Basalt

Physical Properties

Hardness: 6-7 on Mohs scale. Color: Dark grey to black, weathering to brown or green. Luster: Dull to matte. Structure: Fine-grained (aphanitic), often contains micro-crystals of plagioclase and pyroxene. Density: 2.8–3.0 g/cm³.

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the Earth's surface. Common in oceanic crust and huge volcanic plateaus. Ages range from modern eruptions to billions of years old.

Uses & Applications

Used extensively in construction as crushed stone for road base, railroad ballast, and concrete aggregate. Can be used for high-quality floor tiles and as a component in rock wool insulation.

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 found on the Moon and Mars, forming the dark 'Maria' plains seen from Earth.

Field Identification & Locations

Identify in the field by its heavy weight (density), dark color, and lack of visible large crystal grains. Look for smooth, rounded edges if found in riverbeds or coastal areas (like the specimen shown).

Identified on: 4/14/2026

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