LIWA

Types of Rocks

ICSE Class 7 · Geography · Chapter 7

What this chapter covers

Key definitions

Rocks are natural solid materials that make up the Earth's crust, each formed from one or more minerals joined together.

A mineral is a naturally occurring solid substance with a definite chemical make-up and a regular inner structure.

Magma is molten rock found below the Earth's surface.

Lava is magma that has come out onto the Earth's surface, usually through a volcano or a crack in the ground.

Igneous rocks are rocks formed when molten magma or lava cools and becomes solid.

Intrusive igneous rocks are igneous rocks formed when magma cools slowly deep inside the Earth's crust.

Extrusive igneous rocks are igneous rocks formed when lava cools quickly on the Earth's surface.

Granite is a hard, coarse-grained intrusive rock, usually pink, grey or white with dark specks.

Basalt is a dark, heavy, fine-grained extrusive rock formed from lava.

Pumice is a light, frothy extrusive rock full of tiny holes left by gas bubbles in cooling lava.

Sediment is the loose material, such as sand, silt, clay, pebbles and the remains of plants and animals, that is carried by water, wind or ice and later dropped in a new place.

Sedimentary rocks are rocks formed when layers of sediment are deposited, pressed together and cemented into solid rock over a very long time.

Key points

Questions answered

What are rocks, and what are they made of?
Rocks are natural solid materials that make up the Earth's crust. They are made of one or more minerals, such as quartz, feldspar and mica.
Name the three main types of rocks and state the basis on which they are grouped.
Igneous, sedimentary and metamorphic rocks. They are grouped by the way they were formed.
Why are igneous rocks called primary rocks?
They were the first rocks to form when the Earth's hot surface cooled, and all other rocks were later made from them.
Granite has large crystals, but basalt has very small ones. Explain why.
Granite formed from magma that cooled slowly deep underground, so its crystals had time to grow. Basalt formed from lava that cooled quickly on the surface, so its crystals stayed tiny.
Give two features that help you recognise a sedimentary rock.
It occurs in layers (strata), and it often contains fossils. It is also usually softer and more porous than igneous rock.
Why are fossils never found in igneous rocks?
Igneous rocks form from molten rock, and any plant or animal remains would be destroyed by that great heat.
Name the metamorphic rock formed from each of these: (a) limestone, (b) shale, (c) sandstone, (d) granite.
(a) marble, (b) slate, (c) quartzite, (d) gneiss.
A rock is heated so strongly that it melts completely. Will the rock formed from it be metamorphic? Give a reason.
No. Metamorphic rocks form without melting. A melted rock becomes magma, and when it cools it forms an igneous rock.