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Learn about the various forms of erosion caused by water, wind, and gravity. Explore topics such as underground erosion, sinkholes, hot springs, river runoff, flood plains, alluvial fans, deltas, ocean waves, wind erosion, and glacial erosion.
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E R O S I O N Based on Discover God’s Creation Chapter 6 By Rebecca Fraker
6-1 Erosion Caused By Water Raindrops splashing on the ground can move an amazing amount of soil and rock. In a single storm, raindrops falling on an area the size of a football field can move more than 100 tons of dirt!
Ground Water Ground water is precipitation, rain or melting snow, that soaks into the soil. Water drains through the soil until it reaches solid rock. There it collects into underground deposits called the aquifer. In some areas, water from the aquifer bubbles up through the ground in springs. In other areas, wells must be dug to tap into the aquifer.
Erosion underground • Erosion can occur underground. In some areas, water moves through the soil and dissolves soft rock. The dissolved minerals are carried away, leaving cracks and openings. • As water continues to dissolve the rock, the cracks and openings dissolve into caves and caverns.
Sinkholes • Some underground caves collapse because they cannot support the weight of the ground above them. • This forms a sinkhole. • Some sinkholes are caused by natural erosion. Others happen where too much water or oil has been pumped out of the ground.
Hot Spring • If water flows down deep cracks and goes inside the earth, it may get near hot magma. • If it does, it heats up. Then it may flow up and out. This is a hot spring.
Geyser • If the underground heated water shoots out of the ground with a lot of force, it is called a geyser. • Some geysers are so powerful, they spray hot water and steam 400 feet in the air.
Runoff Water • Most erosion is caused by runoff water. • Runoff water is the water that does not soak into the ground, but runs off the top of the land downhill to form creeks and streams. • Streams combine to form rivers that eventually empty into the ocean.
Load • As runoff water moves over the land, erosion takes place and the water picks up soil, gravel, mud, humus, and rocks. • This eroded material is called the load. • As the river slows, the boulders and gravel begin to settle out. Pebbles are deposited. • Sand and mud are the last to sink to the bottom and may often form sandbars.
Flood Plain • When a river overflows its banks, the load spreads over a large area called a flood plain. • Many towns and cities will not allow people to build houses on flood plains. • Other areas allow it. These places often become the scenes of disasters, as we saw recently in New Orleans. There, many houses were built on the flood plain.
Alluvial fans • Where rivers flow out of hills and mountains into open valleys or plains, alluvial fans form. • Alluvial fans are fan-shaped deposits of rocks and gravel at the base of hills or mountains.
Delta • By the time a river reaches the ocean, its load usually consists of only very fine particles. These tiny particles settle out at the mouth of the river and form a delta. • The Mississippi River deposits an average of 850,000 tons of material at its delta every day.
Ocean Waves • Ocean waves cause erosion as they hit the shore. • The waves move material from one place and deposit it in another, forming different features. • Ocean waves release tremendous amounts of energy that can move huge rocks.
During one storm, a wave near Tillamook, Oregon, hurled a 130 lb. Boulder 99 feet above the shore and through the roof of a building. • The Mississippi River delta continues to grow out into the Gulf of Mexico at a rate of 350 feet a year.
Erosion Caused by Wind and Gravity • Wind erodes the land in two ways. One way is by scooping out loose material and blowing it away. • The other way is similar to sandblasting. Tiny grains of sand bombard rock surfaces and slowly wear away softer rock.
Deposits made by wind action • Sand dunes form in deserts and along beaches because of the wind. • Often, sand is moved a great distance. Sand dunes on the shores of Lake Michigan move 16-25 feet a year. • Particles of rock and soil carried by the wind grind against each other, often forming an even finer powder. The world’s largest sand dunes are in the Sahara desert.
Gravity • As weathering takes place, gravity pulls the loosened material down. Rock and soil fall from cliffs or roll down steep hillsides, gradually building up at the base.
Landslides/Rockslides • Sometimes water so saturates the ground that the mud or rocks on the side of a hill will slip off in a highly dangerous destructive landslide or rockslide. Earthquakes can also trigger landslides.
6-3 Erosion Caused By Glaciers • Glaciers are large masses of thick ice that move slowly over the land. • They form only in very high mountains or in cold regions near the North and South poles, where the temperatures never rise enough to melt the snow completely.
Glaciers • Year after year, snow builds up and packs into ice. • Large glaciers begin to move slowly over the land, usually only a few meters per year. • However, the ones that are near the sea move much faster because the land’s surface near the sea is warmer. Mendenhall Glacier in Alaska moves up to 90 feet a day!
Two types of glaciers • Continental glaciers • Valley glaciers
Continental Glaciers • These are large enough to cover most of an entire continent. • Today there are only two: one covering most of Greenland, and one covering Antarctica.
Valley Glaciers These are much smaller than continental glaciers. Over 200,000 of these small glaciers are scattered over Earth. Valley glaciers usually occur in high mountains and look like frozen rivers.
What do glaciers do? • You can see the effects of glaciers throughout much of Canada and the United States • As glaciers move, they carve and grind away the landscape like giant bulldozers, scraping away soil and loose rocks and depositing it elsewhere. • Moraines, eskers, glacial polish, erratic boulders, and U-shaped valleys all indicate where glaciers have come and gone.
Moraines • As a glacier moves, long piles of rock and soil called moraines, form along its edges. • When the glacier melts, these moraines remain. • Sometimes they dam rivers and form moraine lakes.
Eskers • Water melting off the glacier forms channel beneath the ice. • As water continues to flow along these channels, long winding ridges called eskers form.
Kettle Lakes • Some glaciers gouge out basins as they move. • When glaciers melt, the basins fill with water to form kettle lakes. • The state of Minnesota, called the “the land of 10,000 lakes” is dotted with thousands of kettle lakes.
Erratic Boulders • As a glacier moves, it often picks up large boulders and carries them to a new location. When the glacier melts, it leaves these erratic boulders behind.
Glacial Polish • As glaciers move over some kinds of rock, the grinding action “polishes” the surface of the rock. These rocks shows glacial polish.
U-Shaped Valleys • As glaciers move between mountains, they gouge out U-shaped valleys. Yosemite Valley in California is an example of this feature of glacial action.
