Vesicular Basalt (or Scoria)

Physical Properties

Hardness: 5-6 (Mohs); Color: Dark grey, reddish-brown, or black; Luster: Dull to sub-metallic; Structure: Aphanitic with vesicles (gas bubbles); Specific Gravity: 2.8–3.0

Formation & Geological History

Formed from the rapid cooling of magnesium-rich and iron-rich lava at the Earth's surface. Extrusive volcanic environment. The holes (vesicles) are created by trapped gas or steam escaping as the lava solidifies.

Uses & Applications

Used as aggregate in construction, landscaping, road base, and as decorative stone in gardens or aquariums. Occasionally used for pumice-like abrasive tasks if highly porous.

Geological Facts

The vesicles in this rock are essentially frozen snapshots of gas trying to escape a volcanic eruption million of years ago. On the ocean floor, basalt is the most common rock type, forming the entirety of the oceanic crust.

Field Identification & Locations

Identify in the field by its pitted 'Swiss cheese' appearance, heavy weight compared to pumice, and dark color. Commonly found in volcanic fields, mid-ocean ridges, and island arcs.

Identified on: 5/1/2026

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