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.
The
rock cycle
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.
Sedimentary
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.
Metamorphic
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.
Igneous
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!
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