What is weathering? Weathering is a set of physical, chemical and biological processes that change the physical and chemical properties of rocks and soil at or near the earth's surface. More about weathering Definition – the breakdown of rock to form sediment [very small pieces of rock] Weathering happens to rocks that are NOT MOVING Weathering is part of the Rock Cycle There are three types of weathering Mechanical [sometimes called physical] Chemical Biological Mechanical weathering breaks rocks down into smaller pieces. Types of mechanical weathering include frost wedging, exfoliation, and thermal expansion. Chemical weathering breaks rocks down chemically adding or removing chemical elements, and changes them into other materials. Chemical weathering consists of chemical reactions, most of which involve water. Biological weathering is the breakdown of rock caused by the action of living organisms, including plants, burrowing animals, and lichens. A lichen is a combination of fungus and algae, living together in a symbiotic relationship. Lichens can live on bare rock, and they break down rocks by secreting acids and other chemicals. Let’s investigate mechanical weathering first. In mechanical weathering, a rock is broken down into smaller pieces without changing its mineral composition. There are several different processes that cause mechanical weathering. Look on page 325 and 326 of your science book to discover how plants, animals, and ice wedging cause mechanical weathering. Observe the effects of mechanical weathering Click on the link below and follow the instructions to observe mechanical weathering: http://www.classzone.com/books/earth_scie nce/terc/content/visualizations/es1201/es12 01page01.cfm?chapter_no=visualization Observe photographs of the different types of mechanical weathering. Click on the link below: http://www.geosci.unc.edu/faculty/glazner/ Images/Weathering/weathering.html Now, let’s look at chemical weathering. In chemical weathering, a rock is broken down by chemical reactions that change its mineral composition and physical and chemical properties Chemical weathering happens when the minerals that make up a rock are changed, leading to the disintegration of the rock Chemical weathering happens quickly in warm, moist environments because water is needed for the chemical reactions. The warm weather speeds up the reactions. Not all minerals are prone to chemical weathering. For example, feldspar and quartz, are common minerals in the rock granite, have very different levels of resistance to chemical weathering. Quartz doesn’t weather very easily, but feldspar does. Over a long time, it chemically changes into clay minerals. Biological weathering – What is it? Biological weathering involves processes that can be either chemical or physical in nature. Biological weathering can be considered special types of mechanical or chemical weathering. Some biological weathering processes are: 1. Rocks can break because of animal burrowing. 2. Tree roots grow into cracks and widen them, which helps physical weathering. 3. Bacteria, lichens and other organisms secrete acidic solutions, which helps chemical weathering. Lichens on rocks cause biological weathering. Plants growing in rocks break the rocks apart. Now let’s move on to EROSION What is erosion? Erosion is defined as the removal and movement of earth materials by natural agents. Some of these agents include glaciers, wind, water, earthquakes, volcanoes, tornadoes, hurricanes, mud flows, and avalanches. How are erosion & weathering different? Weathering involves two processes [mechanical, chemical] that often work together to break down rocks. Both processes occur in place. No movement is involved in weathering. As soon as a rock particle (loosened by one of the two weathering processes) moves, we call it erosion or mass wasting. Mass wasting is simply movement down slope due to gravity. Rock falls, slumps, and debris flows are all examples of mass wasting. We call it erosion if the rock particle is moved by some flowing agent such as air, water or ice. In a nutshell: if a particle is loosened, chemically or mechanically, but stays put, we call it weathering. Once the particle starts moving, we call it erosion. Water is the most important erosional agent and erodes most often as running water in streams or rivers. Water in all its forms is erosional. Raindrops create splash erosion that moves tiny particles of soil. Water collecting on the surface of the soil collects as it moves towards tiny streams and creates sheet erosion. Click on the link below to observe river erosion causing waterfalls: http://www.classzone.com/books/earth_scie nce/terc/content/visualizations/es1305/es13 05page01.cfm?chapter_no=visualization Erosion by wind is known as aeolian erosion (named after Aeolus, the Greek god of winds) and usually occurs in deserts. Aeolian erosion of sand in the desert is partially responsible for the formation of sand dunes. Painted Desert National Monument The erosive power of moving ice is actually a greater than the power of water, however since water is much more common, it is responsible for a greater amount of erosion on the earth's surface. Glaciers cause erosion two ways - they pluck and abrade. Plucking takes place by water entering cracks under the glacier, freezing, and breaking off pieces of rock that are then moved by the glacier. In abrasion, the glacier cuts into the rock under the glacier, scooping rock up like a bulldozer and smoothing and polishing the rock surface. Athabasca Glacier - Jasper, Canada Waves in oceans and other large bodies of water cause coastal erosion. The power of ocean waves is awesome; large storm waves can produce 2000 pounds of pressure per square foot. The pure energy of waves along with the chemical content of the water is what erodes the rock of the coastline. Wave action in Calvert County Maryland Watch wave erosion: http://www.classzone.com/book s/earth_science/terc/content/vis ualizations/es1606/es1606page 01.cfm?chapter_no=visualizatio n Observe landscapes formed by erosion: http://www.classzone.com/book s/earth_science/terc/content/vis ualizations/es1205/es1205page 01.cfm?chapter_no=visualizatio n Runoff Runoff is water that doesn't soak into the ground or evaporate but instead flows across Earth's surface. Runoff does not happen randomly, it depends on several factors. For example, if light rain falls for several hours then runoff most likely won't form because the water will have time to evaporate. But, if heavy rain falls for less than an hour the water will not have time to evaporate or soak up and runoff will form. It also depends where the water falls. When it rains on a steep slope, gravity pulls the water down the slope, therefore it becomes runoff. It is runoff because it is roaming the Earth's surface having not been evaporated or soaked up. Runoff mainly causes three types of erosion: rill erosion, gully erosion, and sheet erosion. Rill Erosion Rill erosion occurs when runoff flows and a small stream forms and carries away plants and soil leaving a scar or channel on the side of the slope. A rill is the channel left on the side of the slope. Gully Erosion Gully erosion is when a rill channel becomes much deeper and wider which forms a gully. Gully erosion is basically rill erosion on a much higher level. Sheet Erosion Sheet erosion is when water flowing into lower elevations loses energy and leaves behind sediments that cover the soil like a sheet. Floodwaters spilling our of rivers often flow in sheets and can cause sheet erosion. Deposition As water moves through a river system, it loses some of its energy and slows down. As it slows, the water can no longer carry everything in it. As a result, the water drops sediment to the bottom of the stream. This process is called deposition. Deltas and Alluvial Fans The sediments dropped by the slowing water (deposition) falls to the floor of the water or river bank. The collection of sediments from a lake or ocean is called a delta. The collection of sediments from a river from a mountain valley to a flat plain is called an alluvial fan. These sediments provide lots of nourishment for the vegetation around them. Agriculture around the Nile River and the Mississippi River depend on them. The End… Use any remaining time to double check your worksheet Then, turn it in!
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