Types of Rocks – Igneous, Sedimentary & Metamorphic

WHAT IS ROCK?

When minerals or earthly particles like clay, sand, shale, magma, etc., combine to form a bigger size particle, we call it a rock. Rocks can be as small as a pebble and can be as large as a mountain. If you are told that our planet earth is just a big rock, will you believe it? This is true. It is sometimes called the third rock from the sun.

Minerals or earthly particles do not combine in any fixed ratio or structure to form a rock. Therefore, we cannot represent rocks with any fixed chemical composition or the formula.

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Definition of Rock –

Rock is an agglomeration of minerals and earthly particles like clay, sand, shale, and magma.

The study of the rocks is called petrology (‘Petro’ means ‘rock’ and ‘logy‘ means ‘study‘). The study of rocks provides a record of the passage of time and the evolution of living things on the earth.

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TYPES OF ROCKS

There are three types of rocks present in the earth’s crust – igneous, sedimentary, and metamorphic.

IGNEOUS ROCKS

What is Igneous Rock?

Igneous rocks form when rock minerals are so hot that they have melted, then cool and go solid again. Thus, we also call igneous rocks, as fire rocks.

Igneous rocks are formed either underground or on the ground. The deep under the surface of the earth, rock melts under high pressure and temperature to form magma. Sometimes, this magma gets trapped in small spaces or pockets and cools down slowly. Magma in these pockets solidifies on cooling and forms igneous rocks. In this way, underground igneous rocks are formed. However, when magma comes above the ground during volcanic eruptions, we call it, lava. This cooling of lava on the ground forms igneous rocks.

Igneous rocks comprise about 95% of the total earth’s crust. All other types of rocks viz., sedimentary and metamorphic, originate from igneous rocks, and, therefore, they are also called primary rocks.  There are many types of igneous rocks. Each kind of igneous rocks contains different minerals and has different structures.

To learn more about igneous rocks, please watch this wonderful video by ClickView. Video courtesy ClickView

Types of Igneous Rocks

There are more than 700 types of igneous rocks. A few examples for types of igneous rocks are – granite, scoria or basalt, pumice, and obsidian.

• Granite rocks

These rocks are the major continental rocks. Granite rocks form due to the slow cooling of pockets of magma deep below the earth’s surface. The main minerals present in granite rocks are quartz, feldspar, and mica. Granite is hard and does not wear out easily. Therefore, we use granite rocks for building monuments, making statues, kitchen slabs, and flooring.

• Scoria or Basalt rocks

Scoria rock or basalt rock forms due to the quick cooling of the lava during underwater volcanic eruptions in the sea or the ocean. It is not a mixture of minerals, it is a kind of glass. It has small pits over it, which are the result of the release of air from small pockets.

• Pumice rocks

Pumice rock forms when lava cools quickly above the ground. These rocks have many small holes in them, which give them a sponge-like appearance. Due to air holes, they are very light and sometimes float on water. We use powder of pumice rocks to make soaps, tooth powder and to polish the floors. Pumice rocks also find uses as body scrubbers.

• Obsidian or Volcanic rocks

Obsidian rock forms due to the rapid cooling of molten lava above the ground. They look like black glass. The edges of these rocks are very sharp, thus obsidian rocks are used for making tools.

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SEDIMENTARY ROCKS

What is Sedimentary Rock?

Sedimentary rocks form when layers of sand, mud, organic matter and eroded particles deposit as sediments at the bottom of the rivers, lakes, and oceans. This deposition of layer over layers continues for millions of years until the bottom layers slowly turned into rocks due to compressive forces exerted by the top layers. Sedimentary rocks, mostly consist of weathered remain of igneous rocks along with organic matter from the remains of marine organisms.

These rocks comprise only about 5% of the earth’s crust. In spite of it, sedimentary rocks cover about 75% of the total land surface.

Types of Sedimentary Rocks

There are many types of sedimentary rocks. A few examples of sedimentary rocks are – sandstone, limestone, conglomerate, gypsum, and shale.

• Sandstone rocks

Sandstone rocks form due to the deposition of sand at the bottom of the rivers, lakes, and oceans. They are rich in minerals called quartz and feldspar. Sandstone is soft and is either yellow, brown, red or pink. As it is relatively soft to carve, thus it is widely used around the world for the construction of buildings, statues, and temples. The Red Fort in Delhi is made of red sandstone.

• Limestone rocks

These rocks form due to the deposition of Calcite. Calcite generally comes from the beds of evaporated seas and lakes, and the shells of sea animals. Limestone is used in the manufacture of concrete and glass. It is an excellent building stone for humid regions. The chalk, which we use for writing on blackboards is also a form of limestone.

• Conglomerate rocks

Conglomerate rocks are made of sand and pebbles which are held together with minerals like silica and calcium carbonate. These rocks are either gray or orange. Conglomerates sometimes find uses as ornamental stones.

• Gypsum rocks

Gypsum rocks are very soft and mainly comprise of sulfate minerals. They form within layers of sedimentary rock, typically found in thick beds or layers.  Gypsum mostly forms in lagoons where ocean waters high in calcium and sulfate content can slowly evaporate and be regularly replenished with new sources of water. Gypsum finds common uses in the form of plaster of Paris, casts, and molds.

• Shale rocks

Shale rocks form due to compression of silt and clay that is piled in layers due to pressure. They typically form in environments where muds, silts, and other earthly sediments deposit them with the help of gentle transporting currents. Most of the shale rocks occur in sheets several meters thick. Shale rocks comprise about 70% of the total sedimentary rocks available in the earth’s crusts. Shale rock provides the valuable raw material for the manufacturing of bricks, tiles, and Portland cement.

Sedimentary Rocks – Layers of Time

Sedimentary rocks form in horizontal layers, at the bottom of seas, in lakes and along rivers, and also in the deserts, as sandstone. These layers represent the passage of time, covering millions of years. Only sedimentary rocks contain fossils. Any fossilized remains of plants and animals are destroyed when they are melted to make igneous rocks, or altered by pressure and temperature to form metamorphic rocks. Over time, sedimentary rocks are bent and folded by earth movements or worn away as particles that become future sedimentary rocks.

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METAMORPHIC ROCKS

What is Metamorphic Rock?

Metamorphosis is a natural process of complete change or transformation without melting. Thus, metamorphic rocks are the rocks that have transformed or changed to another kind of rocks due to pressure, intense temperature, or the action of water and chemical activity. The parent rocks were once igneous or sedimentary rocks. How do sedimentary and igneous rocks change? The rocks are actually under tons and tons of pressure, which builds up heat causing the rock to change. When we examine metamorphic rock samples closely, we find some flattened grains in them.

Types of Metamorphic Rocks

There are many types of metamorphic rocks.

• Marble

Limestone, a sedimentary rock changes to form marble. The limestone, the source material for marble, forms when calcium carbonate precipitates out of water or when organic debris (shells, coral, skeletons) accumulate.

It comes in many colors besides white and black. It is cut and polished to give it a very smooth surface. Marble finds uses in making buildings and statues. The Taj Mahal in Agra, India is an exclusive example of any building constructed of white marble.

• Slate

Shale, a sedimentary rock changes to form slate. Slate has a shiny appearance and can be split into thick sheets. It is either gray or black. We use slate for making blackboards, roofs and stone walls.

• Quartzite

Sandstone, a sedimentary rock changes to form quartzite. It forms when a sandstones rich in quartz undergo metamorphism. Quartzite is used for making statues.

• Gneiss

Granite, an igneous rock changes to form gneiss. In this rock, mineral grains get flattened due to tremendous heat and pressure and get arranged in alternating patterns. They have a glittery appearance because they contain a shining mineral called mica.

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