igneous
Basalt
Basalt (Mafic volcanic rock)
Hardness: 6 (Mohs scale); Color: Dark gray to black, often weathering to brown or green; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None/Irregular; Specific gravity: 2.8–3.0.
- Hardness
- 6 (Mohs scale)
- Color
- Dark gray to black, often weathering to brown or green
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark gray to black, often weathering to brown or green; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None/Irregular; Specific gravity: 2.8–3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. Most basalt is Cenozoic or Mesozoic in age, though it has formed throughout Earth's history through volcanic eruptions and seafloor spreading.
Uses & applications
Extensively used in construction as crushed stone for road base, railroad ballast, and concrete aggregate. High-quality basalt is used for floor tiles, monuments, and as 'lava rocks' in landscaping and grills.
Geological facts
Basalt is the most common rock type in Earth's crust, covering most of the ocean floor. It is also found extensively on the Moon (forming the dark 'maria' or seas) and on Mars.
Field identification & locations
Identify in the field by its dark color, fine-grained texture, and heavy weight relative to its size. It often exhibits 'vesicles' (small gas bubble holes). Commonly found in volcanic regions, oceanic islands (like Hawaii), and mid-ocean ridges.
