igneous
Basalt with Plagioclase Porphyry
Porphyritic Basalt
Hardness: 6.0; Color: Dark grey to charcoal matrix with light grey lath-shaped crystals; Luster: Dull (matrix) to sub-vitreous (crystals); Structure: Aphanitic matrix with phenocrysts; Specific Gravity: 2.8 - 3.0.
- Hardness
- 6
- Luster
- Dull (matrix) to sub-vitreous (crystals)
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Physical properties
Hardness: 6.0; Color: Dark grey to charcoal matrix with light grey lath-shaped crystals; Luster: Dull (matrix) to sub-vitreous (crystals); Structure: Aphanitic matrix with phenocrysts; Specific Gravity: 2.8 - 3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava. The larger crystals (phenocrysts) formed slowly underground before the lava was erupted and the rest of the rock cooled quickly. Likely mid-Proterozoic in age, transported southward by glaciers.
Uses & applications
Used in construction as crushed stone, road base, and as decorative landscaping stones. Also used as a study tool for volcanic cooling rates.
Geological facts
This specific texture—light crystals in dark rock—is sometimes colloquially called 'snowflake' or 'flower' basalt. Finding this in Michigan (42.1 N) is common due to glacial transport from the Lake Superior volcanic basalt flows and the Midcontinent Rift System.
Field identification & locations
Look for a dark, dense matrix with distinct, rectangular or lath-shaped lighter mineral inclusions. Commonly found as rounded cobbles on beaches and in glacial till throughout the Great Lakes region.
