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Transcript
Exogenous Forces and Weathering The motion picture and television industries have treated us to glimpses of many of the natural wonders of the world. We have scanned the great glaciers and ice fields of the Artic, gone on safari in deepest Africa, toured the beautiful national parks of the United States, and flown over the harshest environments known to humanity. Through the marvels of special-effects camera work, we have experienced earthquakes, hurricanes, tidal waves, and erupting volcanoes. Each of these experiences has allowed us to see very different features of the earth. Thee features have been shaped by natural forces active on our planet. Geologists tell us that these forces not only shape the surface, but are also in continual conflict with each other. The forces of weather, waves, rivers, and glaciers are in continual battle with the rocks of the earth’s crust. Since the origin of our planet, these forces have been wearing down the features of the continents. Countering these destructive forces are other forces. These opposing forces are constructive. They lift up the crust. They build geological formations such as mountains. And they create new rocks. The two types of forces together, the tearing down and the building up operate naturally in a geological cycle. The destructive forces are called exogenous. The constructive forces are called endogenous. In this lesson, we will explore the exogenous forces. Weathering Weathering = Fragmentation and breaking up + = Decay and decomposition The exogenous forces that act upon the earth are a combination of several complex natural events. One of these events of major importance is weathering. Weathering, in turn, involves two processes: fragmentation and decay. That is, rocks both break up and decompose. What causes these two processes of weathering? The breaking up occurs by means of physical weathering. The decomposition occurs by means of chemical weathering. Rock exposed to these two processes changes size and shape. It is then transported by gravity, water, wind, and ice. It is the sun that provides the energy for both chemical and physical weathering. Solar energy caused the pattern of winds that gives us our climates and weather. The sun also supplies energy for chemical reactions. Water and gases from the oceans and the air react with the earth’s rocky surface. The surface is broken up. Material is then transported from one place to another. What we have then are two causes of weathering-physical and chemical. The sun energizes events, both wear down the rocks on the earth’s crust, but only one changes the physical appearance of the rocks. The other chemically changes rocks into new substances. Both are exogenous forces. Summary Two opposing types of forces operate naturally on earth: Exogenous Endogenous These are destructive. These are constructive. One major exogenous force is weathering. Weathering involves two processes: a. Physical weathering changes physical features of rocks: fragmentation or breaking up of the rock. b. Chemical weathering changes rock into new substances: decomposition or decaying of the rock. Rock is exposed to both types of weathering. It changes size and shape. Or it’s altered chemically into a new substance. Then it is transported to some other place by: gravity, water, and ice. Link: http://en.wikipedia.org/wiki/Weathering Link: http://dsc.discovery.com/news/2008/03/06/grand-canyon-formation.html Physical Weathering Physical Weathering is the actual breakup of rocks and minerals into smaller pieces or fragments. In this process fragmentation occurs through physical force: In other words, rocks are fragmented by non-chemical or technical methods. They all mean the same—“physical”, “non-chemical” or “technical”. The final product is the same—rock fragment. You are already familiar with most of these non-chemical methods. Rainfall is one of them. It’s less extreme than others. Yet it does act to wear down rock and transport it somewhere else. Here are some more obvious physical agents of weathering: a. Rainfall b. Running Water: River, Streams, waterfalls c. Wave action: lakes and oceans d. Ice e. Earth movements: earthquakes, landslides f. Winds g. Plant growth These agents do not attack only small rocks. They wear down and destroy larger formations, such as boulders and mountains. Chemical Weathering Minerals in rocks decompose (change) into different Chemical Substances. Chemical weathering occurs when minerals react with water and atmospheric gases. Forms of Chemical Weathering Oxidation- occurs when rock material is exposed to free oxygen. This can happen in water or air. When the rock and oxygen combine, they from chemical oxides Hydration-occurs when water is brought into a mineral compound. This is common to many clay compounds. They swell when they absorb water. Then when they dry they crumble into a powdery substance Carbonation-occurs when carbon dioxide combines with water to from carbonic acid (the acid in soda water). Carbon Dioxide comes from decaying plants and root growth. This acid dissolves many rocks, such as limestone. Erosion: Another Major Exogenous Force Weathering is a slow and constant process. It’s important for the breakup of rock masses. The process that actually sculpts or shapes the crust is erosion. Some erosion may be slow. Erosion differs from weathering in that it involves the transportation of material. The agents of erosion that we see the most are: 1. Running water-deposits most of material 2. Moving ice (glaciers)-common in only high mountain regions or near the poles. Are masses of compact snow and ice? Controlled by gravity, which causes them to flow down hill. Changes hillsides because they scrape it. 3. Wind-Travels at high speeds, the wind can carry large amounts of rock fragments. Wind often carries silt and sand. These Particles carried by the wind also strike against rocks and other features. As they continue to hit a surface they erode it. If the Particles are sand the process is called sandblasting. 4. Gravity All four of these have something in common: they all move and deposit material from on place to another. This deposited material is sediment. The agents of Erosion Running Water: Of these four agents, it is running water that deposits the majority of the earth’s sediments: Streams, creeks, and rivers erode and carry sediment into many of our country’s rivers. This is how the muddy Mississippi got its name. A more extreme example of erosion by running water is a flash flood. A flash flood is swift and powerful. Erosion thus occurs very quickly in a flash flood. Glaciers: The second agent glaciers, is common only in high mountain regions or near the north and south poles. Glaciers and masses of compact snow and ice move soil. They’re formed in regions that have heavy snowfalls and snowfields the year round. Gravity controls glaciers. That is, gravity causes glaciers to flow downhill. As they move downhill, the glaciers pick up dirt, stones, and boulders. The stones and boulders then become embedded or frozen into the ice. As the ice continues moving downhill, the stones and boulders scrape and gouge the land’s surface. This changes the hillside’s shape. Wind: The third agent, wind, is very familiar. When it travels at high speeds, the wind can carry large quantities of rock fragments. Silt and sand are often carried by the wind. Near most natural beaches you’ll find sand dunes huge piles of sand deposited by the wind. The particles carries by the wind also strike against rocks and other features. As they continue to hit a surface, they erode it. If the particles are grains of sand, for example, the process of erosion is called sandblasting. Gravity: Gravity is largely responsible for the erosion of land. When gravity alone pulls soil and rocks downhill, the movement is called mass wasting. But usually another force acts with gravity to speed up the movement. For instance, movements of the earth hasten downhill soil movement to cause rapid landslides. In 1964 a major earthquake occurred in Alaska. Most of damage that resulted was from rock and landslides. This image is in the public domain because it contains materials that originally came from the United States Geological Survey, an agency of the United States Department of Interior. For more information, see the official USGS copyright policy Mudflows are another example. They occur in hilly regions after a series of rainstorms. Mudflows sound harmless, but a mudflow moves down a hillside like a river of mud. It can carry boulders, trees, or even whole houses. Summary Weathering Breaks up rocks. Usually slow and constant. We think of the exogenous forces of weathering and erosion as being destructive. They break down, wear down, and transport the rocks and minerals of the crust. But these events do result in one very beneficial product soil. Soil is crucial to us, of course, because it supports our crops. Soil is formed very slowly. “Slowly” here means hundreds of thousands of years of evolution. You can see why farmers get concerned about soil erosion. We already know that rocks and minerals are found in soil. But what else? We find water and air, and we have a top layer of humus. This is made of tiny bits of decayed leaves, twigs, and animal remains. These are sediments, as are the rocks and minerals. The composition of the humus and rocks and minerals depends on what kinds of minerals were eroded to form the soil. If a soil is sandy, we know sand is or was situated nearby. Not all soils are good for growing plants. Harmful materials, such as pollutants, may have eroded into the soil. The best soil is rich in humus and minerals that support plant growth. These soils are usually found in the temperate regions of the world. Summary Exogenous Forces Weathering and Erosion Sediments Soils In hundreds or thousands of years, the soil matures: The composition of the humus and minerals is dependent on the original sediments. Humus and rocks and minerals are considered sediments. The best soils usually form in climates that are temperate.