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The last cycle we will discuss is the rock cycle. This involves
the recognition of three main classes of rocks found on the Earth.
The rock cycle consists of the production and transformation of
one type of rock into another: lava hardens to form an igneous rock,
which subsequently is eroded into a sedimentary rock, which in turn
can transform into a metamorphic rock.
- Igneous: These rocks, such as basalt and granite, are
formed from when magma or lava from volcanoes hardens. These were
the first types of rock to appear on the Earth.
- Sedimentary: These rocks, such as sandstone, shale, and
limestone, are formed from the erosion of other types of rocks
by weather and water.
- Metamorphic: When sedimentary rocks get buried
deep within the Earth, they are subject to intense pressure and
heat. This changes the rock into a metamorphic rock, examples
of which are quartzite.