ROCKS

Rocks

• 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 Rocks• 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 animals.

• 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 through them.

Intrusive igneous rocksPlutonic or intrusive rocks result when magma cools and crystallizes slowly within the Earth's crust. A common example of this type is granite.

Extrusive igneous rockVolcanic or extrusive rocks result from magma reaching the surface either as lava or fragmental ejecta, forming minerals such as pumice orbasalt.

Sedimentary rocks• 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 rocks

• 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• 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 rocks

• These rocks are very hard and closed, banded structures.

• These are very hard and have a high specific gravity.

• These rocks do not have empty spaces in them.

• After changes, these rocks remain in the original position.

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.

Rock Cycle• 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.