Igneous
Basalt with Plagioclase Phenocrysts
Porphyritic Basalt (primarily SiO2, MgO, FeO, and CaO)
Hardness: 6-7 on Mohs scale; Color: Dark grey to black matrix with light white/tan inclusions; Luster: Dull to sub-vitreous; Structure: Aphanitic (fine-grained) matrix with visible phenocrysts; Cleavage: None (conchoidal fracture).
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to black matrix with light white/tan inclusions
- Luster
- Dull to sub-vitreous
Identified More igneous →
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black matrix with light white/tan inclusions; Luster: Dull to sub-vitreous; Structure: Aphanitic (fine-grained) matrix with visible phenocrysts; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed through the rapid cooling of magnesium-rich and iron-rich lava at or very near the Earth's surface. The larger white crystals (phenocrysts) grew slowly at depth before the final eruption. Common throughout the Cenozoic era to the present.
Uses & applications
Used primarily as crushed stone for aggregate in construction, road base, and railroad ballast. Larger decorative specimens are used in landscaping and aquascaping.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. Porphyritic varieties like this one provide a 'snapshot' of a volcano's plumbing system by showing minerals that crystallized at different temperatures.
Field identification & locations
Identify in the field by its high density (heavy for its size) and the contrast between the dark, fine-grained background and larger white feldspar crystals. Frequently found in volcanic fields, riverbeds draining volcanic highlands, and glacial till.
