igneous
Basalt with Plagioclase Phenocryst
Porphyritic Basalt
Hardness: 6 (Mohs); Color: Dark grey to black matrix with white/grey crystals; Luster: Dull to vitreous; Structure: Fine-grained/aphanitic with larger crystals (phenocrysts); Cleavage: None; Specific Gravity: 2.8-3.0
- Hardness
- 6 (Mohs)
- Color
- Dark grey to black matrix with white/grey crystals
- Luster
- Dull to vitreous
Identified More igneous →
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Physical properties
Hardness: 6 (Mohs); Color: Dark grey to black matrix with white/grey crystals; Luster: Dull to vitreous; Structure: Fine-grained/aphanitic with larger crystals (phenocrysts); Cleavage: None; 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. The large internal crystal suggests a two-stage cooling process where the crystal formed slowly underground before being erupted.
Uses & applications
Used in construction for road base, concrete aggregate, and railroad ballast. Porphyritic varieties are often collected as geological curiosities or used as landscape stones.
Geological facts
Basalt is the most common rock type in the Earth's crust and covers most of the ocean floor. Porphyritic basalt provides a 'time capsule' look at the magma's journey to the surface.
Field identification & locations
Identify by its heavy weight, dark color, and fine texture. Look for the presence of distinct, visible crystals (phenocrysts) against a dark, featureless background. Found globally in volcanic regions.
