ciencias

The external forces of earth.

Wind, water and people erode the relief of earth surface.

These changes occur slowly, over millions of years.

External changes happen in three phases:

1. Erosion: is the constant wearing away of rocks and soil.

Temperature, water, wind and people cause erosion.

· Sharp changes in the temperature: break rocks into fragments. This occurs in areas where there is a big difference between daytime and nighttime temperatures, such as mountains and deserts. Sometimes water that has filtered through cracks in rocks freezes. This act like a wedge and breaks the rock.

· Rainwater, rivers and oceans: constantly erode the ground.

Rain and snow dissolve the materials in the soil. Rivers wear away banks and erode materials from the rivers bed.

They create deep valleys and canyons. The waves of the ocean wear away the coasts. Underground water hollows out large caves.

· The wind: mostly erodes soft rocks. It also erodes places where there is a lot of sand, like deserts and beaches.

· The people: cut down forests. They cause fires, build slopes, etc. this actions cause erosion of the soil.

2. Transporting materials: eroded materials are transported from one place to another. The wind, river currents and ocean water transport these materials. Wind curries sand and small rocks. Rivers and oceans carry large amounts of dissolved materials. They also carry larger materials floating on the surface.

3. Sedimentation: occurs when the materials carried by water, ice or wind are deposited. The accumulation of these materials leads to changes in the lanscape.

· A river transports the materials. It deposits them on its banks to create terraces. It deposits at is mouth to create deltas.

· Ocean waves deposit materials on the low shores. The deposited materials create beaches.

· The wind transports large quantities of sand. This creates dunes in deserts and on the coast

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