igneous
Porphyritic Basalt
Porphyritic Basalt (containing Plagioclase or Alkali Feldspar phenocrysts)
Hardness: 6-7 (Mohs scale); Color: Dark gray to black groundmass with reddish-pink or white crystals; Luster: Dull/earthy matrix with vitreous crystals; Structure: Porphyritic (fine-grained matrix with larger visible crystals); Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 (Mohs scale)
- Luster
- Dull/earthy matrix with vitreous crystals
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark gray to black groundmass with reddish-pink or white crystals; Luster: Dull/earthy matrix with vitreous crystals; Structure: Porphyritic (fine-grained matrix with larger visible crystals); Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava. The large pinkish crystals (phenocrysts) grew slowly deep underground before the lava erupted and the rest of the rock cooled quickly on the surface. Common in volcanic regions and oceanic crust.
Uses & applications
Used in construction as crushed stone, road base, and concrete aggregate. Attractive specimens are used as decorative garden stones or for geological education.
Geological facts
Basalt is the most common rock type in the Earth's crust, making up most of the ocean floors. Porphyritic textures like this serve as a 'natural timeline' of a volcano's cooling history.
Field identification & locations
Identify by looking for a dark, dense, fine-grained background containing large, distinct rectangular or blocky crystals. Commonly found in basaltic lava flows and volcanic fields (e.g., Pacific Northwest, Hawaii, or Lake Superior region).
