D. NOMENCLATURE 53

 Names of igneous rocks are based on texture and composition.

Texture: aphanitic = extrusive

phaneritic = intrusive = plutonic

Composition: there are two end members:

Mafic (Mg & Fe) These rocks are relatively low in Si, high in Mg, Fe, Ca. minerals = olivine, pyroxene, plagioclase, and biotite > dark-colored rocks

Felsic (feldspar & silica) These rocks are relatively high in Si, Al, Na, & K. minerals = quartz, orthoclase > light-colored rocks

COMPOSITION

felsic

intermediate

mafic

 

granite (Sierras)

granodiorite

diorite

gabbro

phaneritic

rhyolite

dacite

(Mount St. Helens)

andesite

(Andes)

basalt

(Hawaii)

aphanitic

Phaneritic ultramafic rocks = peridotite (makes up the upper mantle) - composed mostly of olivine.

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