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Weathering and Topography Help (page 2)

By — McGraw-Hill Professional
Updated on Sep 4, 2011

Weathering Factors

Rocks are formed at high temperatures and pressures deep within the Earth. Through tectonic activity, they can become exposed to surface conditions like low temperatures and pressures, as well as air and water. When exposed rock sees these new conditions, they react and begin to change. When this happens, it is known as weathering .

There are a few weathering rules-of-thumb to remember when trying to figure out the weathering pattern in a certain area. These include, rock type and structure, existing soil and weathering, slope, climate, and time. These factors, alone or combined, can make a difference in how an area of rock weathers.

Different minerals react differently to weathering action. Quartz is very resistant to chemical weathering and so are rocks containing any quartz. That is why mountains made up of rock with high amounts of quartz are still standing in areas where their sedimentary neighbors were long ago flattened.

Weathering rinds on basaltic stones tell geologists a lot about rates of weathering. Rinds with depths of 1–4 mm thick can point to the weathering history of a certain rock giving geologists information about surface exposure.

Texture is also an important player in weathering. A rock that is almost totally quartz, like quartzite, will be pretty open to weathering if it has a lot of joints and spaces that allow water to seep in.

Along with texture is the presence or absence of soil. The presence of soil, a breakdown product of rock, seems to add to weathering as well. When more and more surface area is exposed, either through the washing away of soil from bare rock or as rock gets increasingly jointed, then weathering can reach into more places.

Depending on composition and texture, weathering can go on at different rates in the same geographical region. This is known as differential weathering . It includes variable weathering of rocks with different compositions and structures, as well as a change in the intensity. That is why some cliffs with alternating hard and soft rock have curves and ridges where the harder rock has resisted weathering and the softer rock has given way.

Weathering is the breakdown and disintegration of rock into smaller pieces of sediment and dissolved minerals.

An area’s slope can speed or slow weathering. When a slope is steep, loosened mineral grains are washed down to the bottom with the help of gravity. They are often carried far away. Additionally, a steep, constantly exposed rock face will lose minerals as weathered grains are removed and new ones exposed. This type of slope, depending on composition of the exposed rock, may weather much faster than a less inclined slope. Gentle slopes also experience less vertical gravity pull. Loosened grains usually collect in piles, many meters thick, along the length of the slope.

Climate can also contribute to weathering changes. Heat and humidity speed up chemical and biological weathering and assist water’s penetration of rock. Geologists usually find much greater weathering of limestone and marble in hot, wet climates because of the dissolving effect on calcite. Cold, dry climates have more frost-driven fracturing and weathering. We will look at these weathering factors in more detail.

Time is the biggest factor of weathering. Some hard rocks like granite are changed by different weathering factors only over thousands and millions of years. But knowing a rock’s composition and its vulnerability to weathering, gives geologists important information about a region’s geological history. Table 15-1 shows the different factors that affect the rate of speed of rock weathering.

Table 15-1 Weathering happens at different rates depending on different conditions.

Rock characteristics

Weathering rate

Solubility

low

medium

high

Structure

immense

has weak points

highly fractured

Rainfall

low

medium

high

Temperature

cold

moderate

hot

Soil layer

no soil (bare rock)

thin to medium

thick

Organic activity

negligible

moderate

plentiful

Exposure time

brief

moderate

lengthy

 

Weathering takes place in two ways: physical weathering and chemical weathering . Physical and chemical weathering can go on at the same time. This is like a one-two punch. When a rock gets broken into pieces, more of its surface is exposed to the air. This causes more of its surface area to be uncovered to chemical weathering, which in turn breaks it down into smaller bits of rock.

Environmental factors can also cause weathering to take place at different speeds. Mechanical methods like rock smashing and cracking take place in cold and/or arid climates where water is meager. Chemical methods take place in warm and/or moist climates where there is lots of water. Biological methods take place in many different environments across the planet.

Practice problems of this concept can be found at: Weathering and Topography Practice Test

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