Download Rocks and the Rock Cycle

Rocks and the Rock Cycle
There are three rock families: Igneous, Sedimentary and Metamorphic
Igneous Rocks
Igneous rocks form when hot magma or lava cools and solidifies
Magma is melted rock that is only found below the Earth’s crust
Lava is magma that has made it to the surface of the Earth in the form of a volcano –
this is called extrusive rock
Magma sometimes doesn’t make it to the surface, but cools and hardens
underground – this called intrusive rock
The longer it takes for the intrusive rock to cool underground, the larger the crystals
are in the rocks – large collections of intrusive rock are called plutons.
Examples: granite, basalt, obsidian, pumice, tuffa, porphyry
Sedimentary Rocks
Sedimentary rocks make up about 75% of all the rocks we see on the surface of the
Earth
Sedimentary rocks are made from sediments – loose bits of rock, minerals, plant
and animal remains
When the sediments are packed down in layers it is called stratification – happens
mostly in lakes and oceans
As layers pile up on top of each other, the lower layers get squeezed together – this
is called compaction
In some rocks, mineral dissolve as the water soaks into the rock, forming a natural
cement that sticks sediment together – process called cementation
Examples: shale, mudstone, sandstone, limestone, conglomerate, breccia
Metamorphic Rocks
Metamorphic rocks are formed deep under the Earth’s surface with extremely high
pressure and heat
Metamorphic means “changed rock” – the parent rock undergoes changes while
being heated and squeezed
The type of rock depends on the amount of pressure that is applied
For example, shale can be squeezed and heated to form slate and then schist as time
goes on
Examples: slate, schist, gneiss, marble,