by Harold L. Levin
Chapter 4 - page 4
Rocks and Minerals: Documents that Record Earth's History
A rock is an aggregate of one or more minerals.
Rocks are the building blocks of the Earth's crust.
There are three great families of rocks:
- Igneous - crystallized from hot, molten rock (lava or magma)
Examples: granite, basalt
- Sedimentary - fragments of sediment laid down by water or wind become compressed or cemented over time
Examples: sandstone, shale, limestone
- Metamorphic - rocks changed by heat and or pressure or chemical activity
Examples: gneiss, schist, slate, marble
The Rock Cycle
The rock cycle describes the processes through which rocks of one category may be
altered or recycled to form rocks of another category.
The rock cycle.
Here is a simplified example:
- Igneous rocks can be weathered and eroded to form sediment
(gravel, sand, silt, clay).
- Sediment is cemented and compacted to form sedimentary rocks.
- Sedimentary rocks can be deeply buried and subjected to heat and pressure
that converts them to metamorphic rocks.
- Metamorphic rocks may be heated to temperatures so great that they melt.
- The molten rock (magma or lava) crystallizes to form igneous rocks.
This completes the simplified cycle.
Note also that all three types of rocks can be weathered and eroded.
Igneous rocks can also be metamorphosed or melted.
The rock cycle. Public domain image from U.S. Geologic Survey.
Through the rock cycle, one type of rock can be converted into another.
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Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA
September 9, 2005