igneous
Porphyritic Basalt
Mafic extrusive igneous rock with porphyritic texture
Dark gray to black microcrystalline groundmass with light-colored plagioclase phenocryst crystals scattered throughout. Hardness 6-6.5 on the Mohs scale, dull to sub-vitreous luster, high specific gravity (approx. 2.9-3.1). Smooth, river-worn rounded cobble shape.
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Physical properties
Dark gray to black microcrystalline groundmass with light-colored plagioclase phenocryst crystals scattered throughout. Hardness 6-6.5 on the Mohs scale, dull to sub-vitreous luster, high specific gravity (approx. 2.9-3.1). Smooth, river-worn rounded cobble shape.
Formation & geological history
Formed from rapid cooling of mafic magma at or near the Earth's surface during volcanic eruptions, where larger plagioclase crystals cooled slowly at depth before extrusion. Rounded shape developed through water erosion in riverbeds or coastal environments.
Uses & applications
Used as crushed stone in construction, road base, asphalt aggregate, and landscape decor. River cobbles like this are popular in garden landscaping and decorative stonework.
Geological facts
The distinct pattern of small light specks (phenocrysts) floating in a dark matrix is called a porphyritic texture, often referred to as 'snowflake' or 'Chinese writing' pattern when phenocrysts are elongated.
Field identification & locations
Identified by the fine-grained dark gray groundmass with distinct small light-colored feldspar inclusions and smooth, water-rounded exterior. Commonly found along river beds, beaches, and volcanic terrains worldwide.
