How Do Rocks Form Anyway?
Igneous Rocks (Rocks from Fire)
 Rocks formed from the cooling of hot, molten rock (magma or
lava)
 Two Types
o Intrusive
 Cooling of magma deep beneath the surface
 Magma contained in batholiths (large), dikes (angled), sills
(horizontal)
 Magma intrudes (advances by burning its way into existing
rock)
 Cool very slowly - form large crystals
 Examples: Granite, Gabbro, Diorite
o Extrusive
 Cooling of magma (near surface) or lava (at surface)
 Cool quickly - form small crystals
 Examples: Basalt, Rhyolite, Andesite, Obsidian, Pumice, Scoria
Sedimentary Rocks (Rocks from the Surface)
 Rocks formed from the lithification (compaction and cementation)
of sediment
 Sediment
o Broken up pieces of rock and/or organic materials
o Mostly sand, silt, and clay
o Made from weathering and erosion of rocks
 Compaction
o As sediments are deposited and buried by layer upon layer
o Force from above pushes sediment together
 Cementation
o Water carries dissolved minerals thru sediment
o Compaction forces water and air out and minerals are left
behind as the cement
 Three Types
o Clastic
 Rock fragments (any combo of size and shape) are
compacted and cemented together (ex. conglomerate,
breccia, sandstone)
 Can be formed from pressure only (ex. shale)
o Chemical
 Minerals that were once dissolved in water
 Evaporation - water leaves minerals behind (ex. rock salt
is left behind by water)
 Precipitation - minerals fall out of water due to
temperature change in the water (ex. chert, compact
limestone)
 Organic
o Remains of once living things
 Ex. Coal from plants
 Ex. Fossiliferous limestone, coquina, and chalk from the
shells of tiny marine animals
Metamorphic Rocks (Rocks from Heat and Pressure)
 Metamorphosis - changing of one rock into another by heat,
pressure, and/or chemical processes
 Minerals in rock change in size & shape and may form parallel
bands or layers
 Two types
o Contact
 Metamorphism due to rock coming into direct
contact with magma
o Regional
 Heat and pressure due to tectonic activity
 Most met. rocks are formed by regional
metamorphism