How does the Surface of the Ground Change?
Weathering is the term used to describe the processes by which rock is disintegrated and decomposed on or near the earth’s surface, due to external forces such as wind, water and temperature. It is the response of earth matter to environmental changes which disturb the equilibrium or balance, which can only be regained by the breakup of rock into smaller pieces, and/or removal of rock material from one place to another. Because of these changes, rock is fragmented by various forces, decomposed by chemical action, or both.
This occurs when physical forces break the rock into smaller and smaller piece3s without changing its mineral compositions. The broken rock material now has more surface material on which chemicals in the environment can act. Therefore, mechanical weathering speeds up the processes of chemical weathering.
Some mechanical weathering processes are:
- Frost wedging – where alternate freezing and thawing of water between rock particles causes them to shift, or break apart. It is most common in mountain regions where it creates large piles of fallen rock material called talus slopes;
- Unloading – when concentric slabs of igneous rock break loose from large bodies of igneous 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.
- Thermal expansion – occurs especially in areas where daily variation is great, and temperature changes weaken the rock, causing it to shatter. This is most likely to occur after rock has been previously weakened by chemical action.
- Organic activity – occurs, for example, when growing plant roots wedge rocks apart, or burrowing animals move material to the surface where weathering can cause it to disintegrate.
Processes that alter the internal structure of minerals by removing or adding minerals result in chemical changes that disintegrate the rock. This can be done by:
- Solution – the dissolving of certain materials (such as very soluble halite or table salt) in water. Acids in the water will increase the solubility (ability to form a solution) of many minerals. Therefore, if acids such as carbonic acid are present in rain, the rate of chemical weathering is greatly increased.
- Oxidation – the process of rusting when oxygen combines with iron the form iron oxide. Mining can hasten this process, and as mine acid breaks up rock, acidic sediments make their way to water systems, killing fish and polluting the water.
This process refers to the downslope movement of rock and soil under the influence of gravity. As rock is broken apart and weakened by weathering, the weight of unstable masses of rock or soil can cause them to roll or fall downslope in avalanches, earthflows, mudflows, or slumps. Slopes that are too steep also cause mass wasting. Material involved in mass wasting includes debris, earth, mud or rock. It may move downward in a fall, a slide or a flow.
Water is moved in the hydrological cycle that results in evaporation and rain. Water is also moved along the earth’s surface by the processes of erosion, which create streams, lakes, valleys, levees, deltas, alluvial fans and a host of other landforms. Erosion achieves its effects by sculpting or moulding the landscape through the action of water (or wind), and by transporting rock and sediment from one place to another. Areas of rich soil, such as the Nile Valley, can result from these processes.
Erosion is also largely responsible for the formation of deserts. In hot areas where vegetation is scarce, or in areas where vegetation has been removed by human or animal activity, the soil is easily picked up by winds, which can transport it great distances. Combined with the greater evaporation of water that occurs in these regions, erosion can lead to the desertification of great areas of land. The mineral soils (sands) that result are easily carried in winds to neighbouring areas, dumping great quantities of sand to create new or larger deserts.
Many present day landscapes were altered by the glaciers of the most recent ice age. The Alps in Europe, Yosemite valley in California, the Great Lakes in North America, and the fiords of Norway and New Zealand were formed by great glaciers that no longer exist. Continental glaciers (in Greenland and Antarctica) are ice sheets covering large portions of land, whereas alpine glaciers are generally found in mountain valleys. 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.
Glaciers are responsible for great amounts of erosion. Because the debris that is carried in them cannot settle, glaciers can carry huge blocks that could not be carried by wind or water. Although glacial action no longer occurs on any great scale, previous erosion by glaciers has deepened, widened and straightened previously narrow, V-shaped valleys, stripped or polished great areas of rock, and created some of the world’s most beautiful hanging valleys.
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.
Birth of a stream
Once land has absorbed all the moisture possible from rain or snow, the excess water runs off in thin sheets of water, which, because earth’s surface is usually not perfectly smooth or even, and under the influence of gravity, quickly form little channels called rills. These move downslope to join, forming a stream.
As rills and streams wear deeper and deeper channels into the soil, more and more water joins the stream, which in turn, has more energy to erode the land and transport sediment.
Water may flow as either laminar flow (smooth, straight line flow) or turbulent flow (confused, swirling flow). This is determined mainly by the speed at which the stream is flowing, and partly by the roughness or smoothness of the stream channel. Laminar flow only occurs when water flows very slowly through a smooth channel. When the velocity increases, as on a slope, or when the channel becomes rough (perhaps from many rocks), the flow becomes turbulent. Because turbulent water moves in many directions, it can erode the channel and keep sediment suspended so that it can be carried downstream.
The rate of flow is different in different parts of a stream. In a straight channel, such as a canal, the water moves fastest in the centre, just below the surface, where there is the least friction. It moves most slowly where there is greatest friction: on the side and bottom of the channel. In a curved or bent channel, the stream flows fastest at the outside of each bend. This results in erosion patterns create wider and deeper curves.
The speed of stream flow will determine how much material it can erode from the land or streambed and how much material, and what sort it can carry. The speed is affected by the gradient, the shape, size and roughness of the channel, and by the amount of water. The higher the gradient (slope), the faster the stream flow. Faster flow is also possible in a semicircular channel, or a wider channel. Finally, a smooth channel allows faster flow, where a rough channel can slow it down.
Deposited stream 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. Among these are:
- Floodplains - low areas flooded by the stream when it is overflows because of heavy rain and runoff;
- Deltas - formed by the repeated depositing of sediments at the place where a stream enters a an ocean or lake and slows down;
- Alluvial plains – form when a mountain stream drops its sediments as it leaves its narrow valley and enters a flat plain or valley floor.