Igneous
Porphyritic Basalt or Andesite
Porphyritic Basalt/Andesite
Dark gray to brown groundmass with lighter rounded or subangular phenocrysts (likely plagioclase feldspar). Hardness is generally 5.5 to 6 on the Mohs scale. Dull luster, fine-grained groundmass with macroscopic crystals.
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Physical properties
Dark gray to brown groundmass with lighter rounded or subangular phenocrysts (likely plagioclase feldspar). Hardness is generally 5.5 to 6 on the Mohs scale. Dull luster, fine-grained groundmass with macroscopic crystals.
Formation & geological history
Formed from the cooling of magma or lava. The two distinct crystal sizes indicate a two-stage cooling process: a slow cooling deep within the earth forming the larger phenocrysts, followed by an eruption or rapid cooling forming the finer-grained matrix.
Uses & applications
Often used as aggregate in construction, road base, and sometimes landscaping.
Geological facts
The spotted appearance is characteristic of porphyritic rocks. This specific specimen appears to be water-worn, smoothed by tumbling in a river or ocean environment.
Field identification & locations
Identifiable by the distinct lighter spots (phenocrysts) in a dark, fine-grained matrix. Commonly found in volcanic regions or in riverbeds/beaches where volcanic rock has been transported and smoothed.
