Classification of Igneous Rocks

Igneous Rock FormationIntrusive rock - form when magma cools and crystallizes below Earth’s surface

Also called plutonic rocks

Cools slowly - forms coarse grained rocks

Ex: granite

granitemagma

Extrusive rock - form when magma cools and crystallizes on Earth’s surface

Also called volcanic rocks

lava cools quickly - forms fine grained rocks

Ex: scoria

scoria

Igneous Rock Formation

Intrusive & Extrusive

Mineral CompositionBasaltic rocks (mafic) - dark-colored, lower silica contents, and contain mostly plagioclase and pyroxene

gabbro

Granitic rocks (felsic) - light-colored, high silica contents, and contain mostly quartz and feldspar

Mineral Composition

granite

Mineral CompositionIntermediate rocks - have a composition somewhere in between basaltic and granitic, and consist mostly of plagioclase feldspar and hornblende

diorite

Mineral CompositionUltrabasic (ultramafic) - rocks contain only iron-rich minerals such as olivine and pyroxene and are always dark

peridotite

TextureTexture refers to size, shape, and distribution of crystals or grains that make up a rock

Ex: rhyolite is fine-grained, while granite is coarse-grained

rhyolite

granite

Porphyritic RocksPorphyritic - texture characterized by large, well-formed crystals surrounded by finer-grained crystals of same mineral or different minerals

Indicates a complex cooling history during which a slowly cooling magma suddenly began cooling rapidly

Vesicular RocksMagma contains dissolved gases that escape when pressure decreases

When lava is thick enough to prevent gas bubbles from escaping, holes called vesicles are left behind

Spongy appearance - vesicular texture

pumice

vesicular basalt