The Earth is covered in a layer of solid rock called the
crust. Rocks are either SEDIMENTARY
, IGNEOUS,
or METAMORPHIC. Almost all rocks made of minerals, but different
rocks contain different mixtures of minerals. Granite, for example, consists of
quartz, feldspar, and mica. A rock can be identified by its overall colour, the
minerals it contains, the size of the mineral grains, and its texture (mixture
of grain sizes).
The solid rock that makes up the Earth’s crust is called
bedrock. It can be seen on coasts
and in mountains, where it is being worn away by erosion. Erosion breaks the
bedrock into small pieces, forming soil and sediments (such as mud, sand, and
gravel), which cover up the bedrock in most places. The sediments may later
turn into sedimentary rocks.
Sedimentary rocks are made of particles of sediments such
as sand and clay, or the skeletons and shells of sea creatures. When layers of
loose sediment are buried and pressed down under more layers, the particles
slowly cement together and lithify (form rock). Chemical sedimentary rocks,
such as flint, form when minerals dissolved by water are deposited
again.
Igneous rocks are created when magma (molten rock under
the Earth’s crust) cools and becomes solid. Magma loses heat when it
moves upwards at weak spots, such as cracks, in the crust. Extrusive igneous
rocks form when magma reaches the surface and cools quickly. Fast cooling
produces fine-grained rocks. Intrusive igneous rocks form when magma cools
slowly underground. This allows the minerals to grow into coarse
grains.
Metamorphic rocks are formed when the minerals in rocks
are changed underground by heat and pressure. Contact metamorphic rocks are
produced when rocks are heated by magma rising through the crust. Rocks that
are folded or crushed by immense pressure deep in the crust are called regional
metamorphic rocks. The properties (characteristics) of a metamorphic rock
depend on its parent rock (the original rock type) and how it was formed.
