Rock type, structure, and climate primarily control weathering rates. Rocks composed of minerals that are relatively unaffected by chemical weathering will be the most resistant to weathering. For example, quartz is unaffected by dissolution, hydrolysis and oxidation, therefore, rocks composed almost exclusively of the mineral quartz are more resistant than other common rock types. Rocks such as sandstone and quartzite often form resistant ridges separating valleys formed in weaker rocks. Sand on a beach is made up almost exclusively of quartz grains as chemical weathering alters less resistant feldspar minerals and the resulting clays are deposited elsewhere by streams or shoreline currents.
Rock structures such as fractures represent natural surfaces for physical and chemical weathering processes. Consequently, rocks that contain abundant fractures are typically weathered more rapidly than equivalent unfractured rocks.
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| Weathering has been concentrated along vertical fractures separating these panels of sandstone at Arches National Park, Utah. |
A temperature range that varies between freezing and thawing conditions and plentiful supply of water are essential for physical weathering. Warm temperatures and a plentiful supply of water are necessary for chemical weathering. Areas with little water or low temperatures have relatively slow rates of chemical weathering.
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How would climate influence weathering rates at locations on the map? |
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- Tropical rainforest (equatorial regions including South America, Africa, Indonesia, southeast Asia): chemical weathering rates would be rapid as these regions have both high temperatures and plenty of rainfall.
- Hot desert (subtropical regions including North Africa [Sahara], southwest South America [Atacama], southwest Africa [Namib], Asia [Gobi], southwestern U.S., central Australia): plenty of heat but insufficient water to cause significant physical and/or chemical weathering.
- Temperate mountains (Rocky Mountains, Sierra Nevada Mountains, Alps, Andes Mountains): insufficient temperatures for rapid chemical weathering but elevations contribute to freeze-thaw cycles necessary for ice wedging.
- Polar Regions (Alaska, Antarctica, Siberia): too much cold weather to permit thawing. Water in solid form (ice) unable to react with rock.
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| Inscription carved in granite on Independence Rock, Wyoming. Settlers travelling on the Oregon Trail aimed to arrive at the landmark before July 4 to ensure sufficient time to cross the mountains before winter. The inscription is was carved by someone with the surname Fox and is dated June 29, (18)52. The phrase CIN. Ohio can be seen in the top right corner. The inscription is still preserved due to the dry climate of the region. |
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