igneous
River Rock (Basalt)
Basalt (SiO2 range: 45-52%)
Hardness: 6 on Mohs scale; Color: typically dark gray to black, often weathered to brown; Luster: dull to vitreous (especially when wet); Crystal structure: aphanitic (fine-grained); Cleavage: none; Specific gravity: 2.8-3.0.
- Hardness
- 6 on Mohs scale
- Color
- typically dark gray to black, often weathered to brown
- Luster
- dull to vitreous (especially when wet)
Identified More igneous →
Explore River Rock (Basalt) in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6 on Mohs scale; Color: typically dark gray to black, often weathered to brown; Luster: dull to vitreous (especially when wet); Crystal structure: aphanitic (fine-grained); Cleavage: none; 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. Extrusive igneous rock. Ages vary widely by location (from recent to hundreds of millions of years). The rounded shape is the result of mechanical weathering and abrasion within a high-energy aquatic environment, such as a riverbed or shoreline.
Uses & applications
Used extensively in construction (aggregate, road base), landscaping, and for decorative use in home aquariums or Zen gardens. Sometimes used for therapeutic 'hot stone' massages.
Geological facts
Basalt is the most common volcanic rock on Earth and also makes up most of the ocean floors. It has been found on the Moon and Mars as well. The dark, flat plains seen on the Moon (Maria) are basaltic lava flows.
Field identification & locations
Identify in the field by its dark, uniform color, heavy weight for its size, and smooth, rounded water-worn texture. Commonly found in riverbeds, glacial deposits, and along coastal beaches in volcanic regions.
