Rates of Weathering
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||carbonation, weather-resistant, crystalline, obelisk, calcite, topography, all-terrain, differential, guano, sandstone, prior, quartz, sedimentary, shale, composition, undisturbed
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Rates of Weathering
By Trista L. Pollard
1 As dramatic as the process of weathering sounds, it does not happen overnight. In fact, some instances of mechanical and chemical weathering may take hundreds of years. An example would be the dissolving of limestone through carbonation. Limestone dissolves at an average rate of about one-twentieth of a centimeter every 100 years. If you want to see a layer of limestone (about 150 meters thick) dissolve, plan on watching that layer for about 30 million years.
2 Where we see the effects of weathering often is on our stone monuments and buildings and large rock structures. However, before you can analyze the rate at which these structures are weathering, you need to understand the factors that affect weathering rates. The weathering rate for rocks depends on the composition of the rock; the climate of the area; the topography of the land; and the activities of humans, animals, and plants.
3 A rock's composition has a huge effect on its weathering rate. Rock that is softer and less weather-resistant tends to wear away quickly. What is left behind is harder, more weather-resistant rock. This process is called differential weathering. Quartz is one type of rock whose composition, especially its crystalline structure, makes it resistant to mechanical and chemical weathering. This is why quartz remains unchanged on the Earth's surface after surrounding sedimentary rock has been eroded. There are some rocks, like limestone, that weather more rapidly. Limestone has the compound calcite. It is the carbonization of calcite that causes the increased rate of weathering of limestone. The material found in sediment grains also affects the rate of weathering. The mechanical weathering of rocks like shale and sandstone causes their grains to break up over time and become sand and clay particles. Why? Well, the grains in these two types of rocks are not cemented together firmly. Rocks like conglomerates and sandstones have grains that are cemented strongly with silicates. These rocks and other similar types tend to resist weathering. Geologists have also found that they may resist weathering longer than some types of igneous rocks.
4 A rock's exposure to the weathering elements and its surface area can affect its rate of weathering. Rocks that are constantly bombarded by running water, wind, and other erosion agents, will weather more quickly. Rocks that have a large surface area exposed to these agents will also weather more quickly. As a rock goes through chemical and mechanical weathering, it is broken into smaller rocks. As you can imagine, every time the rock breaks into smaller pieces its surface area or part exposed to weathering is increased. Think about a cube, which has both volume and surface area. To find the surface area of a cube, you need to calculate the sum of the areas for all six sides. Let this cube represent our rock that is exposed to weathering. Already our cube has six sides that are exposed to the elements. If we split our cube into eight smaller cubes, then the total surface area would be doubled. Although the surface area increases, the volume remains constant. Splitting the eight smaller cubes in the same way would have the same effect; the surface area would again be doubled. Increased surface area causes rocks to weather more rapidly.
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