Changing Earth s Surface

Size: px

Start display at page:

Download "Changing Earth s Surface"

Shawn Johnson
6 years ago
Views:

1 Name Date Class Changing Earth s Surface What processes wear down and build up Earth s surface? What causes the different types of mass movement? Erosion is the process by which natural forces move weathered rock and soil from one place to another. Gravity, running water, glaciers, waves, and wind all cause erosion. The material moved by erosion is sediment. When the agents of erosion lay down sediment, deposition occurs. Deposition changes the shape of the land. Weathering, erosion, and deposition act together in a cycle that wears down and builds up Earth s surface. Erosion and deposition are at work everywhere on Earth. Erosion and deposition are never-ending. Gravity pulls everything toward the center of Earth. Gravity is the force that moves rock and other materials downhill. Gravity causes mass movement, any one of several processes that move sediment downhill. The different types of mass movement include landslides, mudslides, slump, and creep. Mass movement can be rapid or slow. The most destructive type of mass movement is a landslide, which occurs when rock and soil slide quickly down a steep slope. Some landslides contain huge masses of rock, while others may contain only a small amount of rock and soil. A mudflow is the rapid downhill movement of a mixture of water, rock, and soil. The amount of water in a mudflow can be as high as 60 percent. Mudflows often occur after heavy rains in a normally dry area. In clay soils with a high water content, mudflows may occur even on very gentle slopes. An earthquake can trigger both mudflows and landslides. In the type of mass movement known as a slump, a mass of rock and soil suddenly slips down in one large mass. It looks as if someone pulled the bottom out from under part of the slope. A slump often occurs when water soaks the base of a mass of soil that is rich in clay. Creep is the very slow downhill movement of rock and soil. It occurs most often on gentle slopes. Creep often results from the freezing and thawing of water in cracked layers of rock beneath the soil. Creep is so slow that you can barely notice it, but you can see its effects in objects such as telephone poles, gravestones, and fenceposts. Creep may tilt these objects at spooky angles.

2 Name Date Class Water Erosion What process is mainly responsible for shaping the surface of the land? What features are formed by water erosion and deposition? What causes groundwater erosion? Moving water is the major agent of erosion that has shaped Earth s land surface. The force of a falling raindrop can loosen and pick up soil particles. As water moves over land, it carries these particles with it. This moving water is called runoff, which is water that moves over Earth s surface. The amount of runoff in an area depends on five main factors: amount of rain, amount of vegetation, type of soil, shape of the land, and how people use the land. As runoff travels, it forms tiny grooves in the soil called rills. Rills flow into one another and form larger grooves, called gullies. A gully is a large groove, or channel, in the soil that carries runoff after a rainstorm. Gullies join together to form streams. A stream is a channel along which water is continually flowing down a slope. A tributary is a stream that flows into a larger stream. Through erosion, a river creates valleys, waterfalls, flood plains, meanders, and oxbow lakes. Rivers often form on mountain slopes. There, a river generally follows a straight, narrow course, creating a deep, V-shaped valley. Lower down, a river usually flows over more gently sloping land. The river spreads out, forming a wide river valley. The flat, wide area of land along a river is a flood plain. A meander is a looplike bend in the course of a river. Sometimes a meandering river forms an oxbow lake, a meander that has been cut off from the river. As water moves, it carries sediment with it. Whenever moving water slows down, it deposits sediment. Deposition creates landforms such as alluvial fans and deltas. It can also add soil to a river s flood plain. When a river flows out of a mountain valley, the water slows down. Then sediments are deposited in an alluvial fan, a wide, sloping deposit formed where a stream leaves a mountain range. A river ends when it flows into a still body of water, such as an ocean or a lake. There the water slows down and deposits sediment. This sediment builds up a landform called a delta. Deposition also occurs during floods. When rain falls, some water soaks into the ground, filling openings in soil and cracks in rocks. This underground water is called groundwater. Groundwater can cause erosion through a process of chemical weathering. Over time, this weathering can develop into caves or caverns and also result in deposition. A deposit that hangs like an icicle from the roof of a cave is a stalactite. Slow dripping can build up a cone-shaped stalagmite on a cave floor. In rainy regions where a layer of limestone is near the surface, groundwater can change the shape of the land. Such a landscape is called karst topography.

3 Erosion and Deposition Name Date Class The Force of Moving Water What enables water to do work? How does sediment enter rivers and streams? What factors affect a river s ability to erode and carry sediment? A river s water has energy. Energy is the ability to do work or cause change. There are two kinds of energy. Potential energy is energy that is stored and waiting to be used later. Kinetic energy is the energy an object has due to its motion. As gravity pulls water down a slope, the water s potential energy changes to kinetic energy that can do work. All along a river, moving water causes changes. A river is always moving sediment from place to place. At the same time, a river is also eroding its banks and its valley. In the process of water erosion, water picks up and moves sediment. Sediment can enter rivers in a number of ways. Most sediment washes or falls into a river as a result of mass movement and runoff. Other sediment erodes from the bottom or sides of the river. Wind may also drop sediment into the water. Abrasion is another process by which a river obtains sediment. Abrasion is the wearing away of rock by a grinding action. The amount of sediment that a river carries is its load. Gravity and the force of moving water cause sediment to move downstream. A river s slope, its volume of flow, and the shape of its streambed all affect how fast the river flows and how much sediment it can erode. A fastflowing river carries more and larger particles of sediment. When a river slows down, it deposits some of its sediment load. Generally, as a river s slope increases, its speed also increases. A river s slope is the amount the river drops toward sea level over a given distance. If a river s speed increases, its sediment load and power to erode may increase. A river s flow is the volume of water that moves past a point on the river at any given time. As more water flows through a river, its speed increases. Friction is the force that opposes the motion of one surface as it moves across another surface. Friction affects a river s speed. Where a river is deep, less water comes in contact with the streambed. This reduces friction and allows the river to flow faster. In a shallow river, there is more friction, which reduces the river s speed. A streambed is often full of boulders and other obstacles. This roughness increases friction and reduces a river s speed. The water moves every which way in a type of movement called turbulence. Turbulence slows a stream s flow, but a turbulent stream has great power to erode. The shape of a river affects the way it deposits sediment. Deposition occurs along the sides of a river, where the water moves more slowly. If a river curves, the water moves fastest on the outside of the curve. There, the river erodes. On the inside of the curve, where the speed is slowest, the river deposits sediment.

4 Name Date Class Glaciers What are the two kinds of glaciers? How does a valley glacier form and move? How do glaciers cause erosion and deposition? A glacier is any large mass of ice that moves slowly over land. There are two kinds of glaciers continental glaciers and valley glaciers. A continental glacier is a glacier that covers much of a continent or large island. Today, continental glaciers cover about 10 percent of Earth s land, including Antarctica and most of Greenland. Continental glaciers can flow in all directions. Many times in the past, continental glaciers have covered large parts of Earth s surface. These times are known as ice ages. For example, about 2.5 million years ago, continental glaciers covered about a third of Earth s land. The glaciers advanced and retreated, or melted back, several times. They retreated for the last time about 10,000 years ago. A valley glacier is a long, narrow glacier that forms when snow and ice build up high in a mountain valley. Valley glaciers are found on high mountains. Glaciers can form only in an area where more snow falls than melts. When the depth of snow and ice reaches more than 30 to 40 meters, gravity begins to pull the glacier downhill. Valley glaciers move, or flow, at a rate of a few centimeters to a few meters per day. Sometimes a valley glacier slides down more quickly in what is called a surge. The movement of a glacier changes the land beneath it. Although glaciers work slowly, they are a major force of erosion. The two processes by which glaciers erode the land are plucking and abrasion. As a glacier flows over the land, it picks up rocks in a process called plucking. Plucking can move even huge boulders. Many rocks remain on the bottom of a glacier, and the glacier drags them across the land. This process, called abrasion, gouges and scratches the bedrock. A glacier gathers huge amounts of rock and soil as it moves. When a glacier melts, it deposits the sediment it eroded from the land, creating various landforms. The mixture of sediments that a glacier deposits directly on the surface is called till. The till deposited at the edges of a glacier forms a ridge called a moraine. A terminal moraine is the ridge of till at the farthest point reached by a glacier. Retreating glaciers also create features called kettles. A kettle is a small depression that forms when a chunk of ice is left in glacial till. When the ice melts, the kettle remains. A kettle that is filled with water is called a kettle lake.

5 Erosion and Deposition Name Date Class Erosion and Deposition Section Summary Waves What gives waves their energy? How do waves erode a coast? What features result from deposition by waves? Ocean waves contain energy sometimes a great deal of energy. The energy in waves comes from wind that blows across the water s surface. The energy that water picks up from the wind causes water particles to move up and down as the waves go by. But the water particles themselves don t move forward. Waves are a major force of erosion along coasts. Waves shape the coast through erosion by breaking down rock and transporting sand and other sediment. One way waves erode the land is by impact. Large waves can hit rocks along the shore with great force. This energy can break apart rocks. Waves also erode by abrasion. As a wave approaches shallow water, it picks up sediment, including sand and gravel. This sediment is carried forward by the wave. When the wave hits land, the sediment wears away rock like sandpaper wearing away wood. Waves coming to shore gradually change direction. The change in direction occurs as different parts of a wave begin to drag on the bottom. The energy of these waves is concentrated on the headlands. A headland is a part of the shore that sticks out into the ocean. Over time, waves erode the headlands and even out the shoreline. Waves shape a coast when they deposit sediment, forming coastal features such as beaches, spits, and barrier beaches. Deposition occurs when waves slow down, causing the water to drop its sediment. As waves reach the shore, they drop the sediment they carry, forming a beach. A beach is an area of wave-washed sediment along a coast. The sediment on a beach usually moves down the beach after it has been deposited. Waves usually hit the beach at an angle instead of straight on. These angled waves create a current that runs parallel to the coastline. As waves repeatedly hit the beach, some of the beach sediment moves down the beach with the current, in a process called longshore drift. One result of longshore drift is the formation of a spit. A spit is a beach that projects like a finger out into the water. Spits occur where a headland or another obstacle interrupts longshore drift. Incoming waves carrying sand may build up sandbars, long ridges of sand parallel to shore. A barrier beach is similar to a sand bar. Barrier beaches are found in many places along the Atlantic coast of the United States.

6 Erosion and Deposition Name Date Class Wind How does wind cause erosion? What features result from deposition by wind? A sand dune is a deposit of wind-blown sand. Wind is the weakest agent of erosion. Yet wind can be a powerful force in shaping the land in areas where there are few plants to hold the soil in place. For example, in deserts, few plants grow, where wind can easily move the grains of dry, light sand. Wind causes erosion by deflation and abrasion. Geologists define deflation as the process by which wind removes surface materials. When wind blows over the land, it picks up particles of sediment. The stronger the wind, the larger the particles that it can pick up and move through the air. Particles that are too heavy to be picked up might skip or bounce for a short distance. Strong winds can even roll heavier sediment particles over the ground. Deflation does not usually have a great effect on the land. In deserts, though, deflation can sometimes create an area of rock fragments called desert pavement. There, wind has blown away the smaller sediment. All that remains are rocky materials that are too heavy to be moved by wind. Abrasion by wind-carried sand can polish rock, but it causes little erosion. At one time, geologists thought such abrasion cut the stone shapes seen in deserts. But evidence now shows that most desert landforms are the result of weathering and water erosion. All sediment picked up by wind eventually falls to the ground. This happens when wind slows down or some obstacle, such as a boulder or clump of grass, traps the windblown sand and other sediment. Wind erosion and deposition may form sand dunes and loess deposits. When wind carrying sand strikes an obstacle, the result is usually a sand dune. Sand dunes can be seen on beaches and in deserts where windblown sediment has built up. Sand dunes come in many shapes and sizes. Sand dunes move over time. Plant roots can help anchor a sand dune in place. Sediment that is finer than sand is sometimes deposited in layers far from its source. This fine, wind-deposited sediment is loess. Large loess deposits are found in central China and in such states as Nebraska, South Dakota, Iowa, Missouri, and Illinois. Loess helps form fertile soil.

Erosion and Deposition

Erosion and Deposition Erosion Sediment natural forces move rock/soil from one place to another. gravity, water, wind, glaciers, waves are causes material moved by erosion Deposition when erosion lays