Igneous Rocks

The structure of igneous rocks is characterized by random arrangement of grains, by ragged crystal borders, by intertwinings and embayments such as one might expect in a mass of crystals growing together and interfering with one another's development. In coarse-grained rocks like granite, this structure is visible to the naked eye; in fine-grained rocks it is revealed by the microscope. The principal constituents of these rocks are always minerals containing silicon: quartz, feldspar, mica, and the ferromagnesian group.

The siliceous liquids from which igneous rocks form are thick, viscous materials resembling melted glass both in properties and in composition. Sometimes, in fact, molten lava has the right composition and cool rapidly enough to form a natural glass – the black, shiny rock called obsidian. Usually, however, cooling is slow enough to allow crystalline minerals to form. If cooling is fairly rapid and if the molten material is highly viscous, the resulting rock may consist of minute crystals or partly of crystals and partly of glass. If cooling is extremely slow, mineral grains have an opportunity to grow large and a coarse-grained rock is formed. The grain size of an igneous rock, therefore, reveals something about its history and gives us one logical basis for classification.

Mineral composition provides a convenient means of further classification. Nearly all igneous rocks contain feldspar and one or more of the ferromagnesian minerals; many contain quartz as well. Thus a coarse-grained rock containing quartz, feldspar, and black mica is granite; a fine-grained rock with no quartz and with feldspar in excess of the dark constituents is andesite, and so on.

This classification is convenient for several reasons:

1 Grain size and usually mineral composition can be determined from inspection in the field. Except for a few fine-grained types, an igneous rock can be named without detailed laboratory study.

2. Even if a rock is too fine for its mineral content to be easily determined, its colour often shows its place in the table. Granite and rhyolite, which contain only a little ferromagnesian material, are nearly always light-coloured; gabbro and basalt, with abundant ferromagnesian minerals, are characterictically dark; diorite and andesite usually have intermediate shades. Granite and rhyolite are sometimes designated as felsic rocks (because of their large feldspar content) and gabbro and basalt as mafic rocks (because of their ferromagnesian content).

3. Grain size usually gives an indication not only of the rate of cooling but also of the environment in which a rock was cooled. Sufficiently rapid cooling to give fine-grained rocks occurs most commonly when molten lava reaches the earth's surface from a volcano and spreads out in a thin flow exposed to the atmosphere. Since fine grain size usually betrays volcanic origin, rhyolite, andesite, and basalt are often called volcanic or extrusive rocks.

Coarse-grained rocks, on the other hand, have cooled sufficiently slowly for large crystals to have formed, which must have occurred well beneath the earth’s surface. Such rocks are now exposed to view only because erosion has carried away the material that once covered them Since these rocks do not reach the surface as liquids but are intruded into spaces occupied by the other rocks, they are often called intrusive rocks.