Rhyolite
Silica-rich (felsic) volcanic rock, SiO2 >69%
A fine-grained, silica-rich volcanic rock that is the extrusive equivalent of granite, often pale, banded, or flow-textured.
- Mohs hardness
- 6-7
- Color
- pale grey, pink, tan, cream, often mottled or banded; green when altered
- Type
- igneous
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Overview
Rhyolite is a fine-grained, silica-rich (felsic) volcanic rock, the extrusive equivalent of granite. It forms from highly viscous, gas-rich lava that cools too quickly for large crystals to grow, giving it a fine groundmass that may enclose visible phenocrysts of quartz and feldspar.
Colors range from pale grey, pink, and tan to mottled and banded varieties. Flow banding, spherulites, and gas cavities (later filled to form agates or thundereggs) are common. Decorative types are sold under names like rainforest jasper and leopardite.
Its high silica content makes rhyolitic eruptions among the most explosive on Earth.
Formation & geology
Rhyolite forms when felsic magma, rich in silica and dissolved gases, erupts at the surface and cools rapidly. Because such magma is highly viscous, it traps gas and often erupts explosively, producing ash, pumice, and lava flows that solidify into fine-grained rhyolite.
The rapid cooling prevents large crystal growth, though earlier-formed quartz and feldspar phenocrysts may be present. Flow banding records the movement of stiff lava. Rhyolite is associated with continental volcanic provinces, calderas, and lava domes, found in regions such as Yellowstone, Iceland, and the western United States.
How to identify it
Identify rhyolite by its fine-grained, often pale (grey, pink, tan) groundmass, sometimes with visible quartz and feldspar phenocrysts and flow banding. Overall hardness is about 6-7 due to its quartz and feldspar content.
It is harder and lighter colored than basalt, which is dark and rich in iron and magnesium. Unlike granite, its crystals are too fine to distinguish without magnification.
Look for banding, spherulites, or gas cavities. Obsidian is the glassy, non-crystalline equivalent; rhyolite has a dull, stony, microcrystalline texture rather than glassy luster.
Uses & significance
Rhyolite is used as crushed stone for construction aggregate, road base, and decorative landscaping. Attractive banded and patterned varieties are cut and polished as ornamental stone and lapidary material, including rainforest jasper cabochons.
Historically, fine-grained rhyolite and related felsic rocks were knapped into tools and arrowheads. Rhyolitic cavities also host prized agates and thundereggs sought by collectors.
Metaphysically rhyolite is associated with growth and resolution, though these claims are not scientific.
Frequently asked questions
Is rhyolite the same as granite?
They share a similar felsic composition, but rhyolite is fine-grained volcanic rock while granite is coarse-grained and cooled slowly underground.
Why are rhyolite eruptions explosive?
Its high silica content makes the magma very viscous, trapping gases that release violently, producing explosive eruptions of ash and pumice.
Is rainforest jasper actually rhyolite?
Yes. Rainforest jasper is a green, patterned variety of rhyolite from Australia, marketed under a jasper trade name.
How do I tell rhyolite from basalt?
Rhyolite is light-colored and silica-rich, while basalt is dark and rich in iron and magnesium.
Rhyolite guides
In-depth guides for identifying, valuing, and understanding Rhyolite.
Rhyolite identified by the community
Real specimens identified with Rock Identifier.
