Updated April 25, 2017 By Ethan Shaw
Weathering and erosion, along with the gravity-driven effect called mass wasting, are the fundamental processes by which rock is broken down and removed, collectively called denudation. The most important agent in both weathering and erosion is water, in both its liquid and solid states. From slightly acidified groundwater gnawing at limestone to a huge, boiling river tearing at bedrock, water dismantles the continents even as they’re built up through deposition, volcanic and tectonic action.
It’s important to distinguish between weathering and erosion, which are sometimes mistakenly transposed. Weathering is an action of rock-breaking or rock-rotting essentially in place; it doesn’t involve significant transport of the resulting fragments. Erosion refers to a larger-scale action in which rock is removed and transported. In mass wasting, meantime, gravity moves rock fragments down slopes by gravity; it’s commonly the intermediary stage between weathering and erosion.
Water is intimately involved with some of the most widespread and important forms of weathering. The interplay between its liquid and solid forms accomplishes frost-wedging mechanical weathering: Water penetrates crevices and joints in rock, then freezes within them when temperatures drop. Because water expands when it transforms to solid ice, it pries the sides of the fracture farther apart. This, in turn, affords deeper access to the liquid water once the ice melts. This cycle continues relentlessly, broadening cracks and eventually shattering off plates and chunks of rock. A similar, though less important process -- salt wedging -- occurs in arid climates where water in rock fractures evaporates and leaves behind salt crystals that expand and exert pressure. Water is a primary medium of chemical weathering, in which rock is altered at its mineral level -- as through oxidation or carbonation, in which dissolved oxygen or carbon dioxide, respectively, in water interacts with and changes rockbound minerals.
Water is by far the most important global agent of erosion. In its solid form, as glacial ice, it is certainly an impressive, bulldozing force, responsible for carving mountain peaks into sharp-toothed horns, knife-edged arête ridges and huge cirque basins while razing lowlands and scouring lakebeds. But moving water -- from ephemeral rivulets and boulder-clattering rivers to pounding ocean waves -- acts on a much larger collective scale, gullying slopes and scouring out canyons and channels while dismantling sandbars and carving seacliffs. A river’s action is intimately bound with weathering and mass wasting, as much of its erosive work is transporting away the products of those operations.
Other agents and processes besides water can accomplish weathering and erosion. Exfoliation is a manifestation of weathering in which plates or slabs of rock slough off a parent dome or boulder, commonly observed in granite. Geologists aren’t entirely agreed as to what causes exfoliation -- chemical weathering via water is a possibility -- but changes in pressure or temperature as an intrusive mass of rock is exposed through erosion have been hypothesized. Biological weathering encompasses the influence of living organisms on rock-breaking. For example, lichens, those symbiotic associations of algae and fungi that commonly colonize bare stone, can leach out minerals from rock and weaken it, as well as grind away miniscule particles by expanding and contracting with wetting and drying. Wind can be a notable agent of erosion, abrading rock with airborne particles and removing ground layers of sand and silt. The footprints that astronauts left on the Moon will be there forever. Why? This is because the Moon has no atmosphere and, as a result, has no weathering. Weathering is one of the forces on Earth that destroy rocks and landforms. Without weathering, geologic features would build up but would be less likely to break down.
Weathering is the process that changes solid rock into sediments. Sediments were described in the Rocks chapter. With weathering, rock is disintegrated. It breaks into pieces. Once these sediments are separated from the rocks, erosion is the process that moves the sediments. Erosion is the next chapter’s topic. The four forces of erosion are water, wind, glaciers, and gravity.
While plate tectonics forces work to build huge mountains and other landscapes, the forces of weathering gradually wear those rocks and landscapes away. Together with erosion, tall mountains turn into hills and even plains. The Appalachian Mountains along the east coast of North America were once as tall as the Himalayas.  Figure 1. A once smooth road surface has cracks and fractures, plus a large pothole. No human being can watch for millions of years as mountains are built, nor can anyone watch as those same mountains gradually are worn away. But imagine a new sidewalk or road. The new road is smooth and even. Over hundreds of years, it will completely disappear, but what happens over one year? What changes would you see (figure 1)? What forces of weathering wear down that road, or rocks or mountains over time? Follow this link to view some animations of different types of weathering processes.
Mechanical weathering (also called physical weathering) breaks rock into smaller pieces. These smaller pieces are just like the bigger rock, just smaller. That means the rock has changed physically without changing its composition. The smaller pieces have the same minerals, in just the same proportions as the original rock. There are many ways that rocks can be broken apart into smaller pieces. Ice wedging is the main form of mechanical weathering in any climate that regularly cycles above and below the freezing point (figure 2). Ice wedging works quickly, breaking apart rocks in areas with temperatures that cycle above and below freezing in the day and night, and also that cycle above and below freezing with the seasons. Figure 2. Ice wedging. Ice wedging breaks apart so much rock that large piles of broken rock are seen at the base of a hillside, as rock fragments separate and tumble down. Ice wedging is common in Earth’s polar regions and mid latitudes, and also at higher elevations, such as in the mountains. Abrasion is another form of mechanical weathering. In abrasion, one rock bumps against another rock.
Abrasion makes rocks with sharp or jagged edges smooth and round. If you have ever collected beach glass or cobbles from a stream, you have witnessed the work of abrasion (figure 3). Figure 3. Rocks on a beach are worn down by abrasion as passing waves cause them to strike each other. Now that you know what mechanical weathering is, can you think of other ways it could happen? Plants and animals can do the work of mechanical weathering (figure 4). This could happen slowly as a plant’s roots grow into a crack or fracture in rock and gradually grow larger, wedging open the crack. Burrowing animals can also break apart rock as they dig for food or to make living spaces for themselves. Figure 4. (a) Human activities are responsible for enormous amounts of mechanical weathering, by digging or blasting into rock to build homes, roads, subways, or to quarry stone. (b) Salt weathering of building stone on the island of Gozo, Malta. Mechanical weathering increases the rate of chemical weathering. As rock breaks into smaller pieces, the surface area of the pieces increases figure 5. With more surfaces exposed, there are more surfaces on which chemical weathering can occur. Figure 5. Mechanical weathering may increase the rate of chemical weathering.
Chemical weathering is the other important type of weathering. Chemical weathering is different from mechanical weathering because the rock changes, not just in size of pieces, but in composition. That is, one type of mineral changes into a different mineral. Chemical weathering works through chemical reactions that cause changes in the minerals. Most minerals form at high pressure or high temperatures deep in the crust, or sometimes in the mantle. When these rocks reach the Earth’s surface, they are now at very low temperatures and pressures. This is a very different environment from the one in which they formed and the minerals are no longer stable. In chemical weathering, minerals that were stable inside the crust must change to minerals that are stable at Earth’s surface. Figure 6. Deforestation in Brazil reveals the underlying clay-rich soil. Remember that the most common minerals in Earth’s crust are the silicate minerals. Many silicate minerals form in igneous or metamorphic rocks. The minerals that form at the highest temperatures and pressures are the least stable at the surface. Clay is stable at the surface and chemical weathering converts many minerals to clay (figure 6). There are many types of chemical weathering because there are many agents of chemical weathering. Water is the most important agent of chemical weathering. Two other important agents of chemical weathering are carbon dioxide and oxygen.
A water molecule has a very simple chemical formula, H2O, two hydrogen atoms bonded to one oxygen atom. But water is pretty remarkable in terms of all the things it can do. Remember from the Earth’s Minerals chapter that water is a polar molecule. The positive side of the molecule attracts negative ions and the negative side attracts positive ions. So water molecules separate the ions from their compounds and surround them. Water can completely dissolve some minerals, such as salt. Follow this link to check out this animation of how water dissolves salt. Hydrolysis is the name of the chemical reaction between a chemical compound and water. When this reaction takes place, water dissolves ions from the mineral and carries them away. These elements have undergone leaching. Through hydrolysis, a mineral such as potassium feldspar is leached of potassium and changed into a clay mineral. Clay minerals are more stable at the Earth’s surface.
Carbon dioxide (CO2) combines with water as raindrops fall through the atmosphere. This makes a weak acid, called carbonic acid. Carbonic acid is a very common in nature where it works to dissolve rock. Pollutants, such as sulfur and nitrogen, from fossil fuel burning, create sulfuric and nitric acid. Sulfuric and nitric acids are the two main components of acid rain, which accelerate chemical weathering (figure 7). Acid rain is discussed in the Human Actions and the Atmosphere chapter. Figure 7. This statue has been damaged by acid rain.
Oxidation is a chemical reaction that takes place when oxygen reacts with another element. Oxygen is very strongly chemically reactive. The most familiar type of oxidation is when iron reacts with oxygen to create rust (figure 8). Minerals that are rich in iron break down as the iron oxidizes and forms new compounds. Iron oxide produces the red color in soils. Figure 8. When iron rich minerals oxidize, they produce the familiar red color found in rust. Now that you know what chemical weathering is, can you think of some other ways chemical weathering might occur? Chemical weathering can also be contributed to by plants and animals. As plant roots take in soluble ions as nutrients, certain elements are exchanged. Plant roots and bacterial decay use carbon dioxide in the process of respiration.
Weathering rates depend on several factors. These include the composition of the rock and the minerals it contains as well as the climate of a region.
Different rock types weather at different rates. Certain types of rock are very resistant to weathering. Igneous rocks, especially intrusive igneous rocks such as granite, weather slowly because it is hard for water to penetrate them. Other types of rock, such as limestone, are easily weathered because they dissolve in weak acids. Figure 9. Devil’s Tower is the central plug of resistant lava from which the surrounding rock weathered and eroded away. Rocks that resist weathering remain at the surface and form ridges or hills. Devil’s Tower in Wyoming is an igneous rock from beneath a volcano (figure 9). As the surrounding less resistant rocks were worn away, the resistant center of the volcano remained behind. Different minerals also weather at different rates. Some minerals in a rock might completely dissolve in water but the more resistant minerals remain. In this case, the rock’s surface becomes pitted and rough. When a less resistant mineral dissolves, more resistant mineral grains are released from the rock.
A region’s climate strongly influences weathering. Climate is determined by the temperature of a region plus the amount of precipitation it receives. Climate is weather averaged over a long period of time. Chemical weathering increases as:
So how do different climates influence weathering? A cold, dry climate will produce the lowest rate of weathering. A warm, wet climate will produce the highest rate of weathering. The warmer a climate is, the more types of vegetation it will have and the greater the rate of biological weathering (figure 10). This happens because plants and bacteria grow and multiply faster in warmer temperatures. Figure 10. Wet, warm tropical areas have the most weathering. Some resources are concentrated by weathering processes. In tropical climates, intense chemical weathering carries away all soluble minerals, leaving behind just the least soluble components. The aluminum oxide, bauxite, forms this way and is our main source of aluminum ore. LESSON SUMMARY
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