Review: There are three basic types of rocks – sedimentary, igneous, and metamorphic.
The rock cycle
The Earth’s rocks do not stay the same forever. They are continually changing because of weathering, erosion and large earth movements. Rocks are gradually recycled and changed over millions of years.
For example, sedimentary rocks can be changed into metamorphic rocks. These then can be weathered, eroded, and the pieces transported away. The pieces could be deposited in a lake or sea, eventually forming new sedimentary rock. Many routes through the rock cycle are possible, shown as arrows on the following diagram.
Three main processes change rocks from one form to another
Cooling and crystallization / Weathering and erosion / Metamorphism
One major process is cooling and crystallization
Deep within the Earth, high temperature can melt rocks into magma.
Later that magma can move to a different, cooler region.
If the magma cools slowly then crystals will form.
A mass of these interlocking crystals is called igneous rock
Slower cooling allows for larger crystals to form.
Faster cooling causes smaller crystals.
Here’s granite (an intrusive igneous rock) from Chennai, India. See all the different crystals?
Say, what exactly does the crystallization process look like?
First, you might want to remind yourself what a crystal is! Done that? Good – so in this next GIF we see hot magma, deep in the Earth, cooling off.
Notice that some of the liquid rock comes together to form crystals, floating in hot yellow magma. But then some of the rest of that magma cools into a different kind of crystal.
Finally, the leftover magma around these cools and solidifies, locking all of these tiny crystals together.
This animation is from Classzone: Igneous Rock Crystallization Animations., which is a part of Exploring Earth flash visualizations. from McDougal Little.