igneous
Basalt with Phenocrysts (Porphyritic Basalt)
Porphyritic Basalt (primarily Plagioclase and Pyroxene)
Hardness: 6-7 on Mohs scale; Color: Dark grey to black matrix with light-colored inclusions; Luster: Dull to sub-vitreous; Structure: Aphanitic (fine-grained) groundmass with visible crystals (phenocrysts); Cleavage: None (conchoidal fracture); Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to black matrix with light-colored inclusions
- Luster
- Dull to sub-vitreous
Identified More igneous →
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black matrix with light-colored inclusions; Luster: Dull to sub-vitreous; Structure: Aphanitic (fine-grained) groundmass with visible crystals (phenocrysts); Cleavage: None (conchoidal fracture); Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from the cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The distinct 'spots' or phenocrysts formed slowly underground before a final eruption caused the remaining lava to cool quickly. Common in volcanic regions and ocean floors.
Uses & applications
Used primarily in construction for road base, concrete aggregate, and railroad ballast. Larger porphyritic specimens are often used for decorative landscaping or architectural facings.
Geological facts
Basalt is the most common rock type in the Earth's crust, making up most of the ocean floor. Porphyritic varieties like this one provide a visual record of a 'two-stage' cooling process in a volcano's history.
Field identification & locations
Identify in the field by its dark, dense matrix and small, embedded light-colored crystals (usually feldspar). This specimen appears water-worn, likely found in a riverbed or on a beach. Look for it in areas with high past or present volcanic activity like the Pacific Northwest or Iceland.
