In
geology,
rock is a naturally occurring solid aggregate of
minerals and/or
mineraloids.
The Earth's outer solid layer, the
lithosphere, is made of rock. In general rocks are of three types, namely,
igneous,
sedimentary, and
metamorphic. The scientific study of rocks is called
petrology, and petrology is an essential component of geology.
Rock classification
Rocks are classified by mineral and
chemical composition, by the
texture of the constituent particles and by the processes that formed them. These indicators separate rocks into
igneous,
sedimentary and
metamorphic. They are further classified according to
particle size. The transformation of one rock type to another is described by the geological model called the
rock cycle.
thumb|Sample of igneous gabbroIgneous rocks are formed when molten
magma cools and are divided into two main categories:
plutonic rock and
volcanic. Plutonic or intrusive rocks result when magma cools and crystallizes slowly within the
Earth's crust (example
granite), while volcanic or extrusive rocks result from magma reaching the surface either as
lava or fragmental ejecta (examples
pumice and
basalt) .
[Blatt, Harvey and Robert J. Tracy, 1996, Petrology, W. H. Freeman, 2nd ed. ISBN 0-7167-2438-3]thumb|Sandstone with iron oxide bandsSedimentary rocks are formed by deposition of either
clastic sediments, organic matter, or chemical precipitates (
evaporites), followed by compaction of the particulate matter and cementation during
diagenesis. Sedimentary rocks form at or near the Earth's surface. Mud rocks comprise 65% (
mudstone,
shale and
siltstone);
sandstones 20 to 25% and
carbonate rocks 10 to 15% (
limestone and
dolostone).
s are formed by subjecting any rock type (including previously-formed metamorphic rock) to different
temperature and
pressure conditions than those in which the original rock was formed. These temperatures and pressures are always higher than those at the Earth's surface and must be sufficiently high so as to change the original minerals into other mineral types or else into other forms of the same minerals (e.g. by
recrystallisation).
The three classes of rocks — the igneous, the sedimentary and the metamorphic — are subdivided into many groups. There are, however, no hard and fast boundaries between allied rocks. By increase or decrease in the proportions of their constituent minerals they pass by every gradation into one another, the distinctive structures also of one kind of rock may often be traced gradually merging into those of another. Hence the definitions adopted in establishing rock nomenclature merely correspond to selected points (more or less arbitrary) in a continuously graduated series.
Impact on society
Rocks have had a huge impact on the cultural and technological advancement of the human race. Rocks have been used by
Homo sapiens and other
hominids for more than
2 million years.
Lithic technology marks some of the oldest and continuously used technologies. The
mining of rocks for their
metal ore content has been one of the most important factors of human advancement, which has progressed at different rates in different places in part because of the kind of metals available from the rocks of a region.
The prehistory and history of civilization is classified into the
Stone Age,
Bronze Age, and
Iron Age. Although the
stone age has ended virtually everywhere, rocks continue to be used to construct buildings and
infrastructure. When so used, rocks are called
dimension stone.
See also
