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

Answer Details

(a) Three differences between plutonic and volcanic rocks

(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.