igneous
Porphyritic Basalt (with Pyrolusite or Iron-oxide staining)
Porphyritic Basalt (Mafic Extrusive)
Hardness: 5-6 Mohs. Color: Reddish-brown (oxidized) matrix with dark grey/black phenocrysts. Luster: Dull to sub-metallic on inclusions. Crystal structure: Result of rapid cooling. Cleavage: Indistinct.
- Hardness
- 5-6 Mohs
- Luster
- Dull to sub-metallic on inclusions
Identified More igneous →
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Physical properties
Hardness: 5-6 Mohs. Color: Reddish-brown (oxidized) matrix with dark grey/black phenocrysts. Luster: Dull to sub-metallic on inclusions. Crystal structure: Result of rapid cooling. Cleavage: Indistinct.
Formation & geological history
Formed from the rapid cooling of magnesium and iron-rich lava at or near the Earth's surface. The distinct 'spots' (phenocrysts) indicate two stages of cooling: a slower stage underground followed by rapid eruption.
Uses & applications
Common basalt is used in road construction, concrete aggregate, and as railroad ballast. This specific weathered specimen is primarily a collector's item or educational sample.
Geological facts
The reddish color is due to the oxidation of iron-rich minerals (chemical weathering or 'rusting' of the rock). Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor.
Field identification & locations
Identify in the field by its heavy weight (high density), pitted/vesicular texture, and the presence of larger crystals embedded in a fine-grained groundmass. Look for it in volcanic fields or riverbeds near volcanic activity.
