igneous
Porphyritic Basalt
Porphyritic Basalt
Hardness: 6-7 (Mohs scale); Color: Dark grey to black groundmass with light-colored phenocrysts; Luster: Dull to sub-vitreous; Structure: Aphanitic groundmass with macroscopic crystals; Specific Gravity: 2.8-3.0
- Hardness
- 6-7 (Mohs scale)
- Color
- Dark grey to black groundmass with light-colored phenocrysts
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark grey to black groundmass with light-colored phenocrysts; Luster: Dull to sub-vitreous; Structure: Aphanitic groundmass with macroscopic crystals; Specific Gravity: 2.8-3.0
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The larger crystals (phenocrysts) formed slowly underground before the magma was erupted, while the fine-grained matrix cooled quickly after eruption.
Uses & applications
Used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Exceptionally hard specimens are used for floor tiles or decorative landscaping.
Geological facts
The term 'porphyritic' refers to the distinct size difference between the larger crystals and the fine-grained surrounding rock. Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor.
Field identification & locations
Identify by looking for a very dark, heavy, fine-grained rock containing distinct larger, lighter-colored grains of plagioclase feldspar or olivine. Found globally in volcanic regions.
