What are rocks and their different types.

”Rocks are composed of mineral assemblages. A few consist of only one mineral, such as Qaurtzite, which is completely Qaurtz, but most rocks contain several minerals and these minerals have much to do with the way rocks break or bend, weather or erode”. (H.J. DBlij & Muller)

Rock is naturally-occurring aggregate of minerals of organic or inorganic origin. The Earth’s lithosphere is made of rocks. Petrology is the scientific study of rocks.


Rocks are classified by minerals and Chemical composition, by the texture of the constituents particles and by the processes that formed them. These indicators separates rocks into Igneous, Sedimentary and Metamorphic. The transformation of one rock type to another is described by the geological model called the ”rock cycle”.


* Minerals are homogeneous, naturally occurring, inorganic solids. Each mineral has a definite chemical composition and characteristic crystalline structure. A mineral may be a single element such as God (Au), Copper (Cu), or it may be compound made up of number of elements. About 2,500 different minerals have been described till today.
* Rocks are made up of one or more elements.


  • Igneous Rocks
  • Sedimentary Rocks
  • Metamorphic Rocks 

1. Igneous Rocks:

Igneous rocked are formed when molten magma cools. Igneous rocks have got their origin from word ”Agni” which means made from fire or heat. When the rock is liquid and inside the earth, it is called magma. When the magma gets hard inside the crust, it turns into granite. It cools very slowly and is very hard.

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When the magma gets up to the surface and flows out, like what happens when a volcano erupts, then the liquid is called Lava. Lava flows down the sides of volcano. When it cools and turns hard it is called obsidian, lava rock or pumice – depending on what it looks like. About 64.7 % of the Earth’s crust by volume consists of Igneous rock; making it the most plentiful category. Of these, 66% are basalts and gabbros while 66% are granite. The oceanic crust is 99% basalt, which is an igneous rock of mafic composition.

Types of Igneous Rocks:

01. Acid/Felsic Igneous Rocks:

It has an access of acid forming radical, silicon which is about 80%. The rest has magnesium, sodium, potassium, etc. It cools fast and does not flow or spread away. High mountains are formed of acid igneous rocks. It is light in colour. The main example of this type of rock is granite. Its erosion is very slow. They are mostly compact and massive. They belong to the portion of the Earth known as Sial.

02. Basic Mafic Igneous Rocks :

The silicon content in these type of rocks is below 40%. These rocks contain 40% magnesium and the rest 20% contains iron, aluminium and potassium. It cools slowly due to low silicon content. The molten lava spreads to far off distance and plateaus are formed from these rocks. Basalt and Dolesite are the main examples. These rocks are not very hard and are eroded comparatively faster. They belong to the portion of the earth known as Sima.

03. Intrusive/ Plutonic Igneous Rocks:

These rocks are formed by solidification of magma at moderate depths beneath the earth’s surface. Since the solidification of magma occurs at great depths below the earth’s surface, the rate of cooling is much slower. As a result of this slow cooling, very large sized crystals are formed in such rocks. These deep seated intrusive are known as plutonic rocks. Highly coarse grained granite is a typical example of a plutonic rock.

04. Extrusive/ Volcanic Igneous Rocks:

These rocks are formed as a result of the solidification of lava and ashes on the surface of earth. This lava, hot gases and water vapours erupt from the weaker portion of the crust and spread on the surface of the earth. These rocks are also known as volcanic rocks. As lava cools down rapidly on coming out of earth’s hot interior, the mineral crystals of extrusive igneous rocks change their structure and are very small, giving them fine grained look. Basalt is a typical example of extrusive igneous rocks.

05. Hypabyssal Igneous Rocks:

Hypabyssal igneous rocks are formed at a depth in between the plutonic and volcanic rocks. These are formed due to cooling and resultant solidification of rising magma just beneath the earth surface. Hypabyssal rocks are less common than plutonic and volcanic rocks and often form dikes, sills, laccoliths or phacoliths.

2. Sedimentary Rocks:

Sedimentary rocks are formed by deposition of either clastic sediments, organic matter, or chemical precipitates (evaporites), followed by compaction of the particular matter and cementation. Sedimentary rocks form at or near the Earth’s surface.

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When mountains are first formed, they are tall and jagged like the Rocky Mountains on the west coast of North America. Over times (millions of years) mountains become old mountains like the Appalachian Mountains of East Coast of Canada and United States. When mountains are old, they are rounded and much lower. What happens in meantime is that lots of rock gets worn away due to the erosion. Rain, freeze/ thaw cycle, wind and running water cause the big mountain to crumble a little bit at a time.
Eventually most of the broken bits of the rock end up in the streams and rivers that flow down from the mountains. These little bits of rocks and sand are sediments. When the water slows down enough, these sediments settle to the bottom of the lake or oceans they run into. Over many years, layers of lakes and oceans. Think of each layer as a page in a book. One piece of paper is not heavy. But a stock of telephone books is very heavy and would squish any thing that was underneath. Over time the layers of sand and mud at the bottom of lakes and oceans turned into rocks. These are called Sedimentary rocks.
Some Examples of Sedimentary rocks are sandstone and shale. Sedimentary rocks often have fossils in them. Plants and animals that have died get covered up by new layers of sediments and are turned into stones. Most of the fossils we find are of plants and animals that lived in the sea. They just settled to the bottom. Other plants and animals died in swamps, marshes or at the edges of lakes. so these rocks are rich in fossil fuels.
When large amount of plants are deposited in sedimentary rocks, they turn into carbon. This gives us our coal, oil, natural gas and petroleum. Sedimentary rocks cover 75% of Earth’s surface. Most of the rocks found on the earth’s surface are sedimentary.

Types of Sedimentary Rocks:

A sedimentary rock is divided into three different types. Each type is formed from a different kind of sediment.

01. Mechanically formed sedimentary rocks:

Clastic rock is a sedimentary rock formed from sediments that have been cemented are compacted together. These sediments are often broken bits of rocks and minerals. They range in size from very large to very small. The texture of Clastic rocks is usually fragmental. Examples are Conglomerates, Sand Stone, Silt Stone, shale etc. Following are important types of these rocks.

02. Arenaceous Rocks:

These rocks are made of sand particles cemented and compacted together. Quartz particles are major part of these types of rocks. The example are Sandstone and pebbles.

03. Argillaceous Rocks:

These rocks are mainly made up of clay. Shale is the best example of such rocks. These rocks are found in excess in Joya Meer and Khewra, Pakistan. According to the mood of formation of these rocks, they are further divided into 3 types.

a.  Aeloian Rocks: Sedimentary Rocks that are created through the deposition of particles carried by wind are Aelian Rocks. For Example Sand Dunes and Loess etc. in Northern China.

b.  Aqueous Rocks: These rocks are formed by water and are found in River Rocks and Lacustrine Rocks.

c.  Marine Deposits: When river falls into a sea the load of river is deposited on the ocean floor, the pebbles and heavier material are deposited near the shore, while lighter load is carried farther to the ocean. At some places the shells of living creatures are accumulated on the ocean floor to make the rock rich in calcium.

04. Chemical sedimentary Rocks:

Chemical rock is a sedimentary rock made of minerals that were once dissolved in water. Some chemical rocks are formed when water evaporates and leaves mineral deposits behind. Rock salt is an example of a chemical rock formed in this way.

05. Organic Sedimentary Rocks:

Organic sedimentary rock is a rock made of substances that were once part of or made by living things. Following are two broad categories of these rocks:

06. Calcareous Rocks:

These rocks are are made from the skeletons or remnants of animals. Chalk and limestone are the example of these types of rocks. Some lime stones are formed by once living things. For example, When animals with shells die, their shells sink to the ocean floor. As layer of shell build up, they may harden into rock.

07. Carbonaceous Rocks:

Carbonaceous rocks are formed from the remains of plants converted by heat and pressure into coal. Peat, Lignite and Coal are Carbonaceous deposits of organic origin. All these rocks are composed of plants debris in different stages of alteration.

Some Common Sedimentary Rocks:

”’Conglomerate rock”’ has rounded rocks(pebbles, boulders) cemented together in a matrix.
”’Sand Stone”’ is soft stone that is made when sand gains gets cemented together.
Sediments and Corresponding Sedimentary Rocks

3. Metamorphic Rocks:

Metamorphic rocks are formed by subjecting any rock type (including previously formed metamorphic rocks) 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 recrystallization).

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Metamorphic rocks are the rocks that have changed. The word comes from the Greek ”Meta” and ”morph” which means to change form. Metamorphic rocks are were originally igneous or sedimentary, but due to movement of Earth’s crust, were changed. If you rub your hands very hard together, you will feel heat and pressure. This is the mechanism of formation of Metamorphic rocks.
Marble is an example of sedimentary rock that has changed into Metamorphic Rock.
”’Metamorphic rocks”’ are formed by alteration of pre-existing rocks by exposure to heat and pressure while remaining in a solid form. Metamorphism also occurs by breaking bonds between atoms in a mineral so that the atoms rearrange themselves into new, or more stable mineral form. Rocks are transformed and remain in a solid state because not all the bonds in the rock minerals are broken. Metamorphism occurs in solid rocks because only some of the bonds between atoms are broken in an unstable mineral. As a result, the freed atoms and ions can migrate to another location with in the mineral, or bond with atoms in a different mineral. The end result is to produce mineral that are more stable under the environmental conditions in which they exist.

Types of Metamorphic Rocks:

The metamorphic are by the least abundant of the three major surface rock categories. They are much more common deep with in the crust, where they have been produced by the tectonic activity.

01. Foliated Rocks:

Foliated Rock is a metamorphic rock whose minerals are arranges in parallel layers. Foliated results when minerals recrystallize, or are flattened under pressure. Foliation also results when minerals of different densities are separated into layers behaving much like a mixture of oil and water. This separation results in a series of alternating light and dark bands. Many foliated rocks break into thin sheets. Slates, Schist and Gneiss are examples of foliated rocks.

02. Non-foliated Rocks:

Non-Foliated Rock is metamorphic rock without layers. This type of rock does not break up into flat sheets. Marble and Quartzite are the example of non-foliated rocks.


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