Igneous RocksAre grouped by mineral composition and texture

1. Texture

Size and arrangement of mineral crystals

igneous crystals often interlocking

A. Intrusive Texture

Intrusive: formed underground from magma that cooled SLOWLY

large, easily seen crystals

Intrusive-coarse-plutonic

Igneous rocks w/ large crystals are known as: Intrusive= plutonic= coarse-grained

–ex: granite

Granite

Intrusive-Plutonic-Coarse GrainedGranite

B. Extrusive Texture

Extrusive: formed above ground from lava that cooled QUICKLY

small crystals, often hard to see

Igneous rocks w/ small crystals are known as:

extrusive= volcanic= fine-grained

–ex: basalt,

–rhyolite

Rhyolite:Extrusive

Fine GrainedVolcanic

Basalt:

ExtrusiveFine GrainedVolcanic

2. Composition

Is the mineral content often shown as color of rock

A. Felsic

Felsic rocks

–are light colored

–have hi silica content

–most intrusive rox are felsic

felsic

Granite

Rhyolite

B. Mafic

Mafic rocks are:–dark colored–low in silica–most extrusive rocks are mafic

•ex: basalt

Basalt

Some igneous rocks are hard to classify: Obsidian: glassy

Porphyritic: full of holes

porphyritic

pumice: full of holes; floats

scoria: full of holes; doesn’t float

Floating Pumice

Scoria

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