igneous
Porphyritic Basalt or Porphyritic Andesite
Porphyritic Extrusive Igneous Rock
Hardness: 6-7 on Mohs scale (due to mineral phenocrysts); Color: Dark brown to black groundmass with light-colored (pink/white/grey) rounded or angular phenocrysts (often plagioclase feldspar or quartz); Luster: Dull to sub-vitreous; Texture: Porphyritic, well-rounded and smoothed by wave action.
- Hardness
- 6-7 on Mohs scale (due to mineral phenocrysts)
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale (due to mineral phenocrysts); Color: Dark brown to black groundmass with light-colored (pink/white/grey) rounded or angular phenocrysts (often plagioclase feldspar or quartz); Luster: Dull to sub-vitreous; Texture: Porphyritic, well-rounded and smoothed by wave action.
Formation & geological history
Formed from two-stage cooling of magma. Initially, crystals formed slowly deep underground, followed by a rapid cooling at or near the surface when the magma erupted as lava, trapping larger crystals (phenocrysts) within a finer-grained groundmass.
Uses & applications
Used as aggregate in construction, road metal, landscaping stone, and collected as an interesting geological specimen by beachcombers.
Geological facts
The spotted appearance is caused by distinct phenocrysts that crystallized earlier than the surrounding matrix. When found on beaches, these rocks are often polished smooth by the constant tumbling action of sand and water.
Field identification & locations
Easily identified by its contrasting texture—light spots embedded in a darker background. Commonly found in riverbeds, glacial tills, and beaches where volcanic rocks have been transported and weathered.
