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.