igneous
Basalt Column Section
Columnar Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6 (Mohs), Color: Dark grey to charcoal black, Luster: Dull/Matte, Crystal structure: Fine-grained/Aphanitic, Hexagonal geometry, Specific gravity: 2.8 - 3.0
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Physical properties
Hardness: 6 (Mohs), Color: Dark grey to charcoal black, Luster: Dull/Matte, Crystal structure: Fine-grained/Aphanitic, Hexagonal geometry, Specific gravity: 2.8 - 3.0
Formation & geological history
Formed from the rapid cooling and contraction of thick basaltic lava flows. As the lava cools, it shrinks and cracks in a pattern of stress-relieving vertical joints, often resulting in hexagonal or polygonal columns.
Uses & applications
Used in construction as crushed stone, for landscaping/architecture, and as characteristic geological specimens for education.
Geological facts
Famous examples of this formation include the Giant's Causeway in Northern Ireland and Devils Tower in Wyoming. The hexagonal shape is the most efficient way to release energy as the rock contracts.
Field identification & locations
Identify in the field by the distinct polygonal (usually 5 or 6-sided) cross-section and dark, dense, fine-grained texture. Commonly found in volcanic provinces.
