igneous
Vesicular Basalt with Feldspar Phenocrysts
Porphyritic Basalt
Hardness of 5-6 on the Mohs scale, dark brown to grayish-black groundmass with whitish/bluish-grey plagioclase feldspar crystals (phenocrysts) and small rounded holes (vesicles) formed by trapped gas bubbles. Luster is dull to subvitreous. Specific gravity ranges from 2.8 to 3.0.
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Physical properties
Hardness of 5-6 on the Mohs scale, dark brown to grayish-black groundmass with whitish/bluish-grey plagioclase feldspar crystals (phenocrysts) and small rounded holes (vesicles) formed by trapped gas bubbles. Luster is dull to subvitreous. Specific gravity ranges from 2.8 to 3.0.
Formation & geological history
Formed from mafic lava flows that cooled rapidly on the Earth's surface. The larger crystals (phenocrysts) formed slowly deeper underground before the magma was erupted, while the fine-grained or glassy groundmass cooled quickly upon extrusion.
Uses & applications
Used primarily in construction, as crushed stone for road bases, railroad ballast, and aggregate in concrete, as well as in landscaping.
Geological facts
Basalt is the most common rock type in the Earth's crust underlying the ocean basins, and it is also found on the Moon and Mars.
Field identification & locations
Can be identified in the field by its dark, fine-grained matrix containing distinctive lighter crystals and small gas cavities (vesicles). Commonly found in volcanic regions worldwide.
