igneous
Basalt with Weathered Rind
Basalt (Aphanitic Mafic Igneous Rock)
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal black (interior), tan/brown (weathered exterior); Luster: Dull to sub-metallic; Crystal Structure: Microcrystalline/Aphanitic; Cleavage: None (conchoidal fracture); Specific Gravity: 2.8 - 3.0
- Hardness
- 6 (Mohs scale)
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal black (interior), tan/brown (weathered exterior); Luster: Dull to sub-metallic; Crystal Structure: Microcrystalline/Aphanitic; Cleavage: None (conchoidal fracture); 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. Basalt is the most common volcanic rock on Earth and forms much of the ocean floor and large plateau regions like the Deccan Traps.
Uses & applications
Extensively used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. In finer forms, it is used for floor tiles, monuments, and 'hot stone' massage therapy due to its high thermal mass.
Geological facts
Basalt is not just Earth-bound; it is the most common rock on the Moon's surface, forming the lunar maria (the dark plains seen from Earth). It also covers much of the surface of Mars and Venus.
Field identification & locations
Identify in the field by its heavy weight (density), dark interior, and fine-grained texture that lacks visible crystals to the naked eye. Often found in volcanic regions or as 'river rocks' transported by water. Use a magnet to test for high iron content; some basalt specimens are slightly magnetic.
