Igneous Rocks
A rock is a group of minerals bound together in some way.
Igneous rocks are formed by the cooling and hardening of molten
rocks.
a) Intrusive (plutonic) - form below the Earth’s surface
b) Extrusive (volcanic) – form above the Earth’s surface
Texture refers to the size of the crystals in an igneous rock
I
Intrusive (plutonic) Igneous Rocks
Magma is molten rock underneath the Earth’s surface.
Intrusive igneous rocks cool slowly deep within the earth and
produces large crystals
Intrusive igneous rocks will result in a very coarse (pegmatite) or
coarse (phaneritic) texture (ex. granite)
II
Extrusive (volcanic) Igneous Rocks
Lava is molten rock that pours out onto the Earth’s surface.
Volcanic rocks lack distinct mineral grains due to rapid cooling of
lava at the Earth’s surface
Extrusive igneous rocks will result in fine (aphanitic) texture (ex.
basalt)
If the cooling is instantaneous a glassy texture may form
(ex. obsidian)
If gas bubbles are trapped as the lava rapidly cools a vesicular
texture may occur (pumice)
III
Porphyritic rocks
contains two stages of cooling.
cools first at great depth forming large crystals still
surrounded by magma
it is then rushed to the surface and the remaining liquid around it
cools quickly forming fine-grained crystals
VI
Common Igneous Rocks
Granite- quartz, orthoclase feldspar, and at least one other mineral;
light-colored; coarse-grained; makes up continents; felsic
Ryolite- fine-grained granite; felsic
Obsidian- volcanic glass; rapid cooling; pyroclastic; felsic
Pumice- sponge-like because of escaping air bubbles during
cooling; felsic
Basalt- fine-grained; dark-colored; makes up the ocean floor; mafic
Gabbro- coarse-grained basalt; mafic
Diabase- grain size inbetween basalt and gabbro; mafic
Basalt glass- mafic obsidian
Scoria- mafic pumice