igneous
Porphyritic Basalt
Porphyritic Aphanitic Basalt
AI-generated identification · may be incorrectHardness: 6 (Mohs scale); Color: Dark grey to black groundmass with light-colored (plagioclase) phenocrysts; Luster: Dull/earthy; Crystal Structure: Aphanitic matrix with visible crystals; Specific Gravity: 2.8-3.0
- Hardness
- 6 (Mohs scale)
- Luster
- Dull/earthy
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to black groundmass with light-colored (plagioclase) phenocrysts; Luster: Dull/earthy; Crystal Structure: Aphanitic matrix with visible crystals; Specific Gravity: 2.8-3.0
Formation & geological history
Formed from the two-stage cooling of mafic lava. The larger crystals (phenocrysts) formed slowly underground, followed by a rapid surface eruption that cooled the remaining liquid into a fine-grained matrix.
Uses & applications
Used primarily in construction for road base, concrete aggregate, and railroad ballast. Porphyritic varieties are sometimes used as decorative stone in landscaping.
Geological facts
The two distinct crystal sizes are a record of a change in cooling speed during the rock's formation. Large crystals indicate slow cooling, while the fine background indicates rapid cooling.
Field identification & locations
Identify by looking for a very dark, fine-grained background containing noticeably larger, distinct crystals. Commonly found in volcanic regions like the Pacific Northwest, Hawaii, and Iceland.
