igneous
Scoria (Lava Rock)
Scoria (Extrusive Igneous Rock / Mafic Silicates)
Hardness: 5-6 Mohs. Color: Dark brown, black, or reddish-brown. Luster: Dull/Earthy. Structure: Vesicular (highly porous with gas bubbles). Cleavage: None. Specific Gravity: Generally less than 2.0 (often feels very light but does not float like pumice).
- Hardness
- 5-6 Mohs
- Color
- Dark brown, black, or reddish-brown
- Luster
- Dull/Earthy
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Physical properties
Hardness: 5-6 Mohs. Color: Dark brown, black, or reddish-brown. Luster: Dull/Earthy. Structure: Vesicular (highly porous with gas bubbles). Cleavage: None. Specific Gravity: Generally less than 2.0 (often feels very light but does not float like pumice).
Formation & geological history
Formed from the cooling of gas-rich mafic magma (basaltic or andesitic). As magma is ejected during explosive volcanic eruptions, the sudden drop in pressure causes dissolved gases to expand into bubbles, which are trapped as the lava solidifies.
Uses & applications
Commonly used in landscaping (volcanic mulch), high-temperature insulation, gas grills (lava rocks), lightweight concrete aggregate, and as a drainage material in horticulture.
Geological facts
Unlike pumice, scoria usually has a basaltic composition and larger vesicle (bubble) walls, which makes it denser than water, so it typically sinks. It is a signature product of cinder cone volcanoes.
Field identification & locations
Identify in the field by its sponge-like appearance and sharp, abrasive edges. Commonly found in volcanic fields such as those in Iceland, Italy, the Canary Islands, and the Western United States (Arizona, New Mexico).
