooler rock and flowing with the heat, melting even more rock in its path.
Some igneous rock forms inside the earth, or underground, while still more forms outside, or aboveground. All igneous rock starts deep in the earth as hot, molten magma. One manifestation of this is that basalt ( formed aboveground) and gabbro (formed underground) are two igneous rocks that are noted for their similar chemical makeup despite where they were formed.
Underground creations, or formations, of igneous rock are called “intrusive,” or “plutonic.” Aboveground formations are called “extrusive,” or “volcanic.”
Intrusive igneous rocks are doomed to complete their formation and cooling completely underground from magma. Such rocks take a long time to cool and form large crystals.
Extrusive igneous rocks form and cool completely aboveground from magma released aboveground through a volcano as lava. Such rocks cool rather quickly, leaving only enough time for very small crystals to form.
Once igneous rocks cool and form completely, they may not remain in the same place. The earth’s continental shelves are under continuous stresses and the release of energy will be manifested as motion. Surface weathering and erosion aids the change and movement of formed rocks. This means that igneous rock may not stay too long in one place, and may eventually change and journey back underground as part of the larger rock cycle process.
The family of intrusive igneous rock includes granite, granodiorite, diorite, gabbro, and tonalite.
The family of extrusive igneous rock includes rhyolite, scoria, pumice, basalt, obsidian, andesite, and dacite.
Andesite is observed in shades of gray and sometimes with red or green. It is fine-grained and may displays evidence of bubbling.
Basalt is the most common rock formed from lava. Its crystals are very small and not easily observed. Hawaii and the ocean floor are composed of basalt.
Dacite is light gray, but can be dark gray to black. It contains plagioclase, quartz, pyroxene, or hornblende. Dacite is one of the most common rock types associated with violent, energetic volcanic eruptions.
Granite is one of our most common rocks. It contains a mixture of quartz, feldspar and mica crystals. Granite crystals are large and easily observed.
Obsidian is formed by very rapid cooling of lava. Volcanic glass, or obsidian, is very hard and shiny. It can break or fracture into sharp edges. Early man used obsidian to construct crude knives and the points for arrows and spears.
Pumice is a very light and porous, and contains tiny crystals. Its texture is rough, with many holes and cavities. Pumice originates from lava containing high concentrations of gas bubbles. Pumice is so light that it can float in water.
Tonalite is coarse-grained and related to granite.
Igneous rocks are normally classified according to the minerals they contain. But, they may also be classified based upon their chemical compositions.