igneous
Vesicular basalt (likely amygdaloidal basalt)
Basalt, a fine-grained volcanic rock composed mainly of plagioclase and pyroxene; pale fillings may be secondary minerals such as calcite or zeolites.
AI-generated identification · may be incorrectThe specimen appears pale gray-green with numerous small cavities and dark pits. Basalt is typically hard (about 6 on Mohs), dull to weakly lustrous, and fine-grained; the photo does not resolve its mineral crystals or confirm the cavity fillings.
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Physical properties
The specimen appears pale gray-green with numerous small cavities and dark pits. Basalt is typically hard (about 6 on Mohs), dull to weakly lustrous, and fine-grained; the photo does not resolve its mineral crystals or confirm the cavity fillings.
Formation & geological history
Formed when basaltic lava cooled rapidly at or near the surface; gas bubbles left vesicles, sometimes later filled by minerals. Northern Michigan includes ancient volcanic rocks, though this small specimen cannot be dated from appearance alone.
Uses & applications
Basalt is widely used as crushed aggregate, paving stone, and building stone. Specimens with attractive mineral-filled cavities may be kept for collecting or lapidary work.
Geological facts
Vesicles record gas escaping from lava; mineral-filled vesicles are called amygdules, and the rock is then termed amygdaloidal basalt.
Field identification & locations
The porous, fine-grained appearance is consistent with vesicular basalt, but identification is tentative from the image. Check for a hard, fine-grained matrix and distinguish natural cavities from pores in concrete; basalt is common in volcanic regions, including parts of the Lake Superior region.
