igneous
Porphyritic Basalt
Porphyritic Basalt (Mafic Igneous Rock)
Hardness: 6-7 on Mohs scale; Color: Dark grey to black matrix with lighter cream or white phenocrysts; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic matrix with visible large crystals (phenocrysts); Cleavage: Poor.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to sub-metallic
Identified More igneous →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black matrix with lighter cream or white phenocrysts; Luster: Dull to sub-metallic; Crystal Structure: Aphanitic matrix with visible large crystals (phenocrysts); Cleavage: Poor.
Formation & geological history
Formed from the cooling of magnesium-rich and iron-rich lava. The large crystals (phenocrysts) formed slowly underground before the remaining magma erupted and cooled quickly on the surface, creating the fine-grained matrix.
Uses & applications
Used primarily as crushed stone for construction, road base, and railroad ballast. Exceptionally durable specimens are used in landscaping or as decorative stone.
Geological facts
The term 'porphyry' comes from the Ancient Greek word for purple, as the first specimens described were deep purple rocks found in Egypt. Basalt is the most common rock type in the Earth's crust, making up most of the ocean floor.
Field identification & locations
Identified by the 'chocolate chip cookie' appearance of large, light-colored crystals embedded in a dark, fine-grained rock. Commonly found in volcanic fields, oceanic islands, and mid-ocean ridges.
