Igneous Rocks

coarse, felsic

granite

granite prehistoric uses

hammerstones and bannerstones

fine to porphyritic (phenocrysts), felsic

rhyolite

rhyolite prehistoric uses

tool manufacture

glassy (fine), felsic

obsidian

obsidian historic/prehistoric uses

glass; stone tools

fine, felsic, vesicular

pumice

pumice prehistoric uses

carving stones, abrasives

coarse, intermediate

diorite

diorite prehistoric uses

hammerstones

porphyritic (fine), intermediate

andesite

andesite prehistoric uses

stone tools

coarse, mafic

gabbro

gabbro prehistoric uses

manos and metates

porphyritic (fine), mafic

basalt

basalt prehistoric uses

hoes, adzes, celts, etc.

fine, mafic, vesicular

vesicular basalt

vesicular basalt prehistoric uses

carvings (Olmec Heads)

fine, mafic, amygdaloidal

amygdaloidal basalt

amygdaloidal basalt prehistoric uses

carvings

fine (frothy), mafic, vesicular

scoria

scoria prehistoric uses

carvings

igneous rocks

formed when silicate minerals cool from lava either above or below the surface

extrusive

lava cools too quickly at the surface to form visible crystals (fine-grained)

intrusive

magma cools slowly underground to form large fractals (coarse-grained)

mafic

dark, high in augite

felsic

light, high in quartz and potassium feldspar

phenocrysts

larger crystallized minerals formed in magmatic reservoir (subsurface) and erupts as part of lava, once on the surface no more crystals form

porphyritic

two distinct mineral grain sizes: fine grained groundmass and large grained phenocrysts

vesicles

form from extrusive processes: lava erupting in the air cools rapidly and allows air to escape in pockets

mafic is high in what?

pyroxene, calcium, plagioclase feldspar, magnesium, iron

felsic is high in what?

quartz, k-spar, sodium, silica