SS1 First Term Geography Lesson Note – Rocks

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WEEK 9 & 10          

TOPIC: Rocks

CONTENT:

• Meaning of rock and types of rocks.
• Mode of formation of rock types.
• Characteristics of rock types.
• Importance of rocks to man.

Sub – Topic 1: Definition and types of rocks.

Definition of Rock    

A rock is any solid substance that forms part of the earth crust. This solid substance may be hard, like granite or even be soft like mud. In other word, rocks are aggregates of minerals, that is, a combination of different minerals.

The most common rocks and their minerals include;

ROCKS	MINERALS
Limestone	quart, feldspar, micas
Sandstone	quart, calcite, feldspar
Granite	feldspar, augite

Types of Rock

There are three major types of rocks according to their origin, mode of formation and physical appearance. They are

1. IGNEOUS ROCKS

Mode of formation:

These rocks are formed from the cooling and solidification of molten rock or magma from the earth beneath. They are crystalline in nature examples are Basalt and Granite.

Igneous rock could be divided into two, namely: Intrusive or plutonic igneous rock and Extrusive or Volcanic Igneous rock

Intrusive Igneous rocks are rocks which solidified deep in the crust and they are only seen on the surface after a prolong erosive action. Examples are granite, diorite and gabbro.

Extrusive Igneous rocks are rocks that formed when the molten rock or magma are ejected and poured on the earth’s surface as Lavas. These rocks do not contain fossils. Example is Basaltic rock and they can be located in North Eastern part of Nigeria.

CHARACTERISTICS OF IGNEOUS ROCKS

1. These rocks are crystalline in nature
2. They are non- stratified rocks i.e. they do not occur in layers.
3. They are resistant to erosion and are usually hard and impervious.
4. They do not contain fossils.

SEDIMENTARY ROCKS

Mode of formation:

These rocks are formed from the accumulation of sediments either by water, ice or wind. They are derived from other pre- existing rocks such as igneous rocks. They are laid in layers or strata. These rocks are said to be stratified rocks only when they become harder in nature by compressional forces.

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