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:
Figure 8: Granite- A hard igneous rock
Obsidian:
Figure 9: Obsidian- Smooth and glassy igneous rock
Pumice:
Figure 10: Pumice- A porous igneous rock
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:
Figure 13: Limestone- A soft sedimentary rock
Sandstone:
(a) (b)
Figure 14: (a) Sandstone- A soft stone made up of sedimentary rock
(b) Red Fort, Delhi- made up of red sandstone
Conglomerate:
Figure 15: Conglomerate- A cemented sedimentary rock
Shale:
Figure 16: Shale- A solidified clay
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:
Figure 19: Marble- A changed sedimentary rock
Figure 20: Taj Mahal
Slate:
Figure 21: Slate- Formed from shale
Gneiss:
Figure 22: Gneiss- Formed from granite
Quartzite:
Figure 23: Quartzite- Formed from sandstone