Pumice

Physical Properties

Color: light gray to tan; Hardness: 5-6 Mohs (though fragile); Luster: dull to pearly; Structure: highly vesicular (bubbly) and glass-rich; Density: very low, often floats in water.

Formation & Geological History

Formed during explosive volcanic eruptions when pressurized, gas-rich lava is ejected and quickly cools, trapping gas bubbles in a frothy glass matrix.

Uses & Applications

Used in construction for lightweight concrete, as an abrasive in polishing and personal care (exfoliation), and in horticulture to improve soil aeration.

Geological Facts

Pumice is the only rock that can float on water due to its air-filled pores. After massive eruptions, 'pumice rafts' can form on the ocean and float for miles.

Field Identification & Locations

Identify by its light weight, porous sponge-like appearance, and ability to float. Commonly found in areas with past or present explosive volcanic activity like the Cascade Range or the Mediterranean.

Identified on: 5/1/2026

Mode: Standard