Erosion
Why is river water sometimes brown?
The water turns brown when it’s full of mud. A brown river is an example of
erosion in action. Flowing water carries away, or erodes, tiny bits of dirt and
rock from the ground it passes over. Erosion moves rock and dirt from place to
place. Earth’s surface can be greatly changed by erosion. Over tremendously long
periods of time, erosion can carry whole mountains into the sea!
EROSION STARTS WITH WEATHERING
The dirt and pieces of rock that
erosion carries away come from weathering. Weathering is the process by which
nature breaks rocks up. Heat from the Sun can make rock expand and crack into
pieces. Ice can also break up rock. Water gets into cracks in the rock and
freezes. Ice expands when it freezes and breaks the rock. Plant roots can also
grow into rocks and crack them. Rainwater can dissolve, or absorb, some rocks,
turning them into liquid. Rainwater can turn other rocks into sand or clay.
Once rocks break into pieces,
erosion takes over. Water, wind, and ice carry away the pieces left behind by
weathering.
EROSION CAUSED BY WATER
Rainwater runs down hillsides and
carries dirt with it. Over time, flowing water can carry away so much rock and
dirt that it cuts into the ground and forms a channel. The Colorado River eroded
a huge cut in the rock of the southwestern United States called the Grand
Canyon.
Ocean waves crash against
seashores. The waves take sand away from some beaches and pile it up on other
beaches.
EROSION CAUSED BY WIND
Wind is the main cause of erosion
in deserts. There are few plants to hold the dry dirt in place. Wind blows away
loose sand and dry dirt.
Windstorms blew away a lot of dirt
on the Great Plains of the United States in the 1930s. There had been no rain
for years, and the dirt was dry. The farm areas destroyed by wind were called
the Dust Bowl.
EROSION CAUSED BY ICE
Glaciers are huge sheets of ice.
Glaciers move slowly across the land. Ice underneath the glacier picks up and
carries away sand, dirt, and pebbles. The sand, dirt, and pebbles grind away
more rock as the glacier moves.
HOW DOES EROSION CHANGE EARTH?
Water flowing through rivers can
carve out deep canyons. The water carries sand and dirt to other places. The
Mississippi River dumps tons of dirt where it empties into the Gulf of Mexico.
The dirt eventually piles up to create land where once there was ocean. This
process makes rich farmland called the Mississippi Delta.
Wind can blow away sand and dirt
leaving bare rock. It can pile up the sand it erodes to make hills called sand
dunes.
Glaciers can make big changes in
Earth. During the last ice age, glaciers covered much of North America. Glaciers
carved out many lakes. They made the five Great Lakes.
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