• In geology, a rock is a naturally occurring solid aggregate of one
or more minerals or mineraloids. For example, the common
rock granite is a combination of the quartz, feldspar,
biotite minerals. The Earth's outer solid layer, the lithosphere, is
made of rock.
• Rocks have been used by mankind throughout history. From
the Stone Age rocks have been used for tools. The minerals and
metals we find in rocks have been essential to human
civilization. The scientific study of rocks is calledpetrology,
which is an essential component of geology. Three major groups
of rocks are defined: igneous, sedimentary, and metamorphic.
• Igneous rock (derived from the Latin word igneus meaning of fire,
from ignis meaning fire) forms through the cooling and solidification
of magma or lava. This magma can 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 rock and volcanic.
• 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.
Characteristics of igneous rocks
• These rock do not have remains of plants and
• These rocks do not occur in distinct beds or strata.
• These rocks have metals and minerals like silver,
gold, iron, etc.
• These rocks do not allow water to percolate
Plutonic or intrusive
rocks result when
magma cools and
within the Earth's
crust. A common
example of this type
Volcanic or extrusive
rocks result from
magma reaching the
surface either as lava
or fragmental ejecta,
forming minerals such
as pumice orbasalt.
• Sedimentary rocks are formed at the earth's surface.
Before being deposited, sediments are formed
by weathering or earlier rocks by erosion in a source
area, and then transported to the place of deposition
by water, wind, ice, mass movement or glaciers.
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.
Characteristics of sedimentary
• These rocks are made up of particles of various sizes.
• These rocks are mostly formed under water and have
marks of waves and mud-cracks.
• They are found in horizontal layers.
• These rocks are softer than igneous rocks.
• Between the layers, these rocks also contain remains
of plants and animals, called fossils. Fossil fuels like
petroleum are obtained from these rocks.
• 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 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.
Characteristics of metamorphic
• These rocks are very hard and closed, banded
• These are very hard and have a high specific
• These rocks do not have empty spaces in them.
• After changes, these rocks remain in the original
Uses Of Rocks And Minerals
• Rocks are majorly used in the construction of roads,
houses and buildings. Soil is formed from rocks. The
chemicals which are obtained from the sedimentary
rocks are used to make fertilizers. Coal and petroleum
provide the sources of power. In fact, in ancient
times, rocks were used to make weapons and tools.
• The rock cycle is an ongoing process, beginning as rocks are
pushed up by tectonic forces, and eroded by wind and rain.
The eroded rocks travel by wind or moving water until they
are deposited, settling into layers. Additional eroded rocks
may bury these layers until heat and pressure change the
underlying layers to metamorphic rock. More eroded rocks
may squeeze and press the layers into sedimentary rocks.
Rocks can also be sunk down into the lower layers of the
earth by plate tectonic processes. Buried rocks may also melt
and recrystallize into igneous rocks. Metamorphic,
sedimentary, and igneous rocks may then be pushed up by
tectonic forces, starting the rock cycle again.