Changing the Earth’s Surface
Weathering, Erosion and Deposition If you were to watch a rock, it would seem that
the rock never changes. But when rocks are
exposed on the Earth’s surface, processes
acting on the rock’s surface break it down into
smaller and smaller pieces called sediment. We
cannot always see this process, because in
some cases, it takes a very long
of time.
The process in which rocks are broken down is called
weathering. There are two types of weathering, physical
and chemical. Physical weathering changes rock without
changing its composition. Plant roots wedging into rock,
water freezing in rock cracks, and rock collisions all cause
rocks to break down.
Chemical weathering is a process in which the
chemical composition of the rock is changed. Gases
dissolved in water, such as weak acids and salts, act on
the rock to break them down into new substances.
Lichens, a fungus in a symbiotic relationship with algae,
produce acids which chemically can break down rock.
The process of erosion includes weathering of rock and the movement of
the sediment to other places. When the sediment reaches its final
destination, it is deposited and often forms a new layer of rock.
www.hi.is/~oi/bruarjo
TEKS 7.8B analyze the effects of weathering, erosion, and deposition on the environment in ecoregions of Texas TEKS 8.9C interpret topographic maps and satellite views to identify land and erosional features and predict how these
features may be reshaped by weathering
Rivers Most landscapes are formed by moving water. As the water collects down a
slope, it forms narrower and deeper channels. The water can erode the soil
to form gullies, valleys and canyons. Flowing
water can cut small channels a few centimeters
deep or great canyons the size of the Grand
Canyon.
Eroded sediment carried by the moving water
moves downhill and toward the ocean. The
sediment is deposited where the river enters a
body of water such as a lake or the ocean. This
area is called a delta.
Most rivers are curvy rather than straight.
Over time, the shape of a river changes
because the river shifts sideways in its
position. Rivers tend to erode one bank
and deposit sediment on the other bank. The river
valley eventually becomes wider and wider and the
river path much more curved. When the valley
becomes broad, almost like a flat plain, it is called a
floodplain. When the river floods, it spills over its
banks and floods the surrounding area. Fine sediment,
called silt, is deposited on the floodplain.
Waves Rivers aren’t the only places where moving water
shapes Earth’s surface. As ocean waves move
toward the shore, they deliver a tremendous
amount of energy. The waves hitting the shore
slowly wear away the rock material along the
shoreline. The wave action can create cliffs,
sea caves and sea arches.
Sand beaches are shaped by
the waves as well. The waves
carry grains of sand over
enormous distances. The
sand can be carried not only up and down the beach, but
along the beach! Over time, huge amounts of sand can be
moved along a beach.
To preserve beaches, engineers construct long wall structures such as jetties
and breakwater barriers. These structures protect shores and beaches, but
they often fail during an especially large storm.
Glaciers
A glacier is a large mass of ice on the Earth’s
surface. The ice in the glacier flows because of its
own weight and gravity. As it moves it scrapes the
surface of the Earth and carries rocks and
sediment along, carving a ‘U’ shaped valley.
Material pushed by the glacier forms a ridge of sediment called a moraine.
When the weather warms and the glacier begins to melt, streams of melt
water flow out from beneath the glacier. These streams carry glacial sand
and gravel for long distances before they deposit the sediment in river
valleys.
Wind In places where the wind blows, the wind causes
small particles of sediment to be lifted and carried
to other places. Sometimes the moving particles
encounter other rock materials or formations and
wear away the softer material. Unusual formations
are left behind.
Wind erosion often occurs where there is little or no
vegetation. An example is the formation of sand
dunes. Sand dunes are constantly moving. How
fast the dune moves depends on the speed of the
wind, size of the dune and how much vegetation is
present. Some dunes are stabilized because they
have plants growing on them that have extensive
root system. The roots help to hold the sand in place.
Time
If the process of erosion has been moving eroded rock from the continents
for long periods of time, why haven’t the continents been worn away? The
answer is that forces within the Earth also uplift the surface. So the
continents are continually being eroded
away, reformed and reshaped. Earth’s
surface looks very different from how it
did millions of years ago. The
processes of weathering, erosion and
deposition constantly recreate the
landscape over time.