Differential Weathering - Rates of weathering depend on several factors. Different types of rocks weather at different rates. Certain types of rock, like granite, are very resistant to weathering. Igneous rocks tend to weather slowly because it is hard for water to penetrate them. Other types of rock, like limestone and marble are easily weathered because they dissolve easily in weak acids. More resistant rocks remain at the surface and form ridges or hills.
Devil’s Tower in Wyoming is an interesting example of how different types of rock weather at different rates. As the softer materials of the surrounding rocks were worn away, the resistant center of the volcano remained behind. Different minerals also weather at different rates. Some minerals completely dissolve in water. As less resistant minerals dissolve away, a rock’s surface becomes pitted and rough. When a less resistant mineral dissolves, more resistant mineral grains are released from the rock.
Most importantly, the climate of a region influences weathering. Climate is determined by the temperature of a region plus the amount of rainfall it receives. As the amount of precipitation increases, so does the rate of solution and the number of chemical reactions. In general, as the amount of rainfall increases, so does the degree of weathering. Remember that water is an agent of both mechanical and chemical weathering, so when water is not available, the rate of weathering slows tremendously. Two amazing examples of preservation include mummification and freezing. Both of these situations occur in the absence of liquid water.
There fore a dry climate will produce the lowest rate of weathering, followed by a very cold climate, regardless of the amount of rainfall it receives. The rates of highest weathering would occur in a wet climate that is also warm or hot. As the temperature of a region increases, so does the rate of chemical reactions. For each 10oC increase in average temperature, the rate of chemical reactions doubles.
The warmer a climate is, the more types of vegetation it will have and the greater the rate of biological weathering. This happens because plants and bacteria grow and multiply faster in warmer temperatures. If you want an easy way to remember these examples, think about where you would put your sandwich if you want it to stay fresh for a while. How quickly does it go bad in your lunch box? Where would you put food from the grocery store if you wanted to save it for a week or more?
Some resources are actually concentrated for us by the actions of weathering. In tropical climates, intense chemical weathering carries away all soluble minerals, leaving behind just the least soluble components. The aluminum oxide, bauxite forms this way and is our main source of ore for producing aluminum. The actions of moving water can also concentrate heavier minerals, like gold. This process fueled the gold rush out west in North America in the 1800’s.
EmoticonEmoticon