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| Source: Science Clarified |
Plateaus
Plateau – an area of
land having a relatively level surface considerably raised above adjoining land
on at least one side, and often cut by deep canyons.
Plateaus can be formed from tectonic movement or the erosion
by water and glaciers. Magma from the mantle can rise up and cause the ground
to swell upward, causing large flat areas of rock
to be uplifted, forming plateaus. Lava may also spread outward from cracks and
weak areas in the crust, building up flat areas of land. These raised areas of
land undergo significant erosion from mechanical and weathering processes
forming the features explained below.
Erosion of the plateau is most effective when a band of more
resistant rock (e.g. de Chelly Sandstone and Shinarump Conglomerate) overlies a
band of less resistant rock (e.g. Organ Shale). Erosion of the less resistant
rock leads to undercutting and cliff collapse causing parallel retreat of the
plateau.
An example of a plateau is the Colorado plateau in the USA.
Mesas,
Buttes and Spires
Mesas, buttes and spires are examples of relict hills that
been formed by weathering and water erosion. Sheet runoff exploits joints and
weaknesses in a plateau, eroding wadis and canyons and isolating large columns
of rock as the cliff retreats. These are called mesas and are defined as being
wider than they are tall. These can be vast, but still experience the same
erosional processes as their larger counterparts.
A butte is formed when a mesa is eroded such that it is
taller than it is wide. Erosion of the butte continues until a thin pillar-like
formation known as a spire is left. Eventually the spire will also be eroded
away to form a pediment. An example of a spire is the Totem Pole in Arizona’s
Monument Valley which rises 130 m into the air.
Pediments
Pediments are gently sloping erosional rock surfaces (rather
like wave cut platforms), with an angle of >7°, found at the base of cliffs
and steep hills in the desert. They are formed by the parallel retreat of
plateaus, mesas and buttes and are usually covered with debris from rock falls
or alluvial fans. Weathering and sheet runoff help to develop the pediments as
eroded material carried by water scours the rock surface.
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