Section 4 - Unit 1: Weathering Processes and Mass Wasting
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Weathering and Mass Wasting
By the end of this unit, you will be able to:
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Weathering is the process that changes solid rock into sediments. With weathering, rock is disintegrated into smaller pieces. Once these pieces (sediments) are separated from the rock, erosion is the process that moves the sediments away from its original position. The disintegration and decomposition of rock occurs through physical and/or chemical weathering, and they usually work together to degrade the landscape, making it easier for the agents of water, wind, ice and waves to erode and transport.
Bedrock is consolidated parent rock , which the forces of weathering act on. The upper surface of bedrock generally undergoes continuous weathering. Joints and fractures in bedrock will increase the surface area of rock exposed to weathering. Differential weathering describes the fact that different rocks weather at different rates. Often the first product of weathering bedrock is regolith - broken down and weathered bedrock. As this regolith breaks down further, sometimes combining with outside sources of sediment brought in by transporting agents, soil formation begins! Soil is the dynamic link between the organic and inorganic world. It is composed of decayed, decaying and broken down rocks, minerals and organic material. Organic is matter that is or was once alive (comes from above or near surface). Inorganic is material that was never alive, and typically comes from below (from bedrock and regolith).
Bedrock is consolidated parent rock , which the forces of weathering act on. The upper surface of bedrock generally undergoes continuous weathering. Joints and fractures in bedrock will increase the surface area of rock exposed to weathering. Differential weathering describes the fact that different rocks weather at different rates. Often the first product of weathering bedrock is regolith - broken down and weathered bedrock. As this regolith breaks down further, sometimes combining with outside sources of sediment brought in by transporting agents, soil formation begins! Soil is the dynamic link between the organic and inorganic world. It is composed of decayed, decaying and broken down rocks, minerals and organic material. Organic is matter that is or was once alive (comes from above or near surface). Inorganic is material that was never alive, and typically comes from below (from bedrock and regolith).
PHYSICAL/MECHANICAL WEATHERING
The backside of Yosemite's Half Dome. Pressure-release jointing has caused sheeting and exfoliation - rounding the rock and creating a 'dome'. Half of the dome was removed by a valley glacier!
Mechanical weathering, also called physical weathering, breaks rock into smaller pieces without chemical alteration. 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. Frost action or ice wedging (also called freeze-thaw weathering), is the main form of mechanical weathering in any climate that regularly cycles above and below the freezing point. This mechanical break-up of rock is due to the expansion of freezing water in cracks and crevices. Water expands in volume by 9% (ice floats on water because it is less dense than water). 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.
Ice wedging breaks apart so much rock that large piles of broken rock are seen at the base of a hillside called talus. Ice wedging is common in Earth’s polar regions and mid latitudes, and also at higher elevations, such as in the mountains.
Plants and animals can do the work of mechanical weathering. 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. Organic wedging is the fracture of rock material through the activity of plants and animals (root wedging and animal burrowing). My dog, Harriet, is increasing the rate of physical weathering in my backyard when she digs and chews on rocks!
Salt weathering is the breaking apart of rocks due to salt crystallization - salty water seeps into pores and cracks, when evaporation of salty solutions occurs, large salt crystals grow and this growth can apply outward pressure on the pores and cracks.
Pressure-Release Jointing occurs when rock formed underground (such as a batholith) is then uplifted. The layer of regolith and sediment above is weathered, eroded and transported away. The removal of this overburden causes the pressure of deep burial to be relieved. Overtime the rock expands, and with this expansion layers of curved slabs and plates weather off in a process called sheeting or exfoliation - the progressive breaking off of concentric slabs due to a lessening of weight on exposed rock (the dome of Yosemite's Half Dome).
Mechanical weathering increases the rate of chemical weathering. As rock breaks into smaller pieces, the surface area of the pieces increases. With more surfaces exposed, there are more surfaces on which chemical weathering can occur.
There are many ways that rocks can be broken apart into smaller pieces. Frost action or ice wedging (also called freeze-thaw weathering), is the main form of mechanical weathering in any climate that regularly cycles above and below the freezing point. This mechanical break-up of rock is due to the expansion of freezing water in cracks and crevices. Water expands in volume by 9% (ice floats on water because it is less dense than water). 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.
Ice wedging breaks apart so much rock that large piles of broken rock are seen at the base of a hillside called talus. Ice wedging is common in Earth’s polar regions and mid latitudes, and also at higher elevations, such as in the mountains.
Plants and animals can do the work of mechanical weathering. 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. Organic wedging is the fracture of rock material through the activity of plants and animals (root wedging and animal burrowing). My dog, Harriet, is increasing the rate of physical weathering in my backyard when she digs and chews on rocks!
Salt weathering is the breaking apart of rocks due to salt crystallization - salty water seeps into pores and cracks, when evaporation of salty solutions occurs, large salt crystals grow and this growth can apply outward pressure on the pores and cracks.
Pressure-Release Jointing occurs when rock formed underground (such as a batholith) is then uplifted. The layer of regolith and sediment above is weathered, eroded and transported away. The removal of this overburden causes the pressure of deep burial to be relieved. Overtime the rock expands, and with this expansion layers of curved slabs and plates weather off in a process called sheeting or exfoliation - the progressive breaking off of concentric slabs due to a lessening of weight on exposed rock (the dome of Yosemite's Half Dome).
Mechanical weathering increases the rate of chemical weathering. As rock breaks into smaller pieces, the surface area of the pieces increases. With more surfaces exposed, there are more surfaces on which chemical weathering can occur.
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. 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. |
Stalactites and Soda Straws in a Limestone Cave
Spheroidal Weathering - the Tors of Dartmoor
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Oxidation of minerals in soil.
CHEMICAL WEATHERING BY WATER
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. 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.
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. As weaker minerals are changed by hydrolysis, it breaks down the interlocking crystal network and disintegration of the rock takes place. This happens first in cracks and corners, creating spheroidal weathering - sharp edges and corners of rocks are rounded by the alteration of minerals through chemical weathering. Spheroidal weathering looks similar to rounding by exfoliation, but different processes (one is mechanical, the other is chemical).
Hydration is the chemical reaction where water molecules are added to a compound, forming new minerals. It expands the rock and allows minerals to become soft, making them more susceptible to breaking down.
CHEMICAL WEATHERING BY CARBON DIOXIDE
Carbon dioxide (CO2) combines with water as raindrops fall through the atmosphere. This makes a weak acid, called carbonic acid. 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.
Carbonic acid is very common in nature where it works to dissolve rock. This acid transforms minerals that contain calcium, magnesium, potassium and sodium. The most common chemical weathering via carbonation occurs with limestone and marble, which is composed of calcium carbonate. You can see this weathering on headstones in cemeteries, on statues and architecture. In areas where calcium carbonate rocks dominate the landscape, this weathering produces distinct landscapes, known as karst topography.
CHEMICAL WEATHERING BY OXYGEN
Oxidation is a chemical reaction that takes place when oxygen reacts with metallic minerals to form oxides. Oxygen is very strongly chemically reactive. The most familiar type of oxidation is when iron reacts with oxygen to create iron oxide, aka rust. Iron oxide produces the red color we see in soils.
Minerals that are rich in iron break down as the iron oxidizes and forms new compounds. When iron is removed from mineral in the rock, this again weakens the rock structure making it more susceptible to additional weathering and disintegration.
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.
TERMS
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PREPARE FOR YOUR UNIT QUIZTo check for understanding, you will take a short quiz on Unit 1. Read the introduction to Weathering and Mass Wasting, Physical Weathering, Chemical Weathering and Mass Wasting (use the button above to view the tutorial on Mass Wasting). You will have two chances to take the quiz on Moodle, and I will record your highest score. Review the following to prepare for the quiz:
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