Rock Classification

At first glance, it is not obvious how rocks can be separated into definite categories. We might decide that a light-colored, coarse-grained rock like granite should belong in a different class from a dark, fine-grained volcanic rock like basalt, but we can find a whole series of rocks with properties transitional between the two, and so we cannot say just where one class ends and the other begins. The basic problem is to make distinctions that are not always clear-cut in nature.

Since rocks are composed of minerals, we might guess first that they could be classified on the basis of the kinds and amounts of minerals they contain. But we find that rocks of widely different structures and origins have nearly the same mineral composition, and so we would be grouping together rocks of obviously different types. A classification based on chemical composition encounters the same difficulty, since it places in the same pigeonhole rocks that have little else in common; it has the further disadvantage that chemical compositions are not evident in the field but require laboratory analysis. We might disregard composition and classify rocks according to their origin. This would be an excellent method if it could be applied to all rocks, but the sad fact is that we simply do not know how some rocks were formed and the origin of many others can be determined only after lengthy study.

Evidently we expect a classification of rocks to fulfil several different purposes. We should like it to summarize something about the origins of different rocks, about their compositions, and about their structures, and at the same time we should like to be able to apply it to rocks as we find them in the field. These objectives cannot all be satisfied at the same time. Our recourse is to adopt a compromise, a classification that will accomplish each purpose as well as possible without slighting the others. The particular compromise we shall use is not the only possible one, but it is justified by its simplicity and convenience.

A fundamental division of rocks into three main groups according to origin is agreed on by nearly all geologists:

1. Igneous rocks are those that have cooled from a molten state. Some of these can be observed in process of formation, for instance when molten lava cools on the side of a volcano. For others an igneous origin is inferred from their composition and structure. Two-thirds of crustal rocks are igneous, and the bedrock under the oceans and continents falls into this category.

2. Sedimentary rocks consist of materials derived from other rocks and deposited by water, wind, or glacial ice. Some consist of separate rock fragments cemented together; others contain material precipitated from solution in water. Although sedimentary rocks make up only about 8 percent of the crust, three-quarters of surface rocks are of this kind.

3. Metamorphic rocks are rocks that have been changed, or metamorphosed, by heat and pressure deep under the earth's surface. The changes produced may involve the formation of new minerals or simply the recrystallization of minerals already present.

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.

Sedimentary Rocks

Sediments laid down by water, wind, or ice are consolidated into rock by the weight of overlying deposits and by the gradual cementing of their grains with material deposited from underground water. The resulting rocks are usually characterized by the presence of distinct, somewhat rounded grains that have not grown together like the crystals of igneous rocks. A few sedimentary rocks, however, consist of intergrown mineral grains formed by precipitation from solution in water. Since sediments are normally deposited in layers, the majority of sedimentary rocks have a banded appearance owing to slight differences in colour or grain size from one layer to the next. Sedimentary rocks may often be recognized at a glance by the presence of fossil – remains of plants or animals interred with the sediments as they were laid down.