The rock
cycle is a general model that describes how
various geological processes create, modify, and
influence rocks (Figure 10a-1). This model
suggests that the origin of all rocks can be ultimately
traced back to the solidification of molten magma.
Magma consists of a partially melted mixture of elements
and compounds commonly found in rocks. Magma exists
just beneath the solid crust of the Earth in an interior
zone known as the mantle.
Figure
10a-1: The
rock cycle.
Igneous rocks form
from the cooling and crystallization of magma as it migrates
closer to the Earth's surface. If the crystallization
process occurs at the Earth's surface, the rocks created
are called extrusive
igneous rocks. Intrusive
igneous rocks are rocks that form within
the Earth's solid lithosphere.
Intrusive igneous rocks can be brought to the surface
of the Earth by denudation and
by a variety of tectonic processes.
All rock types can be physically and chemically
decomposed by a variety of surface processes collectively
known as weathering. The debris that
is created by weathering is often transported through
the landscape by erosional processes
via streams, glaciers, wind, and gravity. When this debris
is deposited as
a permanent sediment,
the processes of burial, compression, and chemical alteration
can modify these materials over long periods of time
to produce sedimentary rocks.
A number of geologic processes, like tectonic folding and faulting, can exert heat and pressure
on both igneous and sedimentary rocks causing them to
be altered physically or chemically. Rocks modified in
this way are termed metamorphic rocks.
All of the rock types described above can
be returned to the Earth's interior by tectonic forces
at areas known as subduction
zones. Once in the Earth's interior, extreme
pressures and temperatures melt the rock back into magma
to begin the rock cycle again.