Igneous Rock Gallery
Extrusive igneous rocks
Volcanic rocks are extrusive, which means that they form when the volcano erupts. Extrusive rocks are usually formed from hot lava that cooled quickly when it reached the surface, or from ash that has been ejected explosively. Lava that cools very fast will be glassy, like obsidian.
obsidian basalt (a'a) basalt (pahoehoe)
pumice tuff volcanic ash (Mount St Helen's)
scoria ashflow tuff volcanic ash (Lassen Peak)
rhyolite rhyolitic tuff pitchstone
dacite andesite trachyte
green slate
Intrusive igneous rocks
Magma can solidify underground without reaching the surface. Rocks formed from magma that cooled slowly underground are called intrusive igneous rocks. They normally have crystals that are big enough to be seen because they have had time to grow during the slow cooling process.
granite gabbro peridotite
porphyry microgabbro (dolerite) diorite
aplite appinitic diorite dunite
granodiorite 'horsetooth' porphyritic granite aplite in quartz dolerite
anorthosite
-> Metamorphic Rock Gallery

-> Sedimentary Rock Gallery
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