| A rock is
defined by our glossary as an aggregate of
minerals. It is fairly difficult to go into any
more detail. In order to describe more fully
exactly what constitutes rock, you have to know a
little about how they form. There are
three main types of rock. The first is
sedimentary - a rock formed by the accumulation
and consolidation of sediment (mineral and
organic material deposited by water, ice or
wind). Particles of sand, for example, buried and
compressed, eventually become sandstone, a
classic sedimentary rock.
The
second category is igneous. Igneous rocks are
thosed formed by magma or lava solidifying on or
below a planet's surface. The word comes from the
latin - "igne" - meaning fire.
The
third is metamorphic. The word means literally
means "altered form". These rocks are
those which have been subjected to intense
pressure and heat, usually deep below the earth's
surface, which have changed the properties of the
rock.
The
text below illustrates the principle of a
"rock cycle", in which sediment can
become all three forms of rock before becoming
sediment again.
1.
Sand is washed up on the beach.
2. The sand is buried under more sand until it
hardens into sedimentary rock.
3. The rock goes still deeper down, until it is
squashed and heated into a metamorphic rock.
4. The rock melts under the pressure and heat and
flows as magma underground.
5. The magma becomes lava as it is ejected from
the volcano.
6. The lava cools and hardens to form igneous
rock, which falls into the river.
7. The river erodes the rock down into sand again
and carries it out to sea.
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