Rivers and Streams
falls on the ground some of it soaks into the soil. It moistens
the soil particles and makes them easier to move. A heavy rain
will stir up the surface of the soil and make mud of the small
particles. The water that is not absorbed runs off the soil,
carrying a little of the soil with it.
Water flows downhill,
and is always seeking a lower path. Little trickles of water
flow together, and soon there is a small stream. A small stream
has more force than individual rain drops or melting snow. It
begins to stir up larger particles and carry them away.
Here you see a stream
bed in the desert. Notice the steep sides of the stream
bed. The stream is dry now, but the water will come back in a
big rush after a rain. The fast moving water will carry earth
away from the sides of this little valley.
The sides of many
valleys are covered by plants. The roots of the plants hold the
soil in place when the rain comes, and their leaves break the
force of the falling raindrops. These valleys do
not erode as quickly as valleys of bare earth, but they also
change over time. Water is an
Rocks are constantly being moved
around by currents in the mantle, and they crack in response
to the strain. Rain seeps into the cracks, and when winter comes
the water freezes. Freezing water expands, and it pushes the
walls of the cracks apart, making the cracks wider. Over centuries
huge rocks can be split. Sometimes rocks are split off from the
sides of canyons.
Glaciers are huge "ice rivers"
sliding slowly towards the sea. The underneath part of the glacier
may have captured rocks in the ice, and as the glacier moves,
it grinds the rocks beneath it. Glaciers move all sizes of rocks,
and grind some of the stones to a very fine dust called
This picture shows an
example of glacial
glacier has smoothed
this rock so that it
feels as smooth as
glass. The rocks
on top of the polished
rock may have been
left behind when the
ice of the glacier
cracks in the polished
opportunities for ice
to break up this
is a continuous
show erosion in action. The constant movement of the waves shows
us how sand grains are tumbled, and the returning water traces
miniature drainage channels in the sand. The rocks and stones
on the beach are rounded from thousands of years of being rolled
against each other or washed by sand-carrying breakers.
Waves also erode cliffs along
the beaches. They undercut the cliffs, and eventually the cliffs
fall. The ocean along the coast of California is slowly pushing
back the cliffs.
picks up dust and sand. Sand storms can act like sand blasters
if the wind is moving fast enough. Sometimes when people are
driving in the desert they drive through a sand storm which erodes
all the paint off their cars! In the desert the wind moves the
sand into dunes, piling it up and covering the surfaces with
There are chemicals in
the air. Some of these chemicals can corrode rocks,
and fairly quickly, too. The names on marble gravestones
can be dissolved away in only a couple of centuries. Carvings on
buildings are being digested by chemicals that cause air pollution.
Another way that chemical
action can break down rock
is through crystallization.
If salt water, for instance,
gets into cracks in rocks,
the salt crystallizes when
the water evaporates.
The tiny crystals push
against the sides of the
cracks and slowly widen them.
from a Corel CD-ROM : for viewing only, not for downloading.
© 1999, 2003. Elizabeth Anne Viau and her licensors. All
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