A rock is any naturally occurring solid mass
or aggregate of minerals or mineraloid matter.
It is categorized by the minerals included,
its chemical composition and the way in which
it is formed.
Rocks are usually grouped into three main
groups: igneous rocks, metamorphic rocks and
sedimentary rocks.
Rocks form the Earth's outer solid layer,
the crust.
Igneous rocks are formed when magma cools
in the Earth's crust, or lava cools on the
ground surface or the seabed.
The metamorphic rocks are formed when existing
rocks are subjected to such large pressures
and temperatures that they are transformed,
something that occurs, for example, when continental
plates collide.
The sedimentary rocks are formed by diagenesis
or lithification of sediments, which in turn
are formed by the weathering, transport and
deposition of existing rocks.The scientific
study of rocks is called petrology, which
is an essential component of geology.
== Classification ==
Rocks are composed of grains of minerals,
which are homogeneous solids formed from a
chemical compound arranged in an orderly manner.
The aggregate minerals forming the rock are
held together by chemical bonds.
The types and abundance of minerals in a rock
are determined by the manner in which it was
formed.
Many rocks contain silica (SiO2); a compound
of silicon and oxygen that forms 74.3% of
the Earth's crust.
This material forms crystals with other compounds
in the rock.
The proportion of silica in rocks and minerals
is a major factor in determining their names
and properties.Rocks are classified according
to characteristics such as mineral and chemical
composition, permeability, texture of the
constituent particles, and particle size.
These physical properties are the result of
the processes that formed the rocks.
Over the course of time, rocks can transform
from one type into another, as described by
a geological model called the rock cycle.
This transformation produces three general
classes of rock: igneous, sedimentary, and
metamorphic.
Those three classes 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 minerals, they pass through gradations
from one to the other; the distinctive structures
of one kind of rock may thus be traced gradually
merging into those of another.
Hence the definitions adopted in rock names
simply correspond to selected points in a
continuously graduated series.
=== Igneous rock ===
Igneous rock (derived from the Latin word
igneus, meaning of fire, from ignis meaning
fire) is formed through the cooling and solidification
of magma or lava.
This magma may be derived from partial melts
of pre-existing rocks in either a planet's
mantle or crust.
Typically, the melting of rocks is caused
by one or more of three processes: an increase
in temperature, a decrease in pressure, or
a change in composition.
Igneous rocks are divided into two main categories:
Plutonic or intrusive rocks result when magma
cools and crystallizes slowly within the Earth's
crust.
A common example of this type is granite.
Volcanic or extrusive rocks result from magma
reaching the surface either as lava or fragmental
ejecta, forming minerals such as pumice or
basalt.The chemical abundance and the rate
of cooling of magma typically forms a sequence
known as Bowen's reaction series.
Most major igneous rocks are found along this
scale.About 64.7% of the Earth's crust by
volume consists of igneous rocks, making it
the most plentiful category.
Of these, 66% are basalts and gabbros, 16%
are granite, and 17% granodiorites and diorites.
Only 0.6% are syenites and 0.3% peridotites
and dunites.
The oceanic crust is 99% basalt, which is
an igneous rock of mafic composition.
Granites and similar rocks, known as meta-granitoids,
form much of the continental crust.
Over 700 types of igneous rocks have been
described, most of them having formed beneath
the surface of Earth's crust.
These have diverse properties, depending on
their composition and the temperature and
pressure conditions in which they were formed.
=== Sedimentary rock ===
Sedimentary rocks are formed at the earth's
surface by the accumulation and cementation
of fragments of earlier rocks, minerals, and
organisms or as chemical precipitates and
organic growths in water (sedimentation).
This process causes clastic sediments (pieces
of rock) or organic particles (detritus) to
settle and accumulate, or for minerals to
chemically precipitate (evaporite) from a
solution.
The particulate matter then undergoes compaction
and cementation at moderate temperatures and
pressures (diagenesis).
Before being deposited, sediments are formed
by weathering of earlier rocks by erosion
in a source area and then transported to the
place of deposition by water, wind, ice, mass
movement or glaciers (agents of denudation).
Mud rocks comprise 65% (mudstone, shale and
siltstone); sandstones 20 to 25% and carbonate
rocks 10 to 15% (limestone and dolomite).
About 7.9% of the crust by volume is composed
of sedimentary rocks, with 82% of those being
shales, while the remainder consists of limestone
(6%), sandstone and arkoses (12%).
Sedimentary rocks often contain fossils.
Sedimentary rocks form under the influence
of gravity and typically are deposited in
horizontal or near horizontal layers or strata
and may be referred to as stratified rocks.
A small fraction of sedimentary rocks deposited
on steep slopes will show cross bedding where
one layer stops abruptly along an interface
where another layer eroded the first as it
was laid atop the first.
=== Metamorphic rock ===
Metamorphic rocks are formed by subjecting
any rock type—sedimentary rock, igneous
rock or another older metamorphic rock—to
different temperature and pressure conditions
than those in which the original rock was
formed.
This process is called metamorphism, meaning
to "change in form".
The result is a profound change in physical
properties and chemistry of the stone.
The original rock, known as the protolith,
transforms into other mineral types or other
forms of the same minerals, by recrystallization.
The temperatures and pressures required for
this process are always higher than those
found at the Earth's surface: temperatures
greater than 150 to 200 °C and pressures
of 1500 bars.
Metamorphic rocks compose 27.4% of the crust
by volume.The three major classes of metamorphic
rock are based upon the formation mechanism.
An intrusion of magma that heats the surrounding
rock causes contact metamorphism—a temperature-dominated
transformation.
Pressure metamorphism occurs when sediments
are buried deep under the ground; pressure
is dominant, and temperature plays a smaller
role.
This is termed burial metamorphism, and it
can result in rocks such as jade.
Where both heat and pressure play a role,
the mechanism is termed regional metamorphism.
This is typically found in mountain-building
regions.Depending on the structure, metamorphic
rocks are divided into two general categories.
Those that possess a texture are referred
to as foliated; the remainders are termed
non-foliated.
The name of the rock is then determined based
on the types of minerals present.
Schists are foliated rocks that are primarily
composed of lamellar minerals such as micas.
A gneiss has visible bands of differing lightness,
with a common example being the granite gneiss.
Other varieties of foliated rock include slates,
phyllites, and mylonite.
Familiar examples of non-foliated metamorphic
rocks include marble, soapstone, and serpentine.
This branch contains quartzite—a metamorphosed
form of sandstone—and hornfels.
== Human use ==
The use of rocks has had a huge impact on
the cultural and technological development
of the human race.
Rocks have been used by humans and other hominids
for at least 2.5 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.
=== Mining ===
Mining is the extraction of valuable minerals
or other geological materials from the earth,
from an ore body, vein or (coal) seam.
This term also includes the removal of soil.
Materials recovered by mining include base
metals, precious metals, iron, uranium, coal,
diamonds, limestone, oil shale, rock salt
and potash.
Mining is required to obtain any material
that cannot be grown through agricultural
processes, or created artificially in a laboratory
or factory.
Mining in a wider sense comprises extraction
of any resource (e.g. petroleum, natural gas,
salt or even water) from the earth.Mining
of rock and metals has been done since prehistoric
times.
Modern mining processes involve prospecting
for ore bodies, analysis of the profit potential
of a proposed mine, extraction of the desired
materials and finally reclamation of the land
to prepare it for other uses once mining ceases.The
nature of mining processes creates a potential
negative impact on the environment both during
the mining operations and for years after
the mine has closed.
This impact has led to most of the world's
nations adopting regulations to manage negative
effects of mining operations.
== See also
