igneous
Porphyritic Basalt
Porphyritic Mafic Volcanic Rock
Hardness: 6-7 on Mohs scale; Color: dark grey to black matrix with reddish-pink phenocrysts (likely feldspar or altered olivine); Luster: dull to earthy; Crystal structure: Aphanitic matrix with visible large crystals; Specific Gravity: 2.8-3.0.
- Hardness
- 6-7 on Mohs scale
- Luster
- dull to earthy
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Physical properties
Hardness: 6-7 on Mohs scale; Color: dark grey to black matrix with reddish-pink phenocrysts (likely feldspar or altered olivine); Luster: dull to earthy; Crystal structure: Aphanitic matrix with visible large crystals; Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from the cooling of magnesium-rich and iron-rich lava. The large crystals (phenocrysts) formed slowly underground before the remaining lava erupted and cooled quickly on the surface. Commonly found in volcanic arcs and oceanic hotspots.
Uses & applications
Used in construction as crushed stone for road base, concrete aggregate, and railroad ballast. Attractive specimens are used as garden stones or for geological study.
Geological facts
The term 'porphyritic' comes from the Greek word for purple. This texture indicates a two-stage cooling history of the magma. Basalt is the most common rock in the Earth's crust, covering most of the ocean floor.
Field identification & locations
Identify in the field by looking for a fine-grained, dark-colored rock (aphanitic) containing distinct, larger individual crystals. Often found in areas with historic volcanic activity like the Pacific Northwest or Hawaii.
