TYPES OF IGNEOUS ROCKS

 They are formed from crystallization and solidification of molten magma. They may be deposited onto the earth’s surface or with in the earth’s crust i.e. extrusive or intrusive rocks respectively. The main process for the formation of igneous rocks is volcanicity. This is due to the radioactivity and connectivity with in the earth’s mantle. Lines of weakness or fault lines are created in the earth’s crust through which magma is either extruded or intruded.

TYPES OF IGNEOUS ROCKS

Igneous rocks are usually described as fire formed rocks, igneous rocks are characterized by being hard, don’t contain fossils and they are crystalline in nature. As magma is pushed from deep in the earth’s interior, it cools in different parts of the earth’s crust thus forming different types of rocks. They include

Volcanic rocks or extrusive rocks

Hypabyssal or intermediate rocks

Plutonic or abyssal or deep seated rocks.

Volcanic or extrusive rocks.

These are igneous rocks formed on top of the earth’s surface. They are formed from fast flowing surface cooling lava with much smaller crystals due to exposure to oxygen. Examples include basalt, rhyolite, trachyte and andesite etc. extrusive rocks are generally hard.

Hypabyssal or intermediate rocks.

These are igneous rocks formed between volcanic and plutonic rocks. They are firmed near the earth’s surface and cool at a moderate rate or average rate with medium sized crystals. Like plutonic rocks, hypabyssal rocks are also intrusive. Examples include quartz and dolerite.

Plutonic rocks or deep seated rocks.

These are igneous rocks that are formed at a greater depth with in the earth’s crust. They are also described as deep seated rocks. The magma which forms these rock type rises for a short distance and cools in the crust very far from the earth’s surface. This magma cools extremely slowly at a greater depth due to lack of oxygen and results into formation of rocks with large crystals and examples include gabbro, granite, diorite among others.

Igneous rocks can be illustrated as below.

Summary of various types of igneous rocks.

Rock type

Where it is found

Cooling rate

Size of crystals

Example

Volcanic or extrusive rocks

On top of the earth’s surface

A fast rate

Small crystals

Basalt, andesite etc.

Hypabyssal or intermediate rocks

Shallow depth or near the earth’s surface

An average rate or medium rate

Medium sized crystals

Quartz, diorite etc.

Plutonic or deep seated rocks

At greater depth

Cools at a slow rate

Forms large crystals

Gabbro, granite etc.

Magma from the mantle which is ejected varies in chemical composition which has an effect on its viscosity or thickness. This in turn influences the cooling rate and the process of crystallization.

Acidic igneous rocks have a higher amount of silica content; intermediate igneous rocks have a mixed structure of minerals (average silica content) where as basic igneous rocks contain less content.

Igneous rocks in East Africa are found around volcanic mountains of mountain Kenya areas, Kilimanjaro areas, Muhavura areas and around the great east African rift valley. It should be noted that formally intrusive rocks may later be exposed by denudation processes and when they are resistant, they form uplands for example the Mubende batholith, Tanganyika batholith and the

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