Mississippi Valley Type (MVT)
Dacite
Dacite is a light-colored, fine-grained (aphanitic) to porphyritic extrusive (volcanic) igneous rock. It occupies the compositional middle ground between the intermediate andesite and the...
Rhyolite
Rhyolite is a light-colored, fine-grained (aphanitic), felsic extrusive (volcanic) igneous rock. If basalt is the dark, fast-flowing lava of the ocean floor, rhyolite is...
Diorite
Diorite is a coarse-grained (phaneritic), intermediate intrusive (plutonic) igneous rock. It represents the perfect petrological middle ground between the light-colored, silica-rich granites of the...
Obsidian
Obsidian is an extrusive igneous rock, but with a fascinating twist: it is a naturally occurring volcanic glass. Unlike gabbro or basalt, which are...
Basalt
Basalt rock is a dark-colored, fine-grained (aphanitic), and mafic extrusive (volcanic) igneous rock. If gabbro is the patient, slowly-cooling giant of the deep crust,...