igneous
Basalt
Extrusive Igneous Rock (Vesicular Basalt)
Hardness of 5-6 on the Mohs scale, typically dark gray to black in color, dull luster, aphanitic (fine-grained) crystal structure with visible vesicles (holes) caused by trapped gas bubbles, specific gravity around 2.8-3.0.
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Physical properties
Hardness of 5-6 on the Mohs scale, typically dark gray to black in color, dull luster, aphanitic (fine-grained) crystal structure with visible vesicles (holes) caused by trapped gas bubbles, specific gravity around 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of low-viscosity, magnesium- and iron-rich lava flows at or very near the Earth's surface. The holes (vesicles) are created by expanding gas bubbles as the lava solidifies.
Uses & applications
Used in construction, as crushed stone for roadbeds, aggregate in concrete, and sometimes in landscaping or saunas.
Geological facts
Basalt is the most common rock type in the Earth's oceanic crust and is also found extensively on the Moon and Mars as dark volcanic plains (maria).
Field identification & locations
Easily identified by its dark, fine-grained matrix peppered with rounded holes (vesicular texture). Often found in regions with historic or active volcanic activity such as Hawaii, Iceland, and the Pacific Northwest of the USA.
