Review classification of igneous rocks based on texture and composition.
Eruptions occur from central orifices or from lengthy fissures .
Former are central orifice type volcanoes, latter on land are plateaux, and under the oceans form oceanic crust. Basalt plateaux are major topographic features ie. San Juan plateau in Colorado is 4,000 km 3 . Mogollon-Datil volcanic field is 40,000 km 2 . Generally low viscosity in hot basalt lava flows, high in rhyolite lavas.