Rocks
The rock cycle Sedimentary rocks Metamorphic rocks Igneous rocks

All of the land on the surface of the Earth is made up of rocks or the products of weathering and erosion of rocks. We use rocks for lots of things including building, sculptures, manufacturing and throwing.

Rocks can be classified as igneous, sedimentary or metamorphic depending on how they form. Geologists further subdivide these major groups into subgroups based on more specific characteristics such as physical properties and chemical composition.

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The rock cycle
Click for larger image Cycles are commonly used by scientists to assist them in understanding the natural processes in our world. Examples of natural cycles include the life cycles of animals such as a butterfly as it undergoes a change from a caterpillar and the cycling of rocks on or near the surface of the Earth.

All rocks on Earth are slowly being changed into other rocks. The rock cycle describes how and where these changes take place. We see evidence of some of these changes every day, but other changes occur deep inside the Earth and thus we can only use indirect evidence to learn more about these changes.

Click for larger imageSedimentary rocks
On the surface of the Earth, wind, water and ice erode existing rocks and carry away the particles of rock along with minerals and the remains of plants and animals. These particles then settle at the bottom of lakes and oceans and compact and harden into sedimentary rocks as they are buried by more sediments. Sediment accumulation can also occur in low lying desert areas. This process may occur over thousands or even millions of years and produce thicknesses of hundreds of metres of sedimentary rocks.

Fossils are only found in sedimentary rocks as they are destroyed in the processes of heating and pressure which create metamorphic and igneous rocks.

Click for larger imageMetamorphic rocks
Rocks that become buried deep in the Earth or near molten rock can be heated under pressure and change their properties. They become harder, more coherent and new minerals may form. Such rocks are called metamorphic rocks.

Click for larger imageIgneous rocks
At even greater depths, the temperature becomes high enough to melt rocks. When this molten rock cools and solidifies again, igneous rocks form. Some of this liquid rock solidifies underground forming the foundations of mountains whilst some reaches the surface as lava before hardening.

Any igneous, sedimentary or metamorphic rock subsequently exposed at the Earth's surface by tectonic processes and uplift is then subjected to the process of weathering and erosion and over time forms the sediments that will go on to form sedimentary rock again, so the cycle continues!

Copyright owned by the State of Victoria (Department of Education and Early Childhood Development). Used with Permission.

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