igneous
Porphyritic Basalt or Porphyritic Andesite Pebble
Extrusive Igneous Rock
Hardness typically 6 on the Mohs scale; dark grey to brownish-black matrix with lighter colored, distinct plagioclase feldspar phenocrysts (spots); fine-grained to aphanitic groundmass with crystalline inclusions; dull to sub-vitreous luster; specific gravity around 2.7-3.0.
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Physical properties
Hardness typically 6 on the Mohs scale; dark grey to brownish-black matrix with lighter colored, distinct plagioclase feldspar phenocrysts (spots); fine-grained to aphanitic groundmass with crystalline inclusions; dull to sub-vitreous luster; specific gravity around 2.7-3.0.
Formation & geological history
Formed from cooling lava flows where a two-stage cooling process occurred: initial slow cooling deep underground allowed larger crystals (phenocrysts) to form, followed by rapid cooling at or near the Earth's surface which created the fine-grained matrix.
Uses & applications
Used primarily in construction, road aggregate, and landscaping as crushed stone, or collected as a natural beach pebble.
Geological facts
Porphyritic textures are common in volcanic rocks, telling a story of two different cooling rates in the magma's history before it erupted onto the surface.
Field identification & locations
Found commonly in glacial drift, riverbeds, and beaches where rocks have been weathered and rounded by water action. Identified by its contrasting lighter spots embedded within a dark volcanic background.
