3.3 Notes
How are igneous rocks classified?
Objective: Identify and describe igneous rocks by their minerals and textures
Combination of minerals:
Igneous rocks are made up of many
different kinds of minerals. When minerals
melt in magma or lava, igneous rocks form.
There are six minerals that are commonly
found in igneous rocks: quartz, feldspar,
mica, olivine, amphibole, and pyroxene.
Igneous rocks are identified by their
minerals. Granite is an igneous rock made
up of quartz, feldspar, and mica.
Granite
Crystal size:
Igneous rocks have mineral crystals of
different sizes. The size of the crystals
depends on how long it takes molten rock to
cool. Large crystals form when rock cools
very slowly. Igneous rocks formed from
magma have large crystals.
Small crystals take less time to form.
Igneous rocks formed from lava have very
small crystals.
Some igneous rocks have no crystals because lava cools so quickly that there is no time for
crystals to form. Obsidian is an igneous rock that does not have any crystals.
How are cooling rate and crystal size related?
Texture:
The size of the crystals in an igneous rock is used to describe its texture. Granite has
large crystals that you can see and feel. Rhyolite has very small crystals that are hard to see.
Basalt is another commonly found rock with small crystals.
Rhyolite
Igneous rocks are classified by their texture.
Igneous rocks with large crystals have a coarse texture.
Igneous rocks with small crystals have a fine texture.
Igneous rocks that do not have crystals have a glassy texture.
Slow cooling  magma  large crystals  coarse texture
Fast cooling  lava  small crystals  fine texture
Very fast cooling  lava  no crystals  glassy texture
Classify granite, obsidian, basalt, and rhyolite according to their textures.