Forces That Might Have Shaped a Local Environment
Weathering
Weathering is the term used to describe the processes by which rock is broken down into smaller rocks, sand or soil, and is a result of the action of wind, water and temperature. Weathering is caused by many different factors, including the following:
Frost - where alternate freezing and thawing of water causes rock particles to break apart. This is most common in mountain regions (e.g. in Berner Oberland, south-central <st1:country-region>Switzerland</st1:country-region>) where it creates large piles of fallen rock material called talus slopes.
Unloading – where slabs of igneous rock (deposited by volcanoes) break loose from large bodies of rock. This sheeting action occurs after erosion of surface soil and plants has removed the pressure from igneous rock (eg. Granite), and outer layers of rock expand more quickly than underlying rock. As erosion. Sheeting or unloading can be seen in many parts of Yosemite National Park in the <st1:country-region>U.S.A.</st1:country-region>
Great temperature variation – where great daily temperature changes occur, as in desert environments, they can cause rock to weaken or shatter. This is most likely to occur after rock has been previously weakened by chemical action. Weathering in deserts, where daytime temperatures can be very hot and drop to very cold night time temperatures, is speeded up in this way.
Organic activity – where growing plant roots wedge rocks apart, or burrowing animals move material to the surface where weathering can cause further disintegration.
Chemical weathering – caused by minerals in the water or air that either eat into the rock or change to chemical makeup of the rock. For instance, mining or industrial development can cause the air to become more acidic, speeding up the natural chemical action of rain.
Mass wasting
This refers to the collapsing or sliding down of large areas of rock, soil or snow, as in avalanches, earthflows, mudflows, or slumps. Mass wasting results when the weight of unstable masses of rock or soil or oversteep slopes cause the downward slide.
Erosion
This occurs when water (and wind) wear away rock or soil and carry them from one place to another. Streams, lakes, valleys, levees, deltas, alluvial fans and a host of other landforms are created by erosion. It can create areas of rich soil, such as the Nile Valley, or spectacular land forms, like the Grand Canyon in North America. Erosion is also largely responsible for the formation of deserts, such as the Simpson Desert in <st1:country-region>Australia</st1:country-region> or the Sahara in Africa.
Glaciers
These were responsible for great amounts of erosion, and have created many of the land forms that we know, including the Alps in Europe, Yosemite Valley in California, the Great Lakes in North America, and the fiords of <st1:country-region>Norway</st1:country-region> and <st1:country-region>New Zealand</st1:country-region>. These were formed by great glaciers that no longer exist. Glaciers are also very important because they hold great amounts of earth’s water supply; almost 2/3 of earth’s fresh water is in Antarctica’s glacier.
Streams
In geological language, the word ‘stream’ is used for any flow of water in a channel, from a tiny trickle to a great river. Streams return water that has been evaporated from the oceans back to the oceans, and carry with them sediments that enrich areas of land. Water runoff from land areas helps maintain the balance of water in the earth’s oceans. In addition, streams erode the earth’s surface, and are the most important agent in sculpting the land.
Streams also change the earth’s surface by depositing sediment. Since the beginnings of agriculture, human beings in some parts of the world have relied on stream deposits to provide them with rich soil able to sustain crops. Landforms created by these deposits from streams include:
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Floodplains - low areas flooded by the stream when it is overflows because of heavy rain and runoff;
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Deltas - formed by the repeated depositing of sediments at the place where a stream enters a an ocean or lake and slows down;
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Alluvial plains – form when a mountain stream drops its sediments as it leaves its narrow valley and enters a flat plain or valley floor.
