Igneous rocks, from the Latin ignis - fire, are those rocks that form from the cooling and crystallization of magma or lava. Magma is the molten rock that hasn't reached the surface, whereas lava is molten rock that has reached the surface.

The dominant rock type of the earth, igneous rocks can be classified based on their chemistry. We can define broad enough groups so that we don't need to know the actual chemistry of the rock, but can estimate it based on the mineral components in the rock.

Composition
 

Just by looking at the chart, a number of things should become apparent to you.

• As crystallization increases, the viscosity increases.

This should make intuitive sense if you think about honey on a cold camping trip - when the honey is cold it doesn't flow very well at all and may need a little heating before use.
• As the weight percent of SiO₂ increases, viscosity increases.

This isn't due any temperature effect, but instead is due to the SiO₂ molecule's ability to modify the bonds within the liquid. SiO₂ is known as a network-builder which will increase viscosity of the melt. Addition of Mg, Fe, Ca, and Na may break this network and are thus called network modifiers.

Texture

When we describe a rock - whether it be igneous, metamorphic, or sedimentary, we should note any crystals that are present, their size, shape, and relationship to other crystals.

• Crystallinity: based primarily on the rate and the total time available to crystallize.
• all crystals - holocrystalline (implies long cooling time, e.g., a granite)
• crystals and glass - hypocrystalline
• no crystals (all glass) - holohyaline (extremely rapid cooling, e.g., an obsidian)

• Granularity - how large are the crystals; based on rate of cooling
• pegmatite - crystals are larger than 5 cm (some can be up to 10 meters long!)
• phaneritic - crystals visible to the naked eye
• aphanitic - small crystals best seen with a hand lens
• glassy - crystals can't be seen at all (e.g., obsidian)

• Grain equality - how one grain relates to others
• equigranular - all roughly the same size
• porphyritic - large grains (phenocrysts) in a fine-grained matrix (groundmass) of smaller crystals

• Grain shape - how well developed the individual grain shapes are
• euhedral - all crystal faces well developed
• subhedral - some crystal faces developed
• anhedral - no crystal faces developed

As a magma, or lava, cools, it will form crystals in a very orderly progression. The ferro-magnesian minerals, those that require Fe and Mg in their formation, will crystallize in order and at the expense of one another. This means that those ferro-magnesian minerals that crystallize first, at high temperatures, will be consumed by later minerals as they crystallize. E.g., olivine will crystallize at high temperatures, but will be consumed by the crystallization of lower-temperature ferro-magnesian minerals such as the pyroxenes, amphiboles, and micas (they crystallize in that order). Bowen named this reaction series the Discontinuous Reaction Series.

Minerals that do not consume each other during cooling, and subsequent crystallization form the Continuous Reaction Series. They first start with calcium-rich plagioclase, then progress through the sodium-rich plagioclase until they finally crystallize the potassium-rich plagioclase.

If, after all of this crystallization, there is still any melt fraction left, quartz may crystallize.

Magma, if it doesn't make it to the surface will eventually crystallize - sometimes taking million of years to do so. Rocks just don't melt in place that often, so intrusive rocks generally originate somewhere else and migrate to new locations.

By far, the largest part of the Earth is its mantle. Based on geophysical studies, as well as the occasional piece that gets caught up in eruptions, we think that it has an ultramafic composition. In the plate tectonic scheme of things, when there are large upwelling currents, for example at mid-ocean spreading centers, there will also be an upwelling of peridotitic material. A partial melt of ~5 percent is enough to produce a melt that has a composition that is basaltic.

For melt at the other end of the compositional range - granites (from the Italian, grano - grainy, in reference to its coarse-grained appearance), it's much more difficult than just applying heat. Under a continent, even one that has a high heat flow, the temperature doesn't get nearly hot enough to melt the material and produce a great deal of melt. Only with the application of water, as can be seen in the graph to the left, can we start to melt material and produce granites. The presence of H₂O acts as a flux, allowing lower temperatures at which the rock will melt. This process becomes important when we discuss rocks of intermediate composition and how they form melts.

Intermediate compositions rocks, such as andesites (named after the Andes Mountains of South America), make use of wet melting reactions and are found at all subduction zones. As the plate, usually oceanic, subducts it takes down with it some amounts of water in the sediments being subducted as well as in the minerals themselves (remember the ^. H₂O minerals). As the subducting plate is taken to deeper and deeper depths, the minerals are heated and the volatiles are driven from the slab and into the overlying material causing it to melt as the flux is introduced.

Once the andesite is generated it must make its way up through the overlying crust. During the ascent it can become contaminated with components of the crust or may even stop and stagnate in the crust before finally erupting at the surface.