Rocks: Types, Significances, Practice Problems and FAQs

Fire is one of the most important innovations in the evolutionary history of humans. Do you know when and how this happened? Fire was discovered in the early stone age and they made it by rubbing two flintstones at each other. Stone age was an important stage in the development of the genus Homo. Researchers found that the stone age began 2.6 million years ago. During the stone age the early man made many things using the stones, like the tools and weapons to hunt and eat, homes, ornaments, containers etc. You know that whatever inventions they made during the stone age are still a part of our lives.

Fig: Stone age man making fire using stones

Even now we make buildings using stones, rocks and bricks. Do you know what is the main difference between a stone and a rock? Rock is made out of stone and minerals. So rocks are found on the Earth’s crust as well as below the ground and they are made up of minerals. But stones can be called non-metallic minerals and we can simply say that stones are the trimmed and polished tiny pieces of rock. This can be formed naturally or artificially.

Fig: Stones and rocks

Now we know that stones are formed from rocks. So the next thought will be, how the rocks are formed? The only thing we know is that they are formed from the Earth. So now think about the shape, size and colour of the rocks. Do all rocks look alike? Obviously all rocks do not look alike because we have seen different types of rocks, even in our surroundings. The rocks seen in your area will be different from that of your friend’s place. How is this possible? To know more about this we have to know more about the types of rocks and the stages of their formation. Let’s discuss more about different types of rocks in this article.

Rocks
Types of rocks
Formation of rocks
Significance of rocks
Practice Problems
FAQs

Rocks

The solid collection of minerals are called rocks. The mineral grains are cemented together and they can grow by the accumulation of more minerals. Rocks can be observed in different shapes and sizes. The small rocks are called pebbles and large rocks are called boulders. Rocks can be classified into different types on the basis of their chemical composition, mineral deposition and the way by which it is formed.

Fig: Pebbles and boulder

Petrology and Geology

The study of rocks is called petrology. It is the branch of geology that deals with the study of rocks and the conditions that lead to the formation of the rocks.

Types of rocks

According to the way of formation of rocks, they can be classified into three types and are as follows:

Igneous rocks
Metamorphic rocks
Sedimentary rocks

Igneous rocks

The rocks that are formed by the cooling of magma are called igneous rocks. Magma is a very hot and molten material and it originates deep inside the Earth. Magma is a molten mixture of minerals that can form rock and the volatiles such as gases and steam. Igneous rocks are formed at high temperatures, because the constituent minerals that form the rocks have to crystallise from the molten material. There are two types of igneous rocks and are as follows:

Extrusive igneous rocks
Intrusive igneous rocks

Fig: Igneous rocks

Extrusive igneous rocks

Magma rises and reaches the surface of the Earth through the fissures or vents, because magma is hotter and lighter than the rocks which surround them. Once the magma reaches the Earth surface then it is called lava. The event of formation of lava is called extrusion.

Fig: Formation of magma and lava

The lava cools quickly due to the exposure to air and water. From this very small and fine grained crystals are formed and it can be called as extrusive igneous rocks. Examples include basalt, scoria, pumice and obsidian.

GIF: Volcanic eruption

Basalt

Basalt is an example of extrusive igneous rock which is formed by the rapid cooling of magnesium and iron rich lava which has low viscosity. More than 90 percent of the volcanic rock of the Earth is basalt. It is mainly used for making structural building materials like tiles, bricks, foundations and sculptures. It is also used within the stonewalls for thermal purposes and in rail tracks.

Fig: Basalt

Scoria

This dark coloured volcanic rock is created when magma containing abundant dissolved gas flows from a volcano during an eruption. Once the molten rock emerges from the Earth, the pressure upon it is reduced and hence the dissolved gas tries to escape in the bubble form. Sometimes the molten rock solidifies before the gas bubbles have escaped and this creates cavities called vesicles in the rock. The colour of the rock ranges from black or dark grey to deep reddish brown. It is commonly used for landscaping, drainage works and gas barbecue grills.

Fig: Scoria

Pumice

The volcanic rock created when there is a super heated and highly pressurised ejection is called pumice or pumicite. It is a light coloured, highly vesicular and rough textured rock. This may or may not contain crystals. Large volumes of pumice under water is a shipping hazard especially for cargo ships. It is used as an abrasive in the making of polishes, pencil erasers and also in the production of stone washed jeans. This rock is used in the construction works because of the high porosity. These air filled rocks serve as a good insulator.

Fig: Pumice

Obsidian

The igneous rock formed from the felsic lava which is rich in elements like silicon, oxygen, aluminium, sodium and potassium are called obsidian rock. It is a hard, brittle and amorphous rock, hence fractures with sharp edges. In the early times, obsidian rocks were used to manufacture cutting and piercing tools. A type of glass knife called obsidian blades are made using the obsidian rocks instead of artificial glasses. It is also used for ornamental purposes as a gemstone.

Fig: Obsidian

Intrusive igneous rocks

If magma does not rise and reach the surface of the Earth, then it will cool deep below the surface. This event may take over thousands or millions of years and it is called an intrusion. The crystals formed in these types of rocks as a result of slow cooling will be large and easily observable. Such rocks are called intrusive igneous rocks. Examples include granite.

Granite

It is a light colured, coarsely grained and clearly visible mineral crystal. It is mainly used to make floor tiles, countertops, paving stones, stair treads, curbings, cemetery monuments and building veneers.

Fig: Granite

Metamorphic rocks

Under the influence of high temperature, pressure and chemically active solutions, changes can happen to the pre-existing rocks. Such rocks which have undergone physical and chemical changes are called metamorphic rocks. This metamorphic process may take millions of years to form a new rock. The metamorphic process might be occurring to the originally formed sedimentary or igneous rocks.

Fig: Formation of metamorphic rocks

Marble

Marble is an example of metamorphic rocks. The metamorphic process of limestone will make it harder and crystalline. This will result in the formation of marble. It is used for making monuments, buildings, statuaries, table tops, interior decorations, and novelties.

Fig: Marble

Slate

Another example of metamorphic rocks is a slate. The metamorphism of shale rock will result in the formation of slate. It is used for making snooker tables, gravestones, roofings, garden decorations and floorings.

Fig: Slate

Quartzite

The hard and non foliated metamorphic rock formed from the sandstone is called quartzite. Sandstone undergoes heating and pressure due to the tectonic compression and results in the formation of quartzite. The colour of pure quartzite is white to grey. It also occurs in the various shades of pink and red. This is because of the presence of hematite in various amounts. It is resistant to chemical weathering. It is used as decorative stones and also in the construction of highways (crushed quartzite).

Fig: Quartzite

Gneiss

The widely distributed and commonly occurring metamorphic rock is genesis. The high temperature and high pressure on the igneous or sedimentary rocks will help in the formation of genesis. It has a banded texture with alternating darker and lighter coloured patches. It does not have distinct cleavage. Genesis is one of the oldest rocks on the Earth. It is commonly used as a building material.

Fig: Gneiss

Sedimentary rocks

The recycled rocks from the eroded and weathered parent rocks that are deposited and lithified at the surface of the Earth are called sedimentary rocks.

Fig: Sedimentary rocks

Formation of sedimentary rocks

These rocks are formed with the help of water, wind, ice or living organisms. Erosion is a major factor for the formation of sedimentary rocks. Hence the sedimentary rocks consist of sand, pebbles, minerals and mud. Most sedimentary rocks are deposited at the bottoms of lakes, oceans and rivers from the land surface. They can also accumulate in the deserts too.

Sedimentary rocks are layered or stratified. Each layer is called strata and can be distinguished by their colour, size of the particle, cement type and internal arrangement. These rocks are soft. The remains of dead animals or plant fossils are found in these rocks. These rocks are generally brittle, lightweight and porous.

Fig: Formation of sedimentary rocks from the parent rock

Significance of sedimentary rocks

Sedimentary rocks form sediments and a cross-section of Earth's crust indicates the arrangement of sediments one over the other during the long history of Earth. Different-aged rock sediments contain fossils of different life-forms which probably died during the formation of the particular sediment. Some of them appear similar to modern organisms. They represent extinct organisms like dinosaurs also.

Figure : Fossil of a lizard on sedimentary rock

Limestone

Limestone (calcium oxides) is an example of sedimentary rocks. They are formed from the shell fragments of dead sea creatures. Calcium carbonate is the major component of the shells. The compression of sea shells will result in the formation of limestones. It is used in mining, paper production, steel manufacturing, plastic production, manufacture of glass, water treatment and purification. It is also used for agriculture purposes like to increase the fertility of the soil.

Fig: Limestone formation from sea shells

Coal

Another example for sedimentary rock is coal which is a black sedimentary rock. It is formed from the accumulation of dead plant materials in large amounts over millions of years. It is used for cement production, medicines, tars, carbon fibres, foams, synthetic petroleum-based fuels, heating in homes and commercial heating.

Fig: Coal

Formation of coal

During the Silurian period of the Paleozoic era (440 mya) Earth was covered with huge swampy forests of giant ferns, reeds and mosses. Plants kept growing and falling into the swamp waters and new plants grew in their place. This formed a thick layer of dead plants rotting in the swamp. The surface of the Earth changed so did the water and dirt washed in and stopped the decaying process of fallen plants. With evolution, different plants came in, fell and formed separate layers. After millions of years like this many layers were formed, one over the other. With the weight of top layers, water and dirt in the lower layers got packed down. Tremendous heat and pressure produced chemical and physical changes in these layers of plants of ferns, reeds and mosses which forced out the oxygen and left only rich carbon deposits. As time progressed, deposited plant material became coal.

Fig: Formation of coal

Gypsum

The soft sulphate rock is called gypsum and is chemically calcium sulphate dihydrate. It is formed when water evaporates from the mineral-rich marine soil environments. It is mined and used mainly as a fertiliser. Gypsum is a major constituent in plaster, drywall, blackboard etc. The dehydrated gypsum is known as plaster of Paris. When water is added to the plaster of Paris, it will return to the form of regular gypsum.

Fig: Gypsum

Sandstone

Sandstones have a clastic origin. The rocks which are composed of fragments of pre-existing minerals and rock are called clastic rocks. Sandstones are formed when the grains of sand are compacted and cemented together by taking thousands or millions of years. Quartz or feldspar are the most resistant minerals to weathering, hence the sandstones are composed of these minerals. They are used in the construction works, decorative art works and tools.

Fig: Sandstone

Shale

The most common sedimentary rock which is formed from mud (mix of clay and minerals) is called shale. They are fine grained and clastic rocks. It can split into thin layers less than 1 cm in thickness and that property is called fissility. These rocks are formed when silts, muds, and other sediments are getting deposited by gentle transporting currents and become compacted. It is used as a synonym for mudrock and is the most common source of rock for natural gas and petroleum.

Fig: Shale

Conglomerate

The clastic sedimentary rock which is composed of rounded and gravel sized clasts are called conglomerate. It is composed of finer grained sediments like slit, sand or clay. The consolidation and lithification of gravel results in the formation of a conglomerate. Slabs of conglomerate have been found on Mars. These are formed when sediment is deposited by the waves on beaches or by the fast-flowing rivers. Crushed conglomerate is used in construction industries for making roads and buildings. It is also used in the cement manufacturing industry.

Fig: Conglomerate

Rock cycle

The continuous process by which rocks are formed, changed from one form to another, destroyed, and then formed again is called a rock cycle. It explains the basic relationship between the three types of rocks; igneous, metamorphic and sedimentary. The mineral crystals and their host rock types pass through various forms to reach the final form of rocks. Each stage of this rock cycle depends on the temperature, pressure, time and changes in the environmental conditions on the Earth crust.

Steps in rock cycle

The formation of surface rocks can be explained in four steps and these are as follows:

Erosion
Diagenesis
Lithification
Uplift

Fig: Rock cycle

Erosion

The igneous rocks formed from the magma from the depth will undergo erosion to form sedimentary rocks. Erosion is a geological process in which the earthen materials are worn away and transported by natural forces like water or wind. Erosion includes two steps and they are as follows:

Weathering

It is the physical and chemical breakdown of minerals or rocks on the Earth’s surface.

Fig: Weathering

Transportation

It is the movement of minerals to the site of deposition. Along with erosion, uplifting and deposition of minerals occurs to form the sedimentary rocks.

Diagenesis

The conversion of sediments into sedimentary rock is called diagenesis. This process includes the compaction and natural cementing of grains.

Fig: Diagenesis

Lithification

The process of conversion of sedimentary rock into the rock present on the Earth surface is called lithification. It involves compaction and cementation.

Fig: Lithification

Uplift

The metamorphic rocks that are formed undergoes uplifting to form the surface rocks. It involves the vertical elevation of the Earth's surface by responding to natural causes like volcanic eruptions, tectonic plates movement etc.

Fig: Uplift

These four steps are for the formation of the rocks which are found on the Earth surface. But there are some other steps for the subduction of the rocks. Subduction is the process of conversion of the oceanic lithosphere into Earth's mantle at convergent boundaries. Then only the cycle will complete. Subduction will occur when the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of the second plate. In this process the heavier plate dives beneath the second plate and sinks into the mantle.

Other steps in rock cycle

The deep burial of surface soil will result in the formation of metamorphic rocks and the re-melting of the metamorphic rocks will result in the formation of the igneous rocks. Erosion and deposition will lead to the formation of sediments. This sediment has two fates. One is to form the sedimentary rocks through diagenesis. Another is the formation of igneous rock through the burial and melting. Through subduction the igneous rocks will return to the Earth's mantle and the cycle continues.

Significance of rocks

Rocks are one of the major natural resources and they are economically important for humans. Some of the significance of rocks are as follows:

Formation of soil occurs from rocks and soil is essential for agriculture and the growth of plants.
Building materials are obtained directly or indirectly from rocks. Examples include granites, gneiss, sandstones, marbles, and slates.
Metals like gold, platinum, silver, copper, aluminium and iron are obtained from different rocks.
Raw materials for many industries are obtained from rocks. Examples include cement industry and limestone kilns.
Graphite is a raw material for pencil manufacturing companies.
Many precious stones like diamonds (metamorphic rock), rubies, gems and sapphires are obtained from different rocks.

Fig: Diamond

Coal, petroleum, natural gas and nuclear minerals are derived from different rocks which are used as fuels.
Rocks can be used to make some fertilisers. Examples include the phosphorite minerals which are the raw materials of phosphatic fertilisers.

Practice Problems

1. Which of the following statements is wrong about the rock?

a) Minerals are cemented together to form rocks.

b) The small rocks are called boulders.

c) The study of rocks is called petrology.

d) The major three types of rocks are igneous rocks, metamorphic rocks and sedimentary rocks.

A and B
B and C
B only
B and C

Solution: The solid collection of minerals are called rocks. The mineral grains are cemented together and they can grow by the accumulation of more minerals. Rocks can be observed in different shapes and sizes. The small rocks are called pebbles and large rocks are called boulders. Rocks can be classified into different types on the basis of their chemical composition, mineral deposition and the way by which it formed. The study of rocks is called petrology. It is the branch of geology that deals with the study of rocks and the conditions that lead to the formation of the rocks. According to the way of formation of rocks, they can be classified into three types and are igneous rocks, metamorphic rocks and sedimentary rocks. Hence the correct option is c.

2. Which of the following rocks has the precursor as the lava?

Extrusive igneous rocks
Intrusive igneous rocks
Metamorphic rocks
Sedimentary rocks

Solution: According to the way of formation of rocks, they can be classified into three types and are igneous rocks, metamorphic rocks and sedimentary rocks. The rocks that are formed by the cooling of magma are called igneous rocks. Igneous rocks are formed at high temperatures, because the constituent minerals that form the rocks have to crystallise from the molten material. There are two types of igneous rocks and are extrusive igneous rocks and intrusive igneous rocks. Magma rises and reaches the surface of the Earth through the fissures or vents, because magma is hotter and lighter than the rocks which surround them. Once the magma reaches the Earth surface then it is called lava. The event of formation of lava is called extrusion. The lava cools quickly due to the exposure to air and water. From this very small and fine grained crystals are formed and it can be called as extrusive igneous rocks. Hence the correct option is a.

3. Find the wrongly matched pair.

Extrusive igneous rocks - Basalt
Intrusive igneous rocks - Granite
Metamorphic rocks - Limestone
Sedimentary rocks - Coal

Solution: The lava cools quickly due to the exposure to air and water. From this very small and fine grained crystals are formed and it can be called as extrusive igneous rocks. Basalt is an example of extrusive igneous rock which is formed by the rapid cooling of magnesium and iron rich lava which has low viscosity. The crystals formed as a result of slow cooling will be large and easily observable. Such rocks are called intrusive igneous rocks. An example of intrusive igneous rock is granite. Under the influence of high temperature, pressure and chemically active solutions, changes can happen to the pre-existing rocks. Such rocks which have undergone physical and chemical changes are called metamorphic rocks. Marble is an example of metamorphic rocks. The metamorphic process of limestone will make it harder and crystalline. This will result in the formation of marble. Another example is a slate. The metamorphic process of shale rock will result in the formation of slate. The recycled rocks from the eroded and weathered parent rocks that are deposited and lithified at the surface of the Earth are called sedimentary rocks. Limestone is an example of sedimentary rocks. The compression of sea shells will result in the formation of limestone. Another example for sedimentary rock is coal. It is formed from the accumulation of dead plant materials in large amounts over millions of years. Hence the correct option is c.

4. In a rock cycle weathering and transportation happens in which step?

Erosion
Diagenesis
Lithification
Uplift

Solution: The continuous process by which rocks are formed, changed from one form to another, destroyed, and then formed again is called a rock cycle. It explains the basic relationship between the three types of rocks; igneous, metamorphic and sedimentary. The mineral crystals and their host rock types pass through various forms to reach the final form of rock. This process occurs as a cycle and each stage depends on the temperature, pressure, time and changes in the environmental conditions on the Earth crust. The formation of surface rocks can be explained in four steps and they are erosion, diagenesis, lithification and uplift. The igneous rocks formed from the magma from the depth will undergo erosion to form sedimentary rocks. Erosion includes two steps and they are weathering which is the physical and chemical breakdown of minerals and transportation which is the movement of minerals to the site of deposition. Along with erosion, uplifting and deposition of minerals occurs to form the sedimentary rock. Hence the correct option is a.

FAQs

1. What is rock mechanics?
Answer: Rock mechanics is the study of mechanical behaviour of rock and rock masses. It studies the deformation of the rocks due to the strain of the rocks in response to stress.

2. What are extra terrestrial rocks?
Answer: The natural objects on Earth which have the origin in outer space are called extra terrestrial rocks. Asteroids are an example of extra terrestrial rocks. They are remnants from the formation of the solar system which orbits the sun. Their size ranges from the length of a car to the size of a large city.

Asteroid vs. Volcanoes: New Modeling Shows What Made the Earth Uninhabitable for Dinosaurs
GIF: Asteroid

3. What is meant by scree in Geology?
Answer: The collection of broken rock fragments is called scree. It is observed at the base of a cliff or on the steep rocky mass that is accumulated through periodic rock fall. Talus deposits are the landforms associated with the scree.

Fig: Scree

4. Rocks can move or not?
Answer: Yes, stones can move and they are called sailing stones or sliding stones or rolling stones or moving stones. They are a part of geological phenomenon. Without animal help, rocks travel and leave extensive traces along a smooth valley floor.

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