igneous
Basalt and Andesite Gravel
Aphanitic Mafic-Intermediate Volcanic Rock
Hardness: 5-7 (Mohs). Colors: dark gray, black, reddish-brown. Luster: dull to earthy. Texture: fine-grained (aphanitic) or vesicular. Structure: massive or slightly porous.
- Hardness
- 5-7 (Mohs)
- Luster
- dull to earthy
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Physical properties
Hardness: 5-7 (Mohs). Colors: dark gray, black, reddish-brown. Luster: dull to earthy. Texture: fine-grained (aphanitic) or vesicular. Structure: massive or slightly porous.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava (basalt) or silica-medium lava (andesite) at or near the Earth's surface. These are typically associated with the Trans-Mexican Volcanic Belt.
Uses & applications
Commonly used in construction aggregate, road base, landscaping, and as an industrial abrasive. Polished varieties are used in decorative gravel.
Geological facts
Basalt is the most common volcanic rock on Earth and also makes up most of the ocean floor and the lunar maria. The reddish-brown specimens often indicate oxidation of iron minerals (forming hematite or limonite).
Field identification & locations
Located in the Trans-Mexican Volcanic Belt (Central Mexico); identify by weight (dense for their size), dark color, and lack of visible crystals without a lens. Look for small holes (vesicles) from gas bubbles.
