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Volcanic (Extrusive) Rock Types


Igneous rocks—those which originate from magma—fall naturally into two categories, extrusive and intrusive. Extrusive rocks are erupted from volcanoes or seafloor fissures, or they freeze at shallow depths. This means that they cool relatively quickly and under low pressures, therefore they are typically fine grained and gassy. The other category is intrusive rocks, which solidify slowly at depth and do not release gases.

Some of these rocks are clastic—composed of rock and mineral fragments, or clasts, rather than solidified melt. Technically that makes them sedimentary rocks, but these volcaniclastic rocks have many differences from other sedimentary rocks—in their chemistry and the role of heat, especially—and geologists tend to lump them with the igneous rocks. I'm being a little strict, then, and calling this whole gallery "volcanic rocks."

Learn more about igneous rocks

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Massive basaltMassive BasaltVesiculated and porphyriticVesiculated Basalt, HawaiiThe taffylike form of lavaPahoehoe LavaA typical chunkAndesite, Sutter Buttes, California
From the beach of St. VincentAndesite from La SoufrièreA banded red lavaRhyolite, Salton Sea Field, CaliforniaLike sugar in taffyRhyolite with Quartz PhenocrystsFrom the Napa ValleyObsidian
Hydrated lava glassPerliteWhere fire meets waterPeperite, ScotlandScoria, Cascade RangeScoria, Cascade RangeStone styrofoamPumice, Alaska
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