SS3 First Term Geography Lesson Note – Vulcanicity

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WEEK 3                                            

TOPIC: VULCANICITY

CONTENT:

• Meaning and Processes of Vulcanicity
• Regions of Volcanic Occurrences

SUB-TOPIC 1: MEANING AND PROCESSES OF VOLCANICITY OR VOLCANISM

DEFINITION: Vulcanicity refers to the rising of Molten Rock or Magma or other gaseous materials beneath the earth’s crust forcing its way into planes of weakness or Vent of the earth’s crust to escape quietly or explosively to the surface.

The magma while moving through the vent to the earth’s surface may cool and solidify within the crust or before it gets to the surface of the earth as Plutonic rocks leading to intrusive landforms.

On the other hand, when the magma reaches the surface of the earth, it cools and solidifies as volcanic rocks and results in extrusive landforms. Rocks formed by either Plutonic or Volcanic activities are called igneous rocks.

TYPES OF LAVAS

There are two main types of lavas namely basic and acid lavas.

1. Basic lavas: These are the hottest lavas, about 1000°C and are highly fluid. They are dark in colour like basalt, rich in iron and magnesium but poor in silica. When poured out of volcano, they flow quietly and are not very explosive. They flow readily with a speed of 16 – 48km per hour due to their high fluidity. They affect extensive areas, spreading out as thin sheets over great distances before they solidify. The resultant volcano is gently sloping with a wide diameter and forms a flattened shield or dome.
2. Acid lavas: These lavas are highly viscous (i.e. semi-fluid) with a high melting point. They are light in colour, of a low density and have a high percentage of silica. They flow slowly and seldom (not often) travel far before solidifying. The resultant cone is therefore steep-sided. The rapid congealing of lava in the vent obstructs the flow of the out-pouring lava, resulting in loud explosions, throwing out many volcanic bombs or pyroclasts. Sometimes the lavas are so viscous that they form a spine or plug at the crater.

DIFFERENCE BETWEEN ACID AND BASIC LAVAS

1. Acid lavas are highly viscous (semi-fluid) while basic lavas are highly fluid.
2. Acid lavas are light in colour while basic lavas are dark in colour.
3. Acid lavas have a high percentage of silica while basic lavas are poor in silica.
4. Acid lavas flow slowly while basic lavas flow readily with a speed of 16-48km per hour.
5. Acid lavas resultant cone is steep-sided while basic lavas resultant volcano is gently sloping.
6. Acid lavas form a spine or plug at the crater while basic lavas form a flattened shield or dome.

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