By Trista L. Pollard /
1As 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.
2Where 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.
3A 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.
4A 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.
5There are very few smooth rocks on our planet. So it should not surprise you that the majority of our rocks have fractures and joints. Both increase the surface area of rocks because they are natural zones of weakness. Fractures and joints provide new natural pathways for running water. Once inside, this water penetrates rocks further causing the rocks to break apart. Water is the number one ingredient in ice wedging; fractures and joints speed up that process. Chemical weathering also occurs more rapidly. As the water and other compounds enter the rock, more material is removed from the fractures and joints. This causes the structure to become weaker, which increases its rate of weathering.
6Climate and topography are other influences on weathering. In areas where the climate changes from hot weather to cold weather frequently, rocks tend to weather more quickly. A major component of ice wedging is the freezing and thawing. Once these rocks are broken down, then chemical weathering begins its job. High temperatures also increase the rate that chemical reactions occur. Rocks found in warm, humid climates tend to go through chemical weathering rapidly. However, not all hot climates cause an increase in the weathering rate. Geologists have found that weathering rates are slower in hot, dry climates.
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8Weathering rates also occur more slowly in very cold climates. The impact of climate can be seen on Cleopatra's Needle in New York City. This structure or obelisk is made of granite. Prior to its journey to America, it was located for 3,000 years in Egypt. Pictures of the structure before 1880 showed ornate markings and drawings on the obelisk's surface. Once it was brought to New York City in 1880, it became exposed to the city's climate, pollution, ice wedging, and acid precipitation. All it took was 100 years for the rapid weathering to wear away this ornate surface.
9A land's elevation and slope of its surface or topography affect the weathering rates of rocks. Rocks found at higher elevations will weather most often from ice wedging than rocks at lower elevations. The rate of weathering on mountainsides and steep slopes occurs more rapidly than rocks located on flatter land surfaces.
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11The last two factors that influence weathering rates are human activities and plant and animal activities. We cause chemical and mechanical weathering of rocks through mining and construction practices. Both expose new rock surfaces to weathering agents. Mining companies use acids and chemical compounds when extracting ores. These acids and compounds, when combined with water and other solvents, cause chemical weathering to occur in the area. Soil is removed on construction sites to make way for new buildings and structures. Once the soil is removed, rock surfaces that weren't exposed become exposed to the elements. The use of recreational vehicles and other activities also speed up the weathering process. When we ride all-terrain vehicles and hike on trails, we expose new rock surfaces to weathering. Human activity weathers rocks more rapidly than rocks in undisturbed areas. The same can be said for rocks disturbed by plants and animals. As plants and trees grow, their roots break apart neighboring rock. Animals that make their homes by burrowing into rocks and soil disturb rock and expose new surfaces. These exposed surfaces weather more rapidly. Animal wastes also accelerate the rate of chemical weathering. Some forms of animal wastes, like bat guano, attract millipedes and other insects. These creatures speed up the process of mechanical weathering on the surfaces below the guano. The compounds in the waste also speed up the chemical weathering process. All of these factors influence weathering processes that usually occur at a very slow pace.
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Rates of Weathering
1. / True or False: Sediment grains that are held together strongly tend to resist weathering better than sediment grains that are held together loosely.False
True
/ 2. / Describe the effect climate change had on Cleopatra's Needle in New York City.
3. / ______increase the surface area of rocks and, as a result, the rocks' weathering rate.
Mineral composition and structure
Joints and nodules
Fractures and joints
Fractures and nodules
/ 4. / What is differential weathering?
5. / Describe how rock composition affects the rate of weathering.
/ 6. / Which do you expect to weather at a faster rate- large boulders or smaller stones? Why?
7. / Rising temperatures cause the rate of chemical weathering to ______.
Accelerate
Decelerate
Stay constant
None of the above
/ 8. / True or False: Ice wedging commonly occurs at lower elevations.