Climate, the long-term weather pattern over a geographic area, affects rocks and soil exposed to the elements. The amount of precipitation, the daily temperature, and the fluctuation in temperatures over a year affect the rate of weathering of rocks. Weathering is the breakdown of the surface minerals of rocks so that they can be transported by erosion. Climate affects both chemical and mechanical weathering rates. Chemical weathering is the process whereby minerals are altered by chemical agents. Mechanical weathering is the physical breakdown of rocks into smaller pieces without a change in chemical composition.
Water and climate act on rock outcrop.
Expansion and Contraction
Expansion and contraction is a form of mechanical weathering. The thermal heating and cooling of the rocks cause them to expand and contract over time. The expansion and contraction can cause the rocks to crack. Rocks that are foliated may break along planes of foliation, sloughing off the exposed rock face. Rocks are susceptible to this type of weathering in climates where there is a large fluctuation in daily temperature. Rock outcrops in the desert will tend to weather by expansion and contraction
Frost Action
Frost action is a type of mechanical weathering that occurs in cool, moist climates. Precipitation falls on the surface, infiltrates into the soil and fractures in the rock face. As the water cools and freezes, it expands up to 9 percent. The expansion in the fractures forces them further apart and breaks up the rock. Frost action occurs most often in wet climates that have wide temperature fluctuations across the freezing point.
Hydrolysis and Dissolution
Hydrolysis and dissolution are forms of chemical weathering. During hydrolysis, water and minerals react with one another creating new minerals or a dissolved material. An example of hydrolysis is when water reacts with feldspar minerals, forming kaolinite, a clay mineral. This process occurs fastest in climates that have rainy seasons or heavy precipitation throughout the year. Dissolution is the breakdown of rocks as minerals dissolve in acid rain. Dissolution will break down limestone quickly, causing sinkholes or karst topography. Dissolution is most rapid in climates that have a lot of acid precipitation.
Mechanical Weathering Rates
Strong mechanical weathering takes place when the annual temperature is 14 degrees and there is about 59 inches of precipitation annually. Moderate mechanical weathering takes place at temperatures between 14 and 32 degrees and about 39 inches of rainfall annually. Slight mechanical weathering takes place where temperatures are above 32 with 20 inches of rainfall annually. A combination of chemical and mechanical weathering takes place when the temperature is between 32 and 50 and there is between 20 and 39 inches of annual rainfall.
Chemical Weathering Rates
Strong chemical weathering takes place when the temperature is between 50 and 68 degrees and the annual rainfall is between 59 and 78 inches. Moderate chemical weathering takes place when the temperature is between 50 and 68 degrees and the annual rainfall is 19 to 59 inches. Very little weathering takes place below 19 inches of annual rainfall.
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