Weathering and Erosion
Weathering and Erosion: Introduction
The following information is included so that teachers have additional background knowledge pertaining to the concepts introduced in the CELL. Teachers may choose to use this information to enrich students during instruction, but this is optional and not necessary for the intended students’ learning outcomes.
Weathering is the alteration of rock materials during exposure to air, moisture, and organic matter. Erosion is a composite of all processes by which rock and soil materials are loosened or dissolved and moved from place to place. These two processes have in common the fact that as a consequence of both, a change takes place. How that change takes place, however, is where the two processes differ. Erosion is associated with large scale changes in the form of the Earth. Erosion is responsible for the formation of rivers, valleys, canyons, etc. Weathering is responsible for much more limited changes in the Earth’s surface. Weathering may, for example, soften a rock bed, whereas erosion is able to disintegrate the rock and transport it great distances.
It is important to note that the two processes, weathering and erosion, interact with each other. Considering the example of the rock bed, weathering starts its disintegration while erosion acts to carry the material away. In this way, weathering aids in the process of erosion. In turn, as will later be pointed out, erosion aids in the process of weathering.
In order to accept the concept of weathering one must first understand the units of time necessary for weathering to take place. If a foot of solid rock has been worn away from a cliff face, it would be wrong to assume that a few years or perhaps even a few hundred years were necessary. A rockslide that occurs tomorrow is probably the climax of a weathering process that started thousands of years ago. So when speaking of a mountain being worn to the ground, or a river gouging a canyon into the landscape, we must remember that these processes take a long, long time – thousands or millions of years.
This is a high-speed video sequence of rain-splash. A single raindrop impacts dry sand. Time between individual video frames is 1/240th of a second. The full video sequence is 0.1 seconds in real-time.
In general, water – either liquid or frozen – is the greatest visible factor in weathering. When rain impacts the Earth, small drops of mud splash up and the Earth’s surface is changed ever so slightly. As rains continue, puddles begin to form and then the water follows natural curves in the Earth’s surface. The path it follows might only be decided by gravity. After time passes, these natural paths are deepened by the running water and form rills. Rills are found everywhere but are most evident on the sides of dirt hills on high land masses without much vegetation. After time, rills coalesce to form a small creek. The creek becomes larger and deeper as time passes and more and more water scrapes at its shores and bottom, until it becomes a river.
Rushing water changes the surface of the Earth in many different ways. Water from a creek or river contains a certain amount of minerals, mud, and other solids. This material is carried by the flow of water until the water flow cannot support the suspended material. This material will fall out of suspension and be deposited to form bars or even islands within a particular river basin. The reason one river may carry more mud than another is because of the type of solid materials it flows over and the velocity of the water.
There are two major types of weathering – chemical weathering and mechanical or physical weathering. When discussing chemical weathering the word decomposition is often used because the material being chemically weathered is actually being chemically destroyed or changed to a different chemical compound. When discussing physical weathering, the word disintegration is often used because the material is broken down into small fragments or pieces. In physical weathering the chemical components of rock remain unchanged.
When discussing physical weathering, the six most important factors are: frost wedging, plants, animals, temperature, exfoliation, and gravity. All contribute to weathering of rock by exposing previously unexposed surfaces to weathering. We will look at these separately although they act in concert in most cases.
When water freezes into ice, it expands in volume by about nine percent. When water is put into a closed vessel and then freezes, its bursting pressure reaches thousands of kilos per square centimeter. Thus, tremendous pressures can develop in frost wedging. Water seeps into cracks and crevasses in solid rock and then, when it freezes, it expands cracking open rocks. The net effect is to expose previously unexposed rock surfaces to weathering. Frost wedging is most common in high mountains where bedrock is exposed on steep slopes and freezing at night happens most of the year.
Plants can also cause physical weathering by wedging. As a plant grows, it forces its roots into the ground to anchor it and to obtain food and water. As roots grow, they may actually crack open rocks exposing more rock surfaces to further physical weathering and chemical weathering. Below is a short video, made by LabLearner Founder Dr. Verner, showing weathering and erosion near a roadside in Wisconsin.
Temperature fluctuations during the day/night cycle contribute to physical weathering. Heat from the Sun followed by cooling at night has a weathering effect on rock materials. Temperature weathering is most common in arid areas where there are wide daily ranges in temperature.
The separation, during weathering, of successive, thin shells from rock is termed exfoliation. Exfoliation occurs most readily in moist areas. In exfoliation, the outermost layers weathers, expands and pulls away from the underlying surfaces beneath exposing new surfaces.
Whenever a rock is loosened so that the force of cohesion is less than the force of gravity on the rock, it will fall. A rock may fall off a cliff and land a thousand feet below. The rock is then said to have been transported or moved by gravity. We must take into account the fact that when the rock hits the ground after falling it will most likely break into pieces. This exposes fresh surfaces to weathering.
The two main forces that affect chemical weathering are temperature and moisture. Warmth and high moisture favor chemical reactions, so chemical weathering is more significant in warm, moist areas. Chemical weathering acts from the surface inward (see weathering of a tombstone to the right). That is, the surface of a rock decomposes before its interior. This is evident when a cross-section of a rock being worn chemically is viewed. The inside is smooth and firm, whereas the outside is rough and flaky. Chemical weathering decomposes different rocks in different ways. This is due to the fact that the different rocks are composed of different components.
Chemical weathering is best illustrated by focusing on one particular type of rock, granite, since it is one of the most abundant rocks on Earth. Falling rain passes through the atmosphere acquiring dissolved carbon dioxide. The water and the dissolved carbon dioxide react to form carbonic acid.
After falling to the surface, the carbonic acid seeps through the regolith to the solid rock below. Regolith is the mantle of weathered debris that covers solid rock. The acid then seeps into cracks in the solid rock and reacts with the minerals in the rock, ultimately weakening it.
In considering the chemical weathering of granitic rock, the chemical compounds present in the rock must be considered. The two major minerals in granite are feldspar and quartz. The compound orthoclase in feldspar reacts with carbonic acid according to the following equation:
The products of this reaction are three new chemicals different from any found in feldspar: Potassium carbonate, Kaolinite, and Silica.
Kaolinite is a common mineral found in clay. Clay is highly insoluble and accumulates below the surface of the ground as part of the regolith. Silica is partly soluble and can be partially washed away. Most of it, however, remains in the clay as part of the regolith. Potassium carbonate is the most soluble of the products and most of it is washed away. The potassium of the potassium carbonate is a necessary plant nutrient so much of it is absorbed from the regolith by surrounding plants.
Quartz is the other material found in the granitic rocks. However, quartz, unlike feldspar resists most chemical reactions. As the feldspar is decomposed and forms clay, the quartz crystals are left separated and detached from each other. This makes it easy for water and wind to carry these quartz particles away. When the quartz is carried away by water it may eventually form deposits and become sandstone. The sandstone can then be weathered into sand. However, it should be noted that there are no chemical changes taking place. So, in chemical terms, the grains of sand on a beach are of the same chemical composition as the quartz found in the original granitic rock.
As was stated above, chemical weathering and physical weathering interact with each other. This is apparent in observing a rockslide (which is a form of physical weathering). The rock was softened and weakened by means of chemical weathering, and the climax of the actual rockslide was achieved by means of physical weathering.
Destruction Caused by Weathering and Erosion
Soil Erosion, the Dust Bowl
In the 1930s, a combination of poor agricultural practices and very severe droughts, much of America’s farmlands in the midwest were turned into a wasteland of sand dunes and dead vegetation. Clouds of dust, towering sometimes hundreds of meters into the sky blew away valuable topsoil, making farming in vast areas of the country unfarmable.
This led to major hardships for a large segment of the US economy. In fact, the dustbowl was a major factor in causing and deepening one of the worse economic catastrophes the country has ever witnessed – The Great Depression.
South Dakota Farm, 1936
Since the time of the dustbowl, farmers have created “shelterbelts”, long lines of trees planted across open fields to cut down on the impact of wind on soil erosion. You can see these shelterbelts all across the country. Look for them.
Another area in which erosion can have a major impact on man is in the form of coastal erosion. The world’s oceans contain incalculable amounts of kinetic energy generated by the Earth’s rotation and gravitational forces from the Moon, causing daily tidal fluctuations. Wind kinetic energy drags across thousands of kilometers of ocean surface to creating waves of containing enormous amounts of energy. This kinetic energy is unleashed on made-made structures built too close to the shore.
The Beauty of Erosion
While erosion may be highly destructive and wreak havoc on agriculture and other human activities, some of the most beautiful natural formations in nature are formed by erosion.
The video at the top of this page and the photograph below are both of Horseshoe Bend in Arizona. Look at the impact of water erosion caused by the Colorado River as it has worn away rock over millions of years, creating one of the world’s largest canyons.
The National Geographic Society has an awesome website with breathtaking photography. It also has a good review of the types of erosion and the factors that influence weathering and erosion. Click on the link below to visit this great NGS website.