igneous
Porphyritic Basalt
Basalt (Mafic Extrusive Igneous Rock)
Hardness: 6 (Mohs scale), Color: Dark grey to black matrix with light-colored phenocrysts, Luster: Dull to sub-vitreous, Crystal structure: Aphanitic (fine-grained) groundmass with visible crystals, Cleavage: None (conchoidal fracture), Specific gravity: 2.8 - 3.0
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Physical properties
Hardness: 6 (Mohs scale), Color: Dark grey to black matrix with light-colored phenocrysts, Luster: Dull to sub-vitreous, Crystal structure: Aphanitic (fine-grained) groundmass with visible crystals, Cleavage: None (conchoidal fracture), Specific gravity: 2.8 - 3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at the Earth's surface. The visible large crystals (phenocrysts) formed slowly underground before the magma was erupted. Commonly found in oceanic crust and volcanic regions.
Uses & applications
Used extensively in construction as road base, concrete aggregate, and railroad ballast. Larger pieces are often used for cobblestones or decorative landscaping stones.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. It is also common on the Moon and Mars. The 'speckled' appearance in this specimen is due to cooling in two distinct stages.
Field identification & locations
Identify in the field by its high density, dark color, and the presence of visible white or green crystals against a dark, fine-grained background. It is commonly found near tectonic plate boundaries or hotspots like Hawaii and Iceland.
