(a) Highlight three differences… | Geography

Question 1 Report

(a) Highlight three differences between plutonic and volcanic rocks
(b) With the aid of diagrams, describe the mode of formation of crater lake

Answer

(a) Three differences between plutonic and volcanic rocks

Plutonic (intrusive) rocks	Volcanic (extrusive) rocks
Form from magma that cools slowly deep within the earth's crust.	Form from lava that cools quickly on or near the earth's surface.
Have large, coarse crystals because of slow cooling.	Have small, fine crystals (or are glassy) because of rapid cooling.
Example: granite.	Example: basalt.
Exposed at the surface only after long erosion of overlying rocks.	Found directly on the surface where they were poured out.

(b) Mode of formation of a crater lake

A crater lake is a body of water that collects in the crater or caldera of a volcano.

During a volcanic eruption, lava, gases and pyroclastic materials are ejected through a central vent, leaving a funnel-shaped hollow called a crater at the top of the volcanic cone.
When the volcano becomes dormant or extinct, eruption ceases and the vent is sealed by cooled, solidified lava which makes the crater floor fairly water-tight.
Over time, rainfall and sometimes underground water collect in the enclosed crater because there is no easy outlet.
The accumulated water fills the depression to form a roughly circular crater lake. Where a very large collapse depression (caldera) is filled, a larger caldera lake is formed.

A well-labelled diagram should show the volcanic cone, the crater/vent, the solidified plug of lava at the base, and the water accumulated in the crater.

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Answer Details

(a) Three differences between plutonic and volcanic rocks

Plutonic (intrusive) rocks	Volcanic (extrusive) rocks
Form from magma that cools slowly deep within the earth's crust.	Form from lava that cools quickly on or near the earth's surface.
Have large, coarse crystals because of slow cooling.	Have small, fine crystals (or are glassy) because of rapid cooling.
Example: granite.	Example: basalt.
Exposed at the surface only after long erosion of overlying rocks.	Found directly on the surface where they were poured out.

(b) Mode of formation of a crater lake

A crater lake is a body of water that collects in the crater or caldera of a volcano.

During a volcanic eruption, lava, gases and pyroclastic materials are ejected through a central vent, leaving a funnel-shaped hollow called a crater at the top of the volcanic cone.
When the volcano becomes dormant or extinct, eruption ceases and the vent is sealed by cooled, solidified lava which makes the crater floor fairly water-tight.
Over time, rainfall and sometimes underground water collect in the enclosed crater because there is no easy outlet.
The accumulated water fills the depression to form a roughly circular crater lake. Where a very large collapse depression (caldera) is filled, a larger caldera lake is formed.

A well-labelled diagram should show the volcanic cone, the crater/vent, the solidified plug of lava at the base, and the water accumulated in the crater.

Read lesson note on Elements Of Physical Geography (WAEC)

Read lesson note on Physical Geography (JAMB)

Elements Of Physical Geography

Physical Geography

View Answer

(a) Highlight three differences between plutonic and volcanic rocks (b) With the aid of diagrams, describe the mode of formation of crater lake

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