3.3 Igneous Rocks Solidify from Liquid Magma to form Crystalline Rock

When magma is at its hottest, ions and groups of ions join together and break apart constantly. Then, as magma cools, the ions begin to move more slowly and eventually join together into orderly crystalline structures. This process, called crystallization, generates various silicate minerals that reside within the remaining melt.

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In any crystalline solid, the ions are arranged in a closely packed regular pattern. However, they are not without some motion. They exhibit a sort of restricted vibration about fixed points. As the temperature rises, the ions vibrate more rapidly and consequently collide with ever-increasing vigor with their neighbors. Thus, heating causes the ions to occupy more space, which in turn causes the solid to expand. When the ions are vibrating rapidly enough to overcome the force of the chemical bonds, the solid begins to melt. At this stage the ions are able to slide past one another, and their orderly crystalline structure disintegrates. Thus, melting converts a solid consisting of tight, uniformly packed ions into a liquid composed of unordered ions moving randomly about.

In the process of crystallization, cooling reverses the events of melting. As the temperature of the liquid drops, the ions pack closer and closer together as they slow their rate of movement. When cooled sufficiently, the forces of the chemical bonds will again confine the ions to an orderly crystalline arrangement.

When magma cools, it is generally the silicon and oxygen atoms that link together first to form silicon-oxygen tetrahedra, the basic building blocks of the silicate minerals. As a magma continues to lose heat to its surroundings, the tetrahedra join with each other and with other ions to form embryonic crystalline nuclei. Slowly each nucleus grows as ions lose their mobility and join the crystalline network.

The earliest formed minerals have space to grow and tend to have better developed crystal faces than do the later ones that fill the remaining space. Eventually all of the melt is transformed into a solid mass of interlocking silicates minerals that we call an igneous rock.

Of course, actual crystallization of magma is much more complex than this description. Whereas a single compound, such as water, crystallizes at a specific temperature, solidification of magma with its diverse chemistry spans a temperature range of 200 degrees Celsius or more. During crystallization, the composition of the melt continually changes as ions are selectively removed and incorporated into the earliest formed minerals.

If the melt should separate from the earliest formed minerals, its composition will be different from that of the original magma. A single magma may generate rocks with widely differing compositions. As a consequence, a great variety of igneous rocks exist.

Although the crystallization of magma is complex, it is nevertheless possible to classify igneous rocks based on their mineral composition and the conditions under which they formed. Their environment during crystallization can be roughly inferred from the size and arrangement of the mineral grains, a property called texture. Consequently, igneous rocks are most often classified by their texture and mineral composition.

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