igneous
Pumice
Pumice (Volcanic glass / Felsic extrusive rock)
Hardness: 6 on the Mohs scale (for the glass framework, though the rock overall is very lightweight and fragile), Color: Light gray, white, yellowish, or cream, Luster: Dull to earthy, Crystal structure: Amorphous (glassy with numerous gas cavities/vesicles), Specific gravity: Less than 1.0 (it floats on water).
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Physical properties
Hardness: 6 on the Mohs scale (for the glass framework, though the rock overall is very lightweight and fragile), Color: Light gray, white, yellowish, or cream, Luster: Dull to earthy, Crystal structure: Amorphous (glassy with numerous gas cavities/vesicles), Specific gravity: Less than 1.0 (it floats on water).
Formation & geological history
Formed from explosive volcanic eruptions involving felsic, gas-rich magma. As the magma is ejected into the atmosphere, pressure drops rapidly, causing dissolved gases to exsolve and form bubbles that are trapped as the lava cools quickly into volcanic glass.
Uses & applications
Used in lightweight concrete, landscaping, abrasive materials (such as pumice stones for skin care, erasers, and polishing compounds), and horticulture as a soil amendment.
Geological facts
Pumice is one of the only rocks that can float on water due to its high porosity and trapped air bubbles. Large underwater volcanic eruptions can create pumice rafts that drift across oceans for months.
Field identification & locations
Easily identified by its extremely low density, light color, and abundance of small vesicles (holes). Commonly found near active or historical volcanic regions around the world, particularly subduction zones and island arcs.
