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.

Source: Microsoft ® Encarta

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