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 Form 1 Geography Online Lessons on Minerals and Rocks

In this class we are going to learn about the definition Rocks and igneous type of rocks and their classification

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Answer Text:
Rocks
-A consolidated material composed of grains of one or more minerals.
Classification of Rocks
1. Igneous Rocks
-Rocks formed when molten material from the earth’s interior cools and solidifies on or beneath the earth’s surface.
Types of Igneous Rocks
a) Intrusive Igneous Rocks
-Rocks formed when magma cools and solidifies below the earth’s surface e.g. granite, diorite, gabbro, peridotite.
-Have coarse texture as a result of slow cooling giving minerals more time to form large crystals.
- Are classified further into two:
(i) Hypabyssal rocks
- intrusive igneous rocks which are near the earth’s surface.
(ii) Plutonic rocks
- Intrusive igneous rocks which are deep below the surface.
b) Extrusive Igneous Rocks
- Rocks formed when lava solidifies on the earth’s surface.
- Have fine texture due to fast cooling giving minerals less time to collect together to form larger crystals.
-They are of two types namely:
(i) Volcanic Ejecta
- Extrusive igneous rocks formed in the following ways:
• When ash and lava ejected from underground as they fall on the earth’s surface e.g. pumice.
• When dust and ash ejected settle on the ground and get compressed to form a rock e.g. tuff.
(ii) Lava Flows
-Extrusive igneous rocks formed when basic lava flows over a considerable distance then cools and solidifies e.g. basalt and obsidian.


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