igneous
Porphyritic Basalt
Porphyritic Mafic Extrusive Rock
Hardness: 6 (Mohs); Color: Dark grey to black groundmass with light-colored phenocrysts; Luster: Dull (groundmass) to vitreous (crystals); Structure: Fine-grained aphanitic matrix with visible crystals (porphyritic); Specific gravity: 2.8 - 3.0
- Hardness
- 6 (Mohs)
- Color
- Dark grey to black groundmass with light-colored phenocrysts
- Luster
- Dull (groundmass) to vitreous (crystals)
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Physical properties
Hardness: 6 (Mohs); Color: Dark grey to black groundmass with light-colored phenocrysts; Luster: Dull (groundmass) to vitreous (crystals); Structure: Fine-grained aphanitic matrix with visible crystals (porphyritic); Specific gravity: 2.8 - 3.0
Formation & geological history
Formed from the rapid cooling of basaltic lava on the Earth's surface. The large crystals (phenocrysts) formed slowly during an initial underground phase before being erupted and encased in a fine-grained matrix.
Uses & applications
Commonly used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Exceptionally durable specimens are used for pavers and tiles.
Geological facts
The word 'basalt' is derived from late Latin 'basaltes', a misspelling of 'basanites' meaning 'very hard stone'. This texture indicates a two-stage cooling process for the magma.
Field identification & locations
Identify by looking for a very dark, fine-grained background containing scattered, distinct, larger crystals (often plagioclase feldspar or olivine). Found in volcanic regions like Hawaii, Iceland, and the Deccan Traps.
