Updated April 11, 2018
By Ethan Shaw
Often proceeding at small, subtle and slow rates, weathering fragments or dissolves rock: a hugely influential geological process that commonly sets the stage for erosion and provides the critical “parent material” for developing soils. The type of rock certainly influences the kind, degree and pace of weathering it will be vulnerable to, although many other factors come into play – not least the surrounding climate.
Weathering breaks down rock through mechanical or chemical processes. Different types of rock have different resistance to weathering, but many other factors besides basic mineral content influence weathering rates, including climate.
Weathering takes apart rock by mechanical disintegration or chemical decomposition. Mechanical (or physical) weathering refers to rock fragmentation by such forces as ice- or salt-wedging and the unloading of pressure on rocks formed far underground and then exposed at the Earth’s surface. Chemical weathering, meanwhile, covers processes that weather rock through chemical reactions, as when minerals in rocks are dissolved or replaced through exposure to air or water.
The relative resistance or “toughness” of a given rock to weathering certainly depends partly on what kind of rock it is. That’s because rock type is determined by the composition and proportion of the constituent minerals, and different minerals vary in how they stand up to weathering. Quartz, for example, is more resistant that micas, which in turn are more resistant than feldspars. But you can’t really make a general ranking of rock types by resistance to weathering because of all the other variables involved.
Not all rocks within a given type, such as granite and limestone, have the same mineralogy, for one thing. Sandstones, for examples, are made of sand grains bound by a wide range of cementing materials, and their toughness hinges on that of their cement: A sandstone cemented by silica is more resistant than one cemented by calcium carbonate.
More massive rocks – those with fewer fractures, joints or bedding planes, which are the borders between individual layers in sedimentary rocks – tend to resist weathering more effectively than less massive ones, because those cuts provide points of entry (or attack) to weathering agents such as water, which in freeze-thaw cycles pries rock apart and which also serves as a medium for chemical weathering.
And then there’s the climate factor. Very roughly speaking, mechanical weathering tends to be a more dominant force in drier climates, while humid climates see more pronounced chemical weathering. Many rocks are resistant to one kind of weathering and weak against the other. Limestone, for instance, is notably prone to chemical weathering given the solubility of its carbonate rock; in humid limestone provinces, caves and caverns – examples of karst landforms – abound. In arid country, by contrast, limestone can be quite resistant and often forms scarps. For example, limestone – along with sandstone and conglomerate – creates bold cliffbands in the Grand Canyon of the Colorado Plateau, while weaker shale weathers to gentle strata between those tougher layers.
In a region containing multiple kinds of rock, their relative weathering resistance or lack thereof helps shape the lay of the land. Roughly speaking, rock layers standing high on the countryside are more resistant to weathering, as well as erosion – the two forces go hand-in-hand – than those underlying valleys and other lowlands. In the Valley and Ridge Province of the Appalachian Mountains, more resistant sandstone and conglomerate serve as “ridge-makers,” while weaker limestones and shales form valleys.
Weathering on certain rock types produces distinctive landforms. Granite outcrops often manifest as domes, walls and boulder fields, terrain that in some cases partly stems from a form of mechanical weathering called exfoliation (though chemical weathering may also contribute) that's best observed in granitic rocks. These form deep below the Earth’s surface; when exposed by uplift or erosion, they may respond to the unloading of pressure by shedding plates or strips of stone to create these monolithic landforms.
By breaking rock into smaller and smaller pieces and freeing minerals, weathering acts as one of the chief soil-making forces. Weathered rock provides what’s called the “parent material,” lending both structure and nutrients to developing soil. Here again, the type of rock matters because of the kinds of minerals and the size of particles that weathering extracts from it. For example, sandstone often weathers into large particles to produce a coarse-textured soil more easily permeated by air and water, as opposed to the finer-textured, less-penetrable soil derived from weathered shale’s smaller particles.
Calcium is closely linked with soil fertility, and calcium-rich rocks tend to both weather fairly quickly and supply soil with plentiful clays – the particles which facilitate a lot of the essential nutrient uptake by plant roots. Soil weathered from calcium-rich ferromagnesium rocks such as basalt, andesite and diorite thus tends to be more fertile than those developed over acidic igneous rocks such as granite and rhyolite.
Weathering is the process that changes solid rock into sediments. With weathering, rock is disintegrated into smaller pieces. Once these sediments are separated from the rocks, erosion is the process that moves the sediments away from it's original position. The four forces of erosion are water, wind, glaciers, and gravity. Water is responsible for most erosion. Water can move most sizes of sediments, depending on the strength of the force. Wind moves sand-sized and smaller pieces of rock through the air. Glaciers move all sizes of sediments, from extremely large boulders to the tiniest fragments. Gravity moves broken pieces of rock, large or small, downslope. These forces of erosion will be covered later.
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? What forces of weathering wear down that road, or rocks or mountains over time?
There are many ways that rocks can be broken apart into smaller pieces. 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. 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. Abrasion is another form of mechanical weathering. In abrasion, one rock bumps against another rock.
CHEMICAL WEATHERING BY WATERA 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.
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. 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.
CHEMICAL WEATHERING BY OXYGEN
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. Minerals that are rich in iron break down as the iron oxidizes and forms new compounds. Iron oxide produces the red color in soils.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.
ROCK AND MINERAL TYPEWeathering 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.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. 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.
CLIMATEA 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:
Once rock material has been broken down into smaller, unstable pieces by weathering, the material has the potential to move downslope called mass wasting (also called a landslide). Before looking into the various types of landslides, the factors that influence them must be examined.
The steeper the slope, the stronger the friction or rock strength must be to resist downslope motion. The steepest angle a slope can be before the ground will slide is about 35 degrees, called the angle of repose. Many times we will cut through a slope to make room for a road or other forms of development. So to help prevent the slope from sliding along these cut areas, retaining walls must be build. More on this later.
COMPOSITION OF SLOPE MATERIAL
WEIGHT AND FRICTION OF SLOPE
Gravity is probably the ultimate driving force of mass wasting. The force of gravity pulls all things on the planet toward the center of the Earth. Without gravity, mass wasting would not occur. But unlike many of the other factors, humans have no influence or control on gravity.
REGIONAL CLIMATE CONDITIONS
WATER CONTENT WITHIN SLOPES
Finally, gravity is the driving force of mass wasting. The force of gravity pulls all things on the planet toward the center of the Earth. But unlike many of the other factors, humans have no influence or control on gravity. For more information on what causes landslides in Utah, click here.
Image source: This image is in the public domain because it contains materials that originally came from the United States Geological Survey, an agency of the United States Department of the Interior.
VOLCANIC MASS WASTING
SLOPE GRADE AND SUPPORT
Slope support is one of most common types of mitigation for potential mass wasting. As mentioned above, a retaining wall can be built to support a steep slope. Next, the retaining wall must be anchored to the bedrock within the slope to hold the wall to the slope. Another type of slope support is simply planting vegetation. The roots of vegetation tend to grab and hold soil in place, so by planting various types of plants and trees can be a simple and cheap way to stabilize a slope. For more on what homeowners can do to minimize your risk to landslides in Utah, click here.
Subsidence occurs when loose, water saturated sediment begins to compact causing the ground surface to collapse. Now there are two types of subsidence.
A dramatic example of fast subsidence occurred in Guatemala City in 2007 when a massive sinkhole formed 300 feet deep. As noted above, the underground region surrounding Guatemala is composed of limestone that and a vast underground network of caverns. It is believed that the water table has been dropping in the region and thus draining the caves. Afterward the caves can not support the overlying weight and collapse in.
DRAINAGE CONTROLSUltimately preventing landslides is impossible because gravity will always exist. But smarter development can help minimize the risk and hazards created by landslides. One component in landslide mitigation is basic drainage control of water. Recall that water can cause slopes to lose their friction as water lubricates individual grains of soil. And if you cut a slope and put a retaining wall for support, you may be preventing the water from filtering through. Thus you will often find drains at the base of retaining walls that allow underground water to within the slopes to drain out.
SLOPE GRADE AND SUPPORTIf people dig into the base of a slope to create a road or a homesite, the slope may become unstable and move downhill. This is particularly dangerous when the underlying rock layers slope towards the area. Ultimately preventing landslides is impossible because gravity will always exist. But smarter development can help minimize the risk and hazards created by landslides. One component in landslide mitigation is basic drainage control of water. Recall that water can cause slopes to lose their friction as water lubricates individual grains of soil. And if you cut a slope and put a retaining wall for support, you may be preventing the water from filtering through. Thus you will often find drains at the base of retaining walls that allow underground water to within the slopes to drain out.Slope support is one of most common types of mitigation for potential landslides. As mentioned above, a retaining wall can be built to support a steep slope. Next, the retaining wall must be anchored to the bedrock within the slope to hold the wall to the slope. Another type of slope support is simply planting vegetation. The roots of vegetation tend to grab and hold soil in place, so by planting various types of plants and trees can be a simple and cheap way to stabilize a slope.Landslides cause $1 billion to $2 billion damage in the United States each year and are responsible for traumatic and sudden loss of life and homes in many areas of the world. To be safe from landslides: