igneous
Basalt with Plagioclase Phenocrysts
Basalt
Dark colored, fine-grained groundmass with visible light-colored rectangular crystals (plagioclase feldspar); hardness 5-6 (Mohs); dull to slightly shiny luster due to polishing.
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Physical properties
Dark colored, fine-grained groundmass with visible light-colored rectangular crystals (plagioclase feldspar); hardness 5-6 (Mohs); dull to slightly shiny luster due to polishing.
Formation & geological history
Formed from the rapid cooling of basaltic lava at or near the Earth's surface. The larger plagioclase crystals (phenocrysts) formed earlier while the magma was cooling slowly underground before eruption.
Uses & applications
Commonly used in construction, road base, and as aggregate. Tumbled stones like this are collected or used for decorative purposes.
Geological facts
Basalt is the most common volcanic rock type on Earth and forms the oceanic crust. The porphyritic texture (large crystals in a fine matrix) indicates a two-stage cooling history.
Field identification & locations
Identify by its dark color, high density, and the presence of light-colored, lath-shaped feldspar crystals against the dark background. Often found in areas of past or present volcanic activity.
