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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 crystals | Natural volcanic glass, razor-sharp edges used for ancient tools, obsidian scalpels sharper than steel, Apache tears |
Pumice | Vesicular (extremely porous) | Felsic to intermediate | Gas-rich explosive eruption, frothy lava solidifies mid-air | Only 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 surface | Most 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 surface | Granite's volcanic equivalent, produces most explosive eruptions, Yellowstone caldera erupts rhyolite, flow-banded textures |
Andesite | Fine-grained, sometimes porphyritic | Intermediate (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 underground | Basalt'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 depths | Salt-and-pepper appearance, Code of Hammurabi carved on diorite pillar, andesite's plutonic equivalent |
Scoria | Vesicular (porous, dark) | Mafic to intermediate | Gas-rich basaltic lava ejected from volcanic vent | Dark 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 eruption | Cappadocia'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 residue | Source of giant crystals and rare minerals, lithium and beryllium mining, crystals can be meters long |
Dacite | Fine-grained, often porphyritic | Intermediate to felsic (63-69% SiO2) | Eruption of intermediate-silica magma at subduction zones | Mount 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 mantle | Makes 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 fast | Imperial 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 heat | Formed by catastrophic eruptions, covers vast areas, welded at 500-600°C, Valley of Ten Thousand Smokes in Alaska |
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