Weathering and erosion are constant processes at Scotts Bluff National Monument that transform its rocky terrain.
Erosion occurs when rocks and minerals are dislodged from their surroundings by means of water, ice or wind; their subsequent deposition elsewhere is known as deposition.
Weathering occurs when a rock gradually wears away over time – an unpredictable process which may last millions or billions of years.
Physical Weathering
Physical weathering refers to any process which breaks apart rocks without altering their chemical makeup, including gravity, water, ice, acids, salts, plants and changes in temperature as the primary agents.
Liquid water seeping through cracks and crevices of rock can freeze when temperatures decrease, acting like a wedge to slowly widen cracks in order to split apart the rock mass – this process is known as cry fracturing or frost weathering.
Weathering occurs frequently in locations with limited soil and plants, particularly where rocks and minerals tend to erode more readily due to less material to resist erosional forces. Erosion transports rock fragments to other locations where they can be deposited for later deposit; erosional forces most frequently include wind, water, gravity, ice or mass wasting (see Chapter 10, Mass Wasting) while its results can be carried away via streams, rivers, glaciers or wind currents (see Chapter 10, Mass Wasting).
Chemical Weathering
Chemical weathering alters the molecular structures of minerals present in rocks by reacting with water or other substances to dissolve their constituent parts. It is most prevalent in wet environments.
Carbon dioxide found in the air can combine with water to form carbonation acid that can break apart rock. Meanwhile, oxygen combined with water forms metal oxides like rust. When left outside in rainy conditions for too long it oxidizes iron and turns it red – another method by which metal oxides form.
At surface conditions, primary minerals found in igneous rocks tend to erode more quickly than sedimentary ones due to being more resilient against chemical weathering than sedimentary ones. Chemical weathering also tends to decline with altitude – something confirmed by previous geomorphological studies which demonstrated it as the major force behind landscape denudation (Rapp 1960). This effect could possibly be attributed to declining vegetative cover at higher elevations as well as increasing snowfall and freezing duration rates at these higher altitudes.
Exfoliation
As rocks are exposed to weather elements, they become vulnerable to erosional processes. Exposure length often determines their level of vulnerability while its location determines its rate of erosion.
Gravity, water, ice, wind and plants all play a part in erosion, the gradual breakdown and movement of sediments over time.
Water’s freezing and thawing cycles loosens rocks’ bonds, enabling chemical weathering to break them down even further. Oxidation, hydrolysis and carbonation are examples of such weathering processes.
Physical weathering can be devastating. Rivers and glaciers scour rocks while abrasion occurs when pebbles grind against river banks and beds similar to how sandpaper wears away wood. Plant roots also play an integral part in mechanically breaking down rocks by growing into cracks in rocks to push them apart, as does rounding, which geologists use as an indicator of what type of weathering and erosion took place over a deposit – well rounded sediment indicating a more intense form of weathering and erosion compared with deposits transported over greater distance and energy transport by the system.
Mechanical Weathering
As forces that cause weathering also break rocks into smaller pieces, mechanical weathering occurs, without altering their chemical composition (for instance abrasion, frost wedging or ice splitting – see Figure 6.1.1). These processes include abrasion, frost wedging or even splitting (figure 6.1.1).
Water freezes to expand by roughly 9%, leading to streets in cold climates to develop potholes at an increased rate. A similar process, known as frost wedging, also breaks apart rock on cliffs and steep slopes.
Abrasion smooths rough and jagged rocks to an even, uniform texture, similar to what happens when water passes over them and knocks them against other rocks and sediments. A similar effect can occur from plants and animals as their roots can find their way into cracks in rocks, forcing the crack widen over time.
Mechanical weathering processes accelerate chemical weathering processes. Rock fragmentation allows more surfaces for chemical weathering reactions to occur and hastens their spread into smaller pieces that will become available for chemical weathering to take place.