Igneous rocks

Geology of National Parks and Monuments

Prof. Kurt Friehauf

Basalt• Mafic• Fine-grained• Black• May have

bubble holes like Swiss cheese

• Volcanic

Obsidian• Volcanic glass• Smooth with

sharp edges• Forms by quick

cooling• Can be mafic or

felsic• Volcanic

Pumice• Froth of volcanic

glass• Light weight

commonly floatsin water

• Rough surface• Forms as foam

when lava slops around

• Volcanic

Scoria• Mafic• Red color due to

oxidation of iron in mafic lava

• Rough, commonly bubbly

• Volcanic

Rhyolite• Felsic• Fine-grained• Pink/tan/white

color• May contain

pumice fragments

• Volcanic

Andesite Tuff

• Intermediate• Fine-grained with

chunks of other rocks

• Green/gray• Forms by

explosive eruption

• Volcanic

Granite• Felsic• Coarse-grained

can see crystals• Pink/tan/white

with dark specks• Plutonic (forms

deep within earth, not volcanic)