Rocks • a rock is a mixture of minerals

• minerals that are always found in a particular rock are called essential minerals

• minerals found in lesser quantities

are called accessory minerals

examples: granite contains mostly feldspar, quartz, hornblende, and mica

basalt contains mostly plagioclase feldspar, augite, and olivine

• some rocks are made of only one mineral

ex: limestone and marble contain mostly calcite

Formed from cooling & crystallizing of either:

lava • forms above ground

• rock formed from lava is called extrusive or volcanic

• most of Earth’s crust (all of sea floor) is volcanic

Igneous Rock

• fine texture:• small crystals (or even no crystals)

examples:

obsidian (volcanic glass)

pumice (volcanic glass foam; it floats in water)

scoria (sold as “lava rock”)

basalt (most common rock on Earth)

rhyolite (light colored)

andesite (medium colored)

or…

magma• like lava, but below ground

• rock is called intrusive or plutonic

• underground igneous rock structures are called plutons

coarse texture • crystals large enough to see with unaided eye

examples: granite (most common rock on continents) gabbro (like coarse grained basalt) diorite (like coarse grained andesite) pegmatite (very large crystals)