Igneous rocks—those which originate from a hot, fluid state—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."
- Massive BasaltMassive basalt
- Vesiculated Basalt, HawaiiVesiculated and porphyritic
- Pahoehoe LavaThe taffylike form of lava
- Andesite, Sutter Buttes, CaliforniaA typical chunk
- Andesite from La SoufrièreFrom the beach of St. Vincent
- Rhyolite, Salton Sea Field, CaliforniaA banded red lava
- Rhyolite with Quartz PhenocrystsLike sugar in taffy
- ObsidianFrom the Napa Valley
- PerliteHydrated lava glass
- Scoria, Cascade RangeScoria, Cascade Range
- Scoria, AlaskaStone styrofoam
- Scoria CloseupLook at those pores
- Graphic Index
- Text Index
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