Geomorphology
Until the early nineteenth century, many natural scientists subscribed to a concept known as catastrophism. According to this idea, floods caused by supernatural forces formed most of the rocks visible at the earth's surface. Late in the eighteenth century, Abraham Gottlob Werner, an influential German professor of Mineralogy, claimed that most rocks formed as a result of the precipitation of minerals from a vast sea that periodically flooded and retreated from the earth's surface. These ideas were entirely speculative; they were unsupported by evidence from the physical world.
Not long after Werner published his ideas, James Hut-ton, a Scottish farmer, established the foundations of uniformitarianism by writing about the origins of rocks in Scotland. Hutton concluded that rocks formed as a result of a variety of processes presently operating at or near the earth's surface—processes such as volcanic activity and the accumulation of grains of sand and clay under the influence of gravity. It was only after extensive debate that Hutton's interpretation of the origins of rocks was generally accepted by the scientific community. Once established, however, uniformitarianism soon dominated the science of geology, gaining almost total acceptance after Charles Lyell, an Englishman, popularized it in the 1830s in a three-volume book entitled Principles of Geology. Let us briefly examine the uniformitarian view of how rocks form.
Rocks consist of interlocking or attached grains that are typically composed of single minerals. A mineral is a naturally occurring inorganic solid element or compound with a particular chemical composition or range of compositions and a characteristic internal structure. Quartz, which forms most grains of sand, is probably the most familiar and widely recognized mineral; other minerals form the materials we call mica, clay, and asbestos. Rocky surfaces that stand exposed and are readily accessible for study are generally designated as outcrops or exposures, although some geologists restrict the first term to rocks laid bare by natural processes and the second to rocks exposed by human activities such as quarrying or road building. Scientists also have access to rocks that are not visible in outcrops or exposures. Well drilling and mining, for example, allow geologists to sample rocks that are still buried beneath the earth's surface.
On the basis of modes of origin, many of which can be seen operating today, early uniformitarian geologists, led by Hutton and Lyell, came to recognize three basic types of rocks: igneous, sedimentary, and metamorphic.
Igneous rocks, which form by the cooling of molten material to the point at which it hardens, or freezes (much as ice forms when water freezes in a refrigerator), are composed of interlocking grains, each consisting of a particular mineral. The most familiar igneous rock to the nongeologist is granite. The molten material, or magma, that turns into igneous rocks comes from great depths within the earth, where temperatures are very high. This material may reach the earth's surface through cracks and fissures in the crust and then cool to form extrusive, or volcanic, igneous rock, or it may cool and harden within the earth to form intrusive igneous rock. Igneous rock that solidifies deep within the earth is sometimes uplifted by subsequent earth movements along with surrounding rock and eventually exposed at the earth's surface by erosion, which is the group of processes that loosen rock and move pieces of loosened rock downhill.
Sedimentary rocks form from sediments, which are materials deposited at the earth's surface by water, ice, or air. Most sediments are accumulations of distinct mineral grains. Some of these grains are products of weathering (i.e., decay and breakup) of older rocks, while others result from the chemical precipitation of minerals from water. Grains of sediment seldom become mutually attached to form a hard rock until long after they have accumulated. The two important agents of this rock-forming process, which is known as unification, are compaction of sediment under the influence of gravity and cementation of grains by the precipitation of mineral cement from solutions that flow between the grains. Lithification is a form of diagenesis,which is the full set of processes, including solution, that alter sediments at low temperatures after burial. Alteration at high temperatures constitutes metamorphism, which will be described shortly.
Most igneous rocks consist of silicate minerals and so do most sedimentary particles, or clasts,derived from them. Sedimentary rocks formed primarily of silicate minerals are thus known as siliciclastic rocks, and these are the most abundant sedimentary rocks of the earth's crust.
Sediments usually accumulate during episodes of deposition, each of which forms a tabular unit known as a stratum (plural, strata). Strata tend to remain distinct from one another even after lithification because the grains of adjacent beds usually differ in size or composition. Because of their differences, the contacting surfaces of the strata usually adhere to each other only weakly, and sedimentary rocks often flake or fracture along these surfaces. The result is that sedimentary rocks exposed at the earth's surface often have a steplike configuration when viewed from the side. Stratification is the word used to describe the layered character of sedimentary rocks. Bedding is stratification in which layers exceed one centimetre in thickness, and lamination is stratification on a finer scale.
Metamorphic rocks form by the alteration, or meta-morphism, of rocks within the earth under conditions of high temperature and pressure. By definition, metamorphism alters rocks without turning them to liquid. If the temperature becomes high enough to melt a rock and the molten rock subsequently cools to form a new solid rock, this new rock is by definition igneous rather than meta- morphic. Metamorphism produces minerals and textures that differ from those of the original rock and that are characteristically arrayed in parallel wavy layers.
Geologists also classify rocks into units called formations. Each formation consists of a body of rocks of a particular type that formed in a particular way—for example, a body of granite, of sandstone, or of alternating layers of sandstone and shale. A formation is formally named, usually for a geographic feature such as a town or river where it is well exposed. Smaller rock units called members are recognized within some formations. Similarly, some formations are united to form larger units termed groups, and some groups, in turn, are combined into supergroups.