igneous
Porphyritic Basalt
Basalt with plagioclase phenocrysts
Hardness: 6.0; Color: Dark grey to black groundmass with prominent white crystals; Luster: Dull (matrix) to vitreous (crystals); Structure: Fine-grained groundmass with large visible crystals (phenocrysts).
- Hardness
- 6
- Color
- Dark grey to black groundmass with prominent white crystals
- Luster
- Dull (matrix) to vitreous (crystals)
Identified More igneous →
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Physical properties
Hardness: 6.0; Color: Dark grey to black groundmass with prominent white crystals; Luster: Dull (matrix) to vitreous (crystals); Structure: Fine-grained groundmass with large visible crystals (phenocrysts).
Formation & geological history
Formed from cooling lava where different cooling rates occurred. The large white crystals grew slowly underground before the remaining lava erupted and cooled quickly on the surface.
Uses & applications
Used primarily in construction for road base, concrete aggregate, and railroad ballast. Exceptionally aesthetic specimens are used as decorative garden stones or paperweights.
Geological facts
The two-stage cooling process that creates this 'chocolate chip' look is known as a porphyritic texture. It is a common volcanic rock type found in the oceanic crust.
Field identification & locations
Identified by large, light-colored crystals (typically plagioclase feldspar) embedded in a very fine-grained, dark volcanic matrix. Common in volcanic regions such as the Pacific Northwest or Iceland.
