Science

Types of Volcanic Rock & Igneous Formation

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Updated:3/7/2026
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Rock Type
Texture
Composition
How It Forms
Known For
Obsidian
Glassy (no crystals)Felsic (silica-rich, >70% SiO2)Rapid cooling of viscous lava, no time for crystalsNatural volcanic glass, razor-sharp edges used for ancient tools, obsidian scalpels sharper than steel, Apache tears
Pumice
Vesicular (extremely porous)Felsic to intermediateGas-rich explosive eruption, frothy lava solidifies mid-airOnly rock that floats on water, pumice rafts can cover ocean after eruptions, used for exfoliation and concrete
Basalt
Fine-grained (aphanitic)Mafic (45-52% SiO2)Rapid cooling of low-viscosity lava at surfaceMost common rock on Earth's surface, forms ocean floor, Giant's Causeway columns, Hawaiian islands are basalt
Granite
Coarse-grained (phaneritic)Felsic (quartz, feldspar, mica)Slow cooling deep underground (plutonic)Backbone of continents, kitchen countertops, Mount Rushmore carved in granite, extremely durable building stone
Rhyolite
Fine-grained (aphanitic)Felsic (same as granite but extrusive)Rapid cooling of silica-rich lava at surfaceGranite's volcanic equivalent, produces most explosive eruptions, Yellowstone caldera erupts rhyolite, flow-banded textures
Andesite
Fine-grained, sometimes porphyriticIntermediate (57-63% SiO2)Eruption at subduction zones (convergent boundaries)Named after the Andes mountains, dominant rock of volcanic arcs, Krakatoa and Mount Pinatubo erupted andesite
Gabbro
Coarse-grained (phaneritic)Mafic (same as basalt but intrusive)Slow cooling of mafic magma deep undergroundBasalt's plutonic twin, forms lower oceanic crust, marketed as 'black granite' for countertops, dense and dark
Diorite
Coarse-grained (phaneritic)Intermediate (plagioclase + hornblende)Slow cooling at intermediate depthsSalt-and-pepper appearance, Code of Hammurabi carved on diorite pillar, andesite's plutonic equivalent
Scoria
Vesicular (porous, dark)Mafic to intermediateGas-rich basaltic lava ejected from volcanic ventDark pumice-like rock but denser (sinks in water), cinder cones built from scoria, used as landscaping rock
Tuff
Pyroclastic (consolidated ash)Varies (depends on eruption)Volcanic ash and fragments cemented together after eruptionCappadocia's cave dwellings carved in tuff, Roman concrete used tuff, Moai of Easter Island carved from tuff
Pegmatite
Very coarse-grained (>2.5 cm crystals)Usually felsic (granitic)Extremely slow cooling of water-rich magma residueSource of giant crystals and rare minerals, lithium and beryllium mining, crystals can be meters long
Dacite
Fine-grained, often porphyriticIntermediate to felsic (63-69% SiO2)Eruption of intermediate-silica magma at subduction zonesMount St. Helens' 1980 eruption was dacite, forms lava domes, between andesite and rhyolite in composition
Peridotite
Coarse-grained (phaneritic)Ultramafic (olivine-rich, <45% SiO2)Crystallization deep in upper mantleMakes up Earth's upper mantle, source rock for diamonds (kimberlite is a type), rare at surface, weathers green
Porphyry
Porphyritic (large crystals in fine matrix)Varies (commonly intermediate)Two-stage cooling — slow then fastImperial Porphyry prized by Roman emperors, born in purple (porphyrogenitos), Egyptian quarries, striking purple-red
Ignimbrite
Welded pyroclastic (compacted)Usually felsic (rhyolitic ash)Pyroclastic flow deposits welded by extreme heatFormed by catastrophic eruptions, covers vast areas, welded at 500-600°C, Valley of Ten Thousand Smokes in Alaska

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