Geology Lab Practicum- Igneous Rocks

Extrusive

form from lava that erupts onto the earth’s surface

Intrusive

form from the magma that cools and solidifies underground

Both extrusive and intrusive

a single body of magma that flows above and below surface level

Phaneritic

coarse-grained, magma cools slowly

Aphanitic

fine-grained, magma cools quickly

Porphyritic

large crystals, called phenocrysts, embedded within a finer-grained matrix of smaller crystals or volcanic glass

Vesicular

presence of numerous small, open cavities called vesicles. These cavities are formed when dissolved gas bubbles are trapped in rapidly cooling lava, creating a porous, cinder-like appearance in the resulting rock

Pyroclastic

formed when molten rock (lava) cools so quickly that crystals do not have time to form, resulting in a smooth, shiny, natural glass texture.

Gabbro

mafic, intrusive, phaneritic, dark with meandering pyroxenes/amphiboles

Diorite

salt and pepper, intermediate, intrusive, and phaneritic

Granite

pink feldspar, felsic, intrusive, phaneritic

Basalt

mafic, extrusive, aphanitic or vesicular, black and grainy

Obsidian

glassy, felsic, black

Rhyolite

felsic, extrusive, aphanitic, light-colored with bits of volcanic glass

Volcanic breccia

angular clasts, extrusive, felsic to mafic, pyroclastic/fragmental

Scoria

mafic, extrusive, vesicular, dark color matrix with various size vesicles

Pumice

felsic, extrusive, fine vesicular, frothy texture, may appeared layered, light color

Tuff

felsic to intermediate, fine vesicular with small clasts, tan to pinkish

Andesite

porphyritic (intrusive and extrusive), intermediate to mafic, dark colors