Theory question 303
Rocks(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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(a) Plutonic rocks cool deep inside the crust, volcanic rocks cool on or near the surface; plutonic cool slowly and have large (coarse) crystals, volcanic cool fast and have small (fine) or no crystals; plutonic are exposed only after erosion, volcanic form at once on the surface; examples: granite, gabbro (plutonic) vs basalt, obsidian (volcanic); plutonic form batholiths, volcanic form lava plateaus and cones.
(b) A volcano erupts and builds a cone with a crater at the top; when activity stops, the vent is blocked by solidified lava; the crater (or a caldera formed when the summit collapses or is blown off) collects rain water, which cannot drain away, forming a crater lake, e.g. Lake Bosumtwi (Ghana), Crater Lake (Oregon), Lake Barombi. Diagrams: volcanic cone with crater; plugged vent with water in the crater.
Name a real crater lake.