5.1 Igneous rocks - Bennett Earth Science

extrusive

formed on the Earth's surface; synonym: volcanic

intrusive

formed within the Earth's crust; synonym: plutonic

mafic

low silica content - dark colored

felsic

high silica content - light colored

vesicular

lava texture formed by gas bubbles being trapped within the rock

glassy

igneous texture formed by lava cooling instantaneously in water

pumice

light-colored vesicular rock

basalt

mafic igneous rock with small (microscopic) crystals

granite

light- colored igneous rock with large (visible) crystals

gabbro

black igneous rock with large (visible) crystals

feldspar

pink mineral found in some granites

mafic minerals

pyroxene, dark plagioclase

felsic minerals

quartz, feldspar, muscovite

ultramafic

very low silica; green in color; found in upper mantle; e.g. olivine

cooling rate

slow = large crystals

fast = small or no crystals

porphyritic texture

complex cooling; large and small crystals

Composition and texture

Qualities used to classify igneous rocks.

Mafic extrusive rocks

Scoria and basalt

Mafic intrusive rock

Gabbro

Felsic extrusive rocks

Rhyolite and pumice

Felsic intrusive rocks

Granite and pegmatite