Compared to less resistant rocks rocks that are more resistant to weathering tend to form

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.

While plate tectonics forces work to build huge mountains and other landscapes, the forces of weathering and mass wasting gradually wear those rocks and landscapes away, called denudation. 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.

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?

MECHANICAL WEATHERING
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, 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.

• Gravity causes abrasion as a rock tumbles down a mountainside or cliff.
  
• Moving water causes abrasion as particles in the water collide and bump against one another.
  
• Strong winds carrying pieces of sand can sandblast surfaces.
  
• Ice in glaciers carries many bits and pieces of rock. Rocks embedded at the bottom of the glacier scrape against the rocks below.
  

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.Remember that the most common minerals in Earth’s crust are the silicate minerals. Many silicate minerals form in igneous or metamorphic rocks deep within the earth. 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. 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.

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

CHEMICAL WEATHERING BY OXYGEN

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:

• Temperature increases: Chemical reactions proceed more rapidly at higher temperatures. For each 10 degrees C increase in average temperature, the rate of chemical reactions doubles.
  
• Precipitation increases: More water allows more chemical reactions. Since water participates in both mechanical and chemical weathering, more water strongly increases weathering.
  

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. 

STEEPNESS OF SLOPEThere are several factors that influence mass wasting, but ultimately it is a battle between friction and gravity. If the friction on a rock is stronger than gravity for a particular slope, the rock material will likely stay. But if gravity is stronger, the slope will fail.

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
Another factor that determines mass wasting is the slope's materia. Mass wasting is more prone on slopes that contain clay and shale. Without going into great detail here, the shape and composition of individual clay particles can absorb water and prevent water from peculating through the ground. A layer of clay on a slope can prevent water from filtering through the slope. Instead, the water stays near the surface and saturates the ground. This can cause the surface layers to lose friction and slide.

WEIGHT AND FRICTION OF SLOPE
A third factor that influences whether a slope will fail is the load or weight of that slope. Adding weight to a weakened slope can obviously cause it to slide easier, especially on steep slopes. This added weight tends to occur by building on top of weak slopes, increasing the steepness of the slope, or over-saturating the slope.Friction has been mentioned as a factor several times already, but there are a few more things must be said here. As already noted, as long as the friction along the slope is stronger than gravity, the ground is unlikely to slide. But if that friction is weakened, slope fail becomes more likely. There are several other ways friction can be reduced along a slope: wildfires, removal of vegetation, or adding too much water.

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
A region's climate can also determine the likelihood of a landslide. Climate is based on temperature and precipitation. Mass wasting is prone in the spring-time when snowmelt, water saturation, and runoff is greatest. Also the type of climate will help determine the type of mass wasting. Humid climates tend to have slides, where water-saturated slopes fail and fall. Drier climates tend to have rocks that fall; especially early spring. Canyons and places prone to wildfires tend to have debris flows.

WATER CONTENT WITHIN SLOPES
The amount of water in the soil is a major factor in the stability of a slope. When you build a sand castle, water is needed to build the walls and towers. That is because water has surface tension and is attracted to each other. This allows you to build towers greater than the angle of repose. So a little water can actually prevent slopes from sliding. But too much water lubricates the individual grains of sediment decreasing friction between each grain, so the possibility of mass wasting increases. The increase of water within the soils can come from over watering, pipe or swimming pool leaks, or prolonged stormy weather. In Utah and many mountainous regions, spring runoff of snow melt increases the water content within the soil. The following is a video from the USGS of the La Conchita, California landslide in 2005. Notice how well it flows down the mountainside. There are two reasons why this landslide occurred. First, this slide occurred on the same slope as a previous landslide in 1995. But the 2005 slide was also influenced by the fact that above is an orchard that was over-watering the vineyards and over-saturated the soil.

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.

ROCK FALL
A rock fall are the fastest of all landslide types and occurs when a rock falls through the air until it comes to rest on the ground - not too complicated. In Utah, they are common in the spring and fall because of what is called freeze-thaw weathering. In the daytime, temperatures in the spring and fall tend to be above freezing, which allows liquid water to enter cracks within rocks.  

TRANSLATIONAL SLIDES
Rather than rotating, a translational slide occurs when slope failure occurs parallel to the slope. Often times the slope failure occurs on soil composed of clay or shale, or along old fault lines, or previous slide areas. What makes translational slides dangerous is that they tend to flow faster and travel farther than rotational slides. The most expensive translational slide in U.S. history actually occurred in Thistle, Utah in 1983. The Utah Geologic Survey also provides a Google Earth file that looks at the Thistle landslide.

DEBRIS FLOWS
Debris flows are one of the most common, but most dangerous of the various types of landslides because of their speed and consistency. Debris flows tend to be a mixture of rock and water with two to three times the density of flooding streams. That density allows debris flows strip away the land and pick up objects as large as school buses. Debris flows are most common at the mouth of canyons along alluvial fans. Lets first explain an alluvial fan. When floods occur within the mouth of a canyon, either because of intense thunderstorms or snow melt, the erosive power of the water can pick up sediment and boulders - a debris flow. Now once the debris flow reaches the mouth of a canyon, the sediment gets deposited in a fan-shaped delta called an alluvial fan. The problem is that people like to live along alluvial fans because of their scenic view on the canyon. Another influence of debris flows is wildfires. When a wildfire strips an area of its vegetation, the bare soil is easily eroded away in either a thunderstorm or snow melt creating these debris flows. Because of Utah's topography and tendency to wildfires, debris flows are quite common.

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
Lahars were mentioned in the module on volcanoes, but in essence they are volcanic landslides. Recall that volcanoes eject pyroclastic material ranging is size from ash to boulders. Now there tends to be two ways lahars occur. One is if a thunderstorm precipitates large amounts of moisture on the pyroclastic material and the pyroclastics flow downslope. The other option is if a volcano is snow-capped and the heat from the volcano causes some of the snow to melt and mix with the pyroclastic material. What makes lahars so dangerous is that they have the consistency of concrete and can travel hundreds of miles.

DRAINAGE CONTROLS
Ultimately preventing mass wasting is impossible because gravity will always exist, but smarter development can help minimize the risk and hazards. 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 SUPPORT
If 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 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. 

SLOW SUBSIDENCE
Slow subsidence occurs when the water within the sediment is slowly squeezed out because of overlying weight. There are several examples of slow subsidence, but the best one is Venice, Italy. Venice was built at sea level on the now submerged delta of the Brenta River. The city is sinking because of the overlying weight of the city and pumping of ground water. The problem now is that sea levels are rising as glaciers melt and water expands due to global warming. An example of slow subsidence in the U.S. includes New Orleans, Louisiana. As we all know from Hurricane Katrina, the Mississippi River has a vast network of levees that prevent the massive river from flooding - most of the time. But by preventing the spring-time flooding, we are preventing the river from depositing sediment onto the land. Instead, the sediment is being transported to the Gulf of Mexico creating the massive Mississippi delta. Below is a Landsat satellite image from NASA of this delta.

FAST SUBSIDENCE
Fast subsidence occurs when naturally acidic water begins to dissolve limestone rock to forma a network of water-filled underground caverns. But if droughts or pumping of ground water reduces the water table below the level of the caves, they caverns collapse creating surface sinkholes.

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 SUPPORT

• Be aware of your surroundings and notice changes in the natural world.
• Look for cracks or bulges in hillsides, tilting of decks or patios, or leaning poles or fences when rainfall is heavy. Sticking windows and doors can indicate ground movement as soil pushes slowly against a house and knocks windows and doors out of alignment.
• Look for landslide scars because landslides are most likely to happen where they have occurred before.
• Plant vegetation and trees on the hillside around your home to help hold soil in place.
• Help to keep a slope stable by building retaining walls. Installing good drainage in a hillside may keep the soil from getting saturated.