Rhyolite

Physical Properties

Hardness: 6-7 (Mohs); Color: Light gray, buff, or pinkish; Luster: Dull or earthy; Crystal structure: Aphanitic (fine-grained) to porphyritic; Cleavage: None; Specific Gravity: 2.4-2.6.

Formation & Geological History

Formed through the rapid cooling of silica-rich (felsic) lava at or near the Earth's surface. It is the volcanic equivalent of granite. It can be found in volcanic arcs and continental rifting zones.

Uses & Applications

Used primarily as crushed stone or aggregate in road construction and concrete when more durable options like basalt are not available. Historically used by indigenous peoples for stone tools due to its conchoidal fracturing.

Geological Facts

Rhyolite has the same chemical composition as granite but cooled much faster. Pumice and Obsidian are also rhyolitic in composition but differ in texture due to gas content and cooling rates.

Field Identification & Locations

Identify by its light color and fine-grained texture. It often shows flow banding (visible layers) from when the viscous lava moved. It won't fizz with acid like limestone and is harder than most sedimentary rocks.

Identified on: 4/29/2026

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