Igneous Rocks - Rocks formed from cooled molten material
Classified by:
Origin
Intrusive - Cools
under the surface of Earth
Cools
slowly thus larger crystals
Extrusive -Cools
on the surface of Earth (obsidian, pumice, basalt)
Cools quickly thus small or no crystals (granite)
Texture (grain size)
no grain - cools so quickly no crystals form (obsidian)
fine grain - cools fast so crystals are small (rhyolite, basalt,
pumice)
coarse grain - cools slowly
so crystals are large (granite)
porphyritic
- cools slowly, then quickly, so crystals are large and small
Mineral Composition
high silica content yields light-colored rock
low silica content yields dark-colored rock
Some uses for some igneous rocks
Basalt - road construction
Pumice - polishes
Granite - building materials