Different types of rocks


DIFFERENT TYPES OF ROCKS


•      Many of the materials we use everyday are made from different kinds of rocks.

•      Depending on the way they are formed, rocks are grouped as:

 

Igneous Rocks:

•      All rocks on Earth were initially igneous in nature.

Figure 6: Igneous rocks

 

Formation:

•      The Earth's core is made of molten rock.

•      When a volcano erupts, this molten rock flows out.

•      It later cools and solidifies to form igneous rock. Inside the Earth, molten rock is called magma and when it flows out onto the surface, it is called lava.

  

(a)                                   (b)

Figure 7: (a) Magma  (b) Lava

•      Almost 95% of the Earth's crust is made up of igneous rocks.

•      These contain minerals such as mica, feldspar, and magnesium.

 

For example:

Granite:

  1. It is a very hard igneous rock that has been formed by the slow cooling of lava.

Figure 8: Granite- A hard igneous rock

  1. It is used extensively as a building material.
  2. We see polished granite on kitchen counters, floors, etc.

 

Obsidian:

  1. It forms by quick cooling of lava.
  2. It is smooth and glassy.

Figure 9: Obsidian- Smooth and glassy igneous rock

  1. It is usually black or dark coloured.
  2. Obsidian is used in making jewellery and ornaments.

 

Pumice:

  1. It is porous, or full of holes.

Figure 10: Pumice- A porous igneous rock

  1. It is formed from lava that is frothy with a lot of air within it.
  2. It is used in making lightweight building materials like concrete blocks.

 

Sedimentary Rocks:

•      These rocks are formed by the deposition of the material present at the Earth’s surface.

Figure 11: Sedimentary rocks

 

Formation:

•      Rain, wind, rivers flowing down mountains, etc., wear down rocks on mountains.

•      These small bits of rocks are then carried down and deposited elsewhere.

•      Repeated deposition presses down the lower layers and hardens them into rock.

•      Such rocks are called sedimentary rocks.

 

Use:

•      These rocks are used in the construction of roads; houses; tunnels; canals; etc.

Figure 12: Road and houses made up of rocks

 

For example:

Limestone:

  1. It is a rock with fine grains that are quite soft.

Figure 13: Limestone- A soft sedimentary rock

  1. It contains the mineral calcium and tiny bits of animal shells.
  2. It is used in making many things such as lime, paper, linoleum, cement, and even chewing gum.

 

Sandstone:

  1. It is a soft stone that is formed of sand particles deposited close together.

(a)                                                     (b)

Figure 14: (a) Sandstone- A soft stone made up of sedimentary rock

(b) Red Fort, Delhi- made up of red sandstone

 

Conglomerate:

  1. This rock has rounded stones such as pebbles, gravel, and even boulders cemented together into a mass.
  2. It has coarse grains and can withstand weathering.

Figure 15: Conglomerate- A cemented sedimentary rock

  1. It is used in construction and also polished to make ornaments.

 

Shale:

  1. It is made of solidified clay and comes apart in thin layers.

Figure 16: Shale- A solidified clay

 

  1. It is used in making bricks and cement.

 

POINT TO REMEMBER:

Many sedimentary rocks have fossils in them because, along with the bits of rocks, animal and plant remains also get deposited during rock formation.

 

Metamorphic Rocks:

•      These rocks are a combination of rock types; compressed together by high pressure which gives them a more hard texture than the other types of rocks.

 

Figure 17: Combination of rock types- Metamorphic rocks

•      They are the rocks that have changed their form to metamorphose.

Figure 18: Different types of metamorphic rocks

 

For example:

Marble:

  1. Marble is an example of changed sedimentary rock formed from limestone.

Figure 19: Marble- A changed sedimentary rock

 

  1. Marble is used in making statues and for other ornamental purposes.

Figure 20: Taj Mahal

 

Slate:

  1. Slate is also metamorphosed sedimentary rock, formed from shale.

Figure 21: Slate- Formed from shale

  1. Like shale, it splits into flat layers.
  2. It is used in making bricks and cement.

 

Gneiss:

  1. Gneiss is formed from granite.

Figure 22: Gneiss- Formed from granite

  1. It has different kinds of minerals in it, giving it a striped or streaked look.
  2. It is used as building stone and for making other structures.

 

Quartzite:

  1. It is formed from sandstone that is rich in the mineral quartz.

 

Figure 23: Quartzite- Formed from sandstone

  1. It is a very hard rock and is weather resistant.
  2. It is used in the glass & ceramic industry, in jewellery & ornaments.