Weathering and Erosional Processes in Deserts

Weathering and Erosional Processes in Deserts

kacimo samy   10:44 ص

Weathering and Erosional Processes in Deserts 

Without the protection of foliage to catch rainfall and slow the wind, and without roots to hold regolith in place, rain and wind can attack and erode the land surface of deserts and soil tends to be sparse. The result, as we have noted, is that hill slopes are typically bare, and plains can be covered with stony debris or drifting sand. 

Arid Weathering and Desert Soil Formation 

In the desert, as in temperate climates, physical weathering happens primarily when joints (natural fractures) split rock into pieces. Joint-bounded blocks eventually break free of bedrock and tumble down slopes, fragmenting into smaller pieces as they fall. In temperate climates, thick soil develops and covers bedrock. In deserts, however, bedrock commonly remains exposed, forming rugged, rocky escarpments.

Chemical weathering happens more slowly in deserts than in temperate or tropical climates, because less water is available to react with rock. Still, rain or dew provides enough moisture for some weathering to occur. This water seeps into rock and leaches (dissolves and carries away) calcite, quartz, and various salts. Leaching effectively rots the rock by transforming it into a poorly cemented aggregate. Over time, the rock will crumble and form a pile of unconsolidated sediment, susceptible to transport by water or wind. 

Although enough rain falls in deserts to leach chemicals out of sediment and rock, there is not enough rain to carry the chemicals away entirely. So they precipitate to form calcite and other minerals in regolith beneath the surface. The new minerals may bind clasts together to form a rock-like material called calcrete.

 Soil formation and chemical weathering in deserts.

Shiny desert varnish, a dark, rusty brown coating of iron oxide, manganese oxide, and clay, may cover the surface of rocks in deserts (figure above a). Desert varnish was once thought to form when water from rain or dew seeped into a rock, dissolved iron and magnesium ions, and carried the ions back to the surface of the rock by capillary action. More recent studies, however, suggest that desert varnish is not necessarily derived from the rock it coats. Instead, the iron and manganese in the varnish may come from dust that settles on the surface of the rock, for in the presence of moisture, micro-organisms (bacteria and archaea) can extract iron and manganese from the dust and transform it into oxide minerals. Such varnish won’t form in humid climates, because rain washes away the dust. 

Desert varnish takes a long time to form. In fact, the thickness of a desert varnish layer provides an approximate estimate of how long a rock has been exposed at the ground surface. In past centuries, Native Americans used desert-varnished rock as a medium for art: by chipping away the varnish to reveal the underlying lighter-coloured rock, they were able to create figures or symbols on a dark background. The resulting drawings are called petroglyphs. 

Because of the lack of plant cover in deserts, variations in bedrock and soil colour visually stand out. Slight variations in the concentration of iron, or in the degree of iron oxidation, in adjacent beds result in spectacular colour bands in rock layers and the thin soils derived from them. The Painted Desert of northern Arizona earned its name from the brilliant and varied hues of oxidized iron in the region’s shale bedrock (figure above b).

Water Erosion 

Evidence of erosion by running water in deserts.

Although rain rarely falls in deserts, when it does come, it can radically alter a landscape in a matter of minutes. Since deserts lack plant cover, rainfall, sheet wash, and stream flow are all extremely effective agents of erosion. It may seem surprising, but water generally causes more erosion than does the wind in most deserts (figure above a). 

Water erosion begins with the impacts of raindrops, which eject sediment from the ground into the air. On a hill, the ejected sediment lands downslope. The ground quickly becomes saturated with water during a heavy rain, so water starts flowing across the surface, carrying the loose sediment with it. Within minutes after a heavy downpour begins, dry stream channels fill with a turbulent mixture of water and sediment, which rushes downstream as a flash flood. When the rain stops, the water sinks into the stream bed’s gravel and disappears such streams are called intermittent, or ephemeral streams. Because of the relatively high viscosity of the water (owing to its load of suspended sediment) and the velocity and turbulence of the flow, flash floods in deserts cause intense erosion. Dry stream channels in desert regions of south-western North America are called dry washes, or arroyos, and in the Middle East and North Africa they are called wadis (figure above b). 

Wind Erosion 

In temperate and humid regions, plant cover protects the ground surface from the wind, but in deserts, the wind has direct access to the ground. Wind, just like flowing water, can carry sediment both as suspended load and as surface load. 

Sediment transport by wind in deserts.

Suspended load (fine-grained sediment such as dust and silt held in suspension) stays in the air for a long time and moves with it. The suspended sediment can be carried so high into the atmosphere (up to several kilometres above the Earth’s surface) and so far downwind (tens to thousands of kilometres) that it may move completely out of its source region. Particularly strong winds can generate a dramatic dust storm, or haboob, that can be 100 km long and 1.5 km high (figure above a). A dust storm, as it approaches, looks like a roiling, opaque wave. A particularly large one engulfed Phoenix, Arizona, in 2011, and shut down the airport. Dust storms are fairly common in the Sahara and Arabian Deserts, where they are known as haboobs.

Surface loads, or bed loads, start moving only in moderate to strong winds, for such winds can roll and bounce sand grains along the ground, a process called saltation. Saltation begins when turbulence caused by wind shearing along the ground surface lifts sand grains. The grains move downwind, following an asymmetric, arch-like trajectory. Eventually, they return to the ground, where they strike other sand grains, causing the new grains to bounce up and drift or roll downwind. The collisions between sand grains make the grains rounded and frosted. Saltating grains generally rise no more than 0.5 m, but where sand bounces on bedrock the grains may rise 2 m. 

The size of clasts that wind can carry depends on the wind velocity. Wind, therefore, does an effective job of sorting sediment, sending dust-sized particles skyward and sand-sized particles bouncing along the ground, while pebbles and larger grains remain behind. In some cases, wind carries away so much fine sediment that pebbles and cobbles become concentrated at the ground surface (figure above b, c). An accumulation of coarser sediment left behind when fine-grained sediment blows away is called a lag deposit. 

The progressive development of a ventifact.

Just as sandblasting cleans the grime off the surface of a building, windblown sand and dust grind away at surfaces in the desert. Over long periods of time, such wind abrasion creates smooth faces, or facets, on pebbles, cobbles, and boulders. If a rock rolls or tips relative to the prevailing wind direction after it has been faceted on one side, or if the wind shifts direction, a new facet with a different orientation forms, and the two facets join at a sharp edge. Rocks whose surface has been faceted by the wind are faceted rocks, or ventifacts (figure above). Wind abrasion also gradually polishes and bevels down irregularities on a desert pavement and polishes the surfaces of desert-varnished outcrops, giving them a reflective sheen. 

Wind erosion in Death valley has left bushes perched on mounds; roots keep soil from blowing away.

Over time, in regions where the substrate consists of soft sediment, wind picks up and removes so much sediment that the land surface becomes lower. The process of lowering the land surface by wind erosion is called deflation. Shrubs can stabilize a small patch of sediment with their roots, so after deflation a forlorn shrub with its residual pedestal of soil stands isolated above a lowered ground surface (figure above). In some places, the shape of the land surface twists the wind into a turbulent vortex that causes enough deflation to scour a deep, bowl-like depression called a blowout.
Figures credited to Stephen Marshak.

The Evolution of Drainage

The Evolution of Drainage

kacimo samy   9:55 ص

Drainage Evolution

Beveling Topography 

Fluvial landscapes evolve over time.

Imagine a place where continental collision uplifts a region; the landscape will evolve (figure above a). At first, rivers have steep gradients, flow over many rapids and waterfalls, and cut deep valleys. But with time, rugged mountains become low, rounded hills; once-deep, narrow valleys broaden into wide floodplains with more gradual gradients. As more time passes, even the low hills are beveled down, becoming small mounds or even disappearing altogether. (Some geologists have referred to the resulting landscape as a peneplain, from the Latin paene, which means almost; it lies at an elevation close to that of a stream’s base level.) 

Though the above model makes intuitive sense, it is an oversimplification. Plate tectonics can uplift the land again, and/or global sea-level rise or fall can change the base level, so in reality peneplains rarely develop before downcutting begins again. Stream rejuvenation occurs when a stream starts to downcut into a land surface whose elevation lies near the stream’s base level. Rejuvenation can be triggered by several phenomena, such as: a drop in base level, as happens when sea level falls; an uplift event that causes the land to rise relative to the base level; or an increase in stream discharge that makes the stream more able to erode and transport sediment. As we've seen, rejuvenation can lead to formation of stream terraces in alluvium. In cases where rejuvenation causes a stream to erode deeply into the land, a new canyon or valley will develop. If the rejuvenated stream had a meandering course, downcutting produces incised meanders (figure above b). 

Stream Piracy and Drainage Reversal 

The concept of stream capture or “piracy.”

Stream piracy sounds like pretty violent stuff. In reality, it’s just a natural process that happens when headward erosion by one stream causes the stream to intersect the course of another stream. When this happens, the pirate stream “captures” the water of the stream that it has intersected, so that the water of the captured stream starts to flow down the pirate stream. Because of piracy, the channel of the captured stream, downstream of the point of capture, dries up (figure above a, b). In some cases, stream capture changes a “water gap” (a stream carved notch through a ridge) into a dry “wind gap.” In 1775, Daniel Boone, the legendary pioneer, led settlers through the Cumberland Gap, a wind gap in the Appalachians, to new homesteads in western Kentucky. 

The pattern of stream flow in an area can also be altered, over time, on a continental scale. For example, when South America and Africa were adjacent to each other in Pangaea, a highland existed along the boundary between the two continents, and the main drainage network of northern South America flowed westward. Later, when South America rifted away from Africa, a convergent plate boundary developed along the western margin of South America, causing the Andes Mountains to rise. The uplift of the Andes caused a drainage reversal, in that the overall slope direction of the drainage network became the opposite of what it once had been. As a consequence, westward flow became impossible, and the eastward-flowing Amazon drainage network developed.

Superposed and Antecedent Streams 

The structure and topography of the landscape do not always appear to control the path, or course, of a stream. For example, imagine a stream that carves a deep canyon straight across a strong mountain ridge why didn't the stream find a way around the ridge? We distinguish two types of streams that cut across resistant topographic highs: 

 Formation of superposed drainage

• Superposed streams: Imagine a region in which streams start to flow over horizontal beds of strata that unconformably overlie folded strata. When the streams eventually erode down through the unconformity and start to downcut into the folded strata, they maintain their earlier course, ignoring the structure of the folded strata. Geologists call such streams superposed streams, because their pre-existing geometry has been laid down on underlying rock structure (figure above a, b). 
• Antecedent streams: In some cases, tectonic activity (such as subduction or collision) causes a mountain range to rise up beneath an already established stream. If the stream downcuts as fast as the range rises, it can maintain its course and will cut right across the range. Geologists call such streams antecedent streams (from the Greek ante, meaning before) to emphasize that they existed before the range uplifted. Note that if the range rises faster than the stream downcuts, the new highlands divert (change) the stream’s course so that it flows parallel to the range face (figure below a–c).

 Development of antecedent and diverted streams.

Credits: Stephen Marshak (Essentials of Geology)

deposition in science (river)

deposition in science (river)

kacimo samy   9:20 ص

Deposition in science (Streams and Their Deposits in the Landscape)

Deposition by river are in different places where slumps, alluvial fans, valleys, canyons, delta, waterfalls, floodplains. 

Valleys and Canyons

About 17 million years ago, a large block of crust, the region now known as the Colorado Plateau (located in Arizona, Utah, Colorado, and New Mexico), began to rise. Before the rise, the Colorado River had been flowing over a plain not far above sea level, causing little erosion. But as the land uplifted, the river began to downcut. Eventually, its channel lay as much as 1.6 km below the surface of the plateau at the base of a steep walled gash now known as the Grand Canyon. The formation of the Grand Canyon illustrates a general phenomenon. In regions where the land surface lies well above the base level, a stream can carve a deep trough, much deeper than the channel itself. If the walls of the trough slope gently, the landforms is a valley, but if they slope steeply, the landforms is a canyon. 

The shape of a canyon or valley depends on the resistance of its walls to erosion slumping.

Whether stream erosion produces a valley or a canyon depends on the rate at which down cutting occurs relative to the rate at which mass wasting causes the walls on either side of the stream to collapse. In places where a stream downcuts through its substrate faster than the walls of the stream collapse, erosion creates a slot (steep-walled) canyon. Such canyons typically form in hard rock, which can hold up steep cliffs for a long time (figure above a). In places where the walls collapse as fast as the stream downcuts, landslides and slumps gradually cause the slope of the walls to approach the angle of repose. When this happens, the stream channel lies at the floor of a valley whose cross-sectional shape resembles the letter V (figure above b); this landforms is called a V-shaped valley. Where the walls of the stream consist of alternating layers of hard and soft rock, the walls develop a stair-step shape such as that of the Grand Canyon (figure above c).

 The evolution of alluvium-filled stream valleys and the development of terraces.

In places where active down cutting occurs, the valley floor remains relatively clear of sediment, for the stream especially when it floods carries away sediment that has fallen or slumped into the channel from the stream walls. But if the stream’s base level rises, its discharge decreases, or its sediment load increases, the valley floor fills with sediment, creating an alluvium-filled valley (figure above a). The surface of the alluvium becomes a broad floodplain. If the stream’s base level later drops again and/or the discharge increases, the stream will start to cut down into its own alluvium, a process that generates stream terraces bordering the present floodplain (figure above b).

Rapids and Waterfalls 

Examples of rapids and waterfalls.

When Lewis and Clark forged a path up the Missouri River, they came to reaches that could not be navigated by boat because of rapids, particularly turbulent water with a rough surface (figure above a). Rapids form where water flows over steps or large clasts in the channel floor, where the channel abruptly narrows, or where its gradient abruptly changes. The turbulence in rapids produces eddies, waves, and whirlpools that roil and churn the water surface, in the process creating white water, a mixture of bubbles and water. Modern-day white water rafters thrill to the unpredictable movement of rapids. 

A waterfall forms where the gradient of a stream becomes so steep that some or all of the water literally free falls above the stream bed (figure above b). The energy of falling water may scour a depression, called a plunge pool, at the base of the waterfall. Though a waterfall may appear to be a permanent feature of the landscape, all waterfalls eventually disappear because headward erosion slowly eats back the resistant ledge. We can see a classic example of headward erosion at Niagara Falls. As water flows from Lake Erie to Lake Ontario, it drops over a 55-m-high ledge of resistant Silurian dolostone, which overlies a weak shale. Erosion of the shale leads to undercutting of the dolostone. Gradually, the overhang of dolostone becomes unstable and collapses, with the result that the waterfall migrates upstream. Before the industrial age, the edge of Niagara Falls cut upstream at an average rate of 1 m per year; but since then, the diversion of water from the Niagara River into a hydroelectric power station has decreased the rate of headward erosion to half that (figure below a, b).

The formation of Niagara Falls, at the border between Ontario, Canada, and New York State. The falls tumble over the Lockport Dolomite, a relatively strong rock layer.

Alluvial Fans and Braided Streams 

 Examples of depositional landforms produced from stream sediment.

Where a fast-moving stream abruptly emerges from a mountain canyon into an open plain at the range front, the water that was once confined to a narrow channel spreads out over a broad surface. As a consequence, the water slows and drops its sedimentary load, forming a sloping apron of sediment (sand, gravel, and cobbles) called an alluvial fan (figure above a). The stream then divides into a series of small channels that spread out over the fan. During particularly strong floods, the water contains so much sediment that it becomes a debris flow that spreads over and smooths out the fan’s surface. 

In some localities, streams carry abundant coarse sediment during floods but cannot carry this sediment during normal flow. Thus, during normal flow, the sediment settles out and chokes the channel. As a consequence, the stream divides into numerous strands weaving back and forth between elongate bars of gravel and sand. The result is a braided stream the name emphasizes that the streams entwine like strands of hair in a braid (figure above b).

Meandering Streams  and Their Floodplains 

The character and evolution of meandering streams and floodplains.

A riverboat cruising along the lower reaches of the Mississippi River cannot sail in a straight line, for the river channel winds back and forth in a series of snake-like curves called meanders (figure above a). In fact, the boat has to go 500 km along the river channel to travel 100 km as the crow flies.

How do meanders evolve? Even if a stream starts out with a straight channel, natural variations in the water depth and associated friction cause the fastest-moving current to swing back and forth. The water erodes the side of the stream more effectively where it flows faster, so it begins to cut away faster on the outer arc of the curve. Thus, each curve begins to migrate sideways and grow more pronounced until it becomes a meander (figure above b). On the outside edge of a meander, erosion continues to eat away at the channel wall, forming a cut bank. On the inside edge, water slows down so that its competence decreases and sediment accumulates, forming a wedge-shaped deposit called a point bar, as noted earlier. 

With continued erosion, a meander may curve through more than 180 degree, so that the cut bank at the meander’s entrance approaches the cut bank at its end, leaving a meander neck, a narrow isthmus of land separating the portions of the meander. When erosion eats through a meander neck, a straight reach called a cut off develops. The meander that has been cut off is called an oxbow lake if it remains filled with water, or an abandoned meander if it dries out (figure above c). Streams that develop many meanders are known, not surprisingly, as meandering streams. The course of a meandering stream naturally changes over time, on a time scale of years to centuries, as new meanders grow and old ones are cut off and abandoned.

Most meandering stream channels cover only a relatively small portion of a broad, gently sloping floodplain (figure above d). Floodplains, as we noted earlier, are so named because during a flood, water over-tops the edge of the stream channel and spreads out over the floodplain. In many cases, a floodplain terminates at its sides along a bluff, or escarpment; large floods may cover the entire floodplain from bluff to bluff. As the water rises above the channel walls and starts to spread out, over the floodplain, friction slows down the flow. This slowdown decreases the competence of the running water, so sediment settles out along the edge of the channel. Over time, the accumulation of this sediment creates a pair of low ridges, called natural levees, on either side of the stream. Natural levees may grow so large that the floor of the channel may become higher than the surface of the floodplain.

Deltas: Deposition at the Mouth of a Stream 

Along most of its length, only a narrow floodplain covered by green, irrigated farm fields borders the Nile River in Egypt. But at its mouth, the trunk stream of the Nile divides into a fan of smaller streams, called distributaries, and the area of green agricultural lands broadens into a triangular patch. The Greek historian Herodotus noted that this triangular patch resembles the shape of the Greek letter delta, and so the region became known as the Nile Delta. Deltas develop where the running water of a stream enters standing water, the current slows, the stream loses competence, and sediment settles out. This can happen in either a lake or the sea.

Delta shape varies depending on current activity, waves, and vegetation.

Geologists refer to any wedge of sediment formed at a river mouth as a delta, even though relatively few have the triangular shape of the Nile Delta (figure above a–c). Some deltas curve smoothly outward, whereas others consist of many elongate lobes formed at different times. Bird’s-foot deltas, so-named because they resemble the scrawny toes of a bird, develop where several distributaries extend far out into relatively calm water; the end of the Mississippi’s active channel ends in a bird’s-foot delta  (figure above a–c). 

 A map showing ancient lobes of the Mississippi Delta. A major flood could divert water from the Mississippi into the channel of the Atchafalaya.

The existence of several overlapping deltas indicates that the main course of the river in the delta has shifted on several occasions. These shifts occur when a toe builds so far out into the sea that the slope of the stream becomes too gentle to allow the river to flow. At this point, the river overflows a natural levee upstream and begins to flow in a new direction, an event called an avulsion. The distinct lobes of the Mississippi Delta suggest that avulsions have happened several times during the past 9,000 years (figure above). New Orleans, built along one of the Mississippi’s distributaries, may eventually lose its riverfront, for a break in a levee upstream of the city could divert the Mississippi into the Atchafalaya River channel. 

The shape of a delta depends on many factors. Deltas that form where the strength of the river current exceeds that of ocean currents have a bird’s-foot shape, since the sediment can be carried far offshore. In contrast, deltas that form where the ocean currents are strong have a $ shape, for the ocean currents redistribute sediment in bars running parallel to the shore. And in places where waves and currents are strong enough to remove sediment as fast as it arrives, a river has no delta at all.

With time, the sediment of a larger delta compacts, and the weight of the delta pushes down the crust below. As a consequence, the surface of a delta slowly sinks. Distributaries can provide sediment that fills the resulting space so that the delta’s surface remains at or just above sea level, forming a broad, flat area called a delta plain. But if people build up artificial levees to constrain the river to its channel, sediment gets carried directly to the seaward edge of the delta and the delta’s interior “starves” (does not receive sediment). When this happens, the delta’s surface drops below sea level. Because of this process, much of New Orleans lies below sea level.
Figures credited to Stephen Marshak.

Soil

Soil

kacimo samy   10:29 ص

What is Soil?

What is soil?. If you've ever had the chance to dig in a garden, you've seen first hand that the material in which flowers grow looks and feels different from beach sand or potter’s clay. We call the material in a garden “dirt” or, more technically, soil. Soil consists of rock or sediment that has been modified by physical and chemical interaction with organic material, rainwater, and organisms over time. Soil is one of our planet’s most valuable resources, for without it there could be no agriculture, forestry, ranching, or home gardening.

How Does Soil Form?

How does soil form?. Three processes taking place at or just below the surface of the Earth contribute to soil formation. First, chemical and physical weathering produces loose debris, new minerals (such as clay), and ions in solution. Second, rainwater percolates through the debris and carries dissolved ions and clay flakes downward. The region in which this downward transport takes place is called the zone of leaching, because leaching means extracting, absorbing, and removal. Farther down, new mineral crystals precipitate directly out of the water or form by reaction of the water with debris. Also, the water leaves behind its load of fine clay. The region in which new minerals and clay collect is the zone of accumulation. Third, microbes, fungi, plants, and animals interact with sediment by producing acids that weather grains, by absorbing nutrient atoms, and by leaving behind organic waste and remains. Plant roots and burrowing animals (insects, worms, and gophers) churn and break up the soil, and microbes metabolise minerals and organic matter and release chemicals. Because different soil-forming processes operate at different depths, soils typically develop distinct zones, known as soil horizons, arranged in a vertical sequence called a soil profile. Let’s look at an idealised soil profile, from top to bottom, using a soil formed in a temperate forest as our example. The highest horizon is the O-horizon (the prefix stands for organic), so called because it consists almost entirely of humus (plant debris) and contains barely any mineral matter. Below the O-horizon, we find the A-horizon, in which humus has decayed further and has mixed with mineral grains (clay, silt, and sand). Water percolating through the A-horizon causes chemical weathering reactions to occur and produces ions in solution and new clay minerals. Downward-moving water eventually carries soluble chemicals and fine clay deeper into the subsurface. The O- and  A-horizons constitute dark-gray to blackish-brown topsoil, the fertile portion of soil that farmers till for planting crops. (In some places, the A-horizon grades downward into the E-horizon, a soil level that has undergone substantial leaching but has not yet mixed with organic material.) Beneath the A-horizon (or the A- and E-horizons) lies the B- horizon. Ions and clay accumulate in the B-horizon, or subsoil. Note from our description that the O-, A-, and E-horizons  make up the zone of leaching, whereas the B-horizon makes up the zone of accumulation. Finally, at the base of a soil profile we find the C-horizon, which consists of material derived from the substrate that’s been chemically weathered and broken apart, but has not yet undergone leaching or accumulation. The C-horizon grades downward into unweathered bedrock, or into unweathered sediment. As farmers, foresters, and ranchers well know, the soil in one locality can differ greatly from the soil in another, in terms of composition, thickness, and texture. Indeed, crops that grow well in one type of soil may wither and die in another nearby. Such diversity exists because the make up of a soil depends on several soil-forming factors:

• Climate: Large amounts of rainfall and warm temperatures accelerate chemical weathering and cause most of the soluble elements to be leached. Small amounts of rainfall and cooler temperatures result in slower rates of weathering and leaching, so soils take a long time to develop and can retain unweathered minerals and soluble components. Climate is the single most important factor in determining the nature of soils that develop. 
• Substrate composition: Some soils form on basalt, some on granite, some on volcanic ash, and some on recently deposited quartz silt. These different substrates consist of different materials, so the soils formed on them end up with different chemical compositions. 
• Slope steepness: A thick soil can accumulate under land that lies flat. But on a steep slope, weathered rock may wash away before it can evolve into a soil. Thus, all other factors being equal, soil thickness increases as the slope angle decreases. 
• Wetness: Depending on the details of local topography and on the depth below the surface at which groundwater occurs, some soil is wetter than other soil in the same region. Wet soils tend to contain more organic material than do dry soils. 
• Time: Because soil formation is an evolutionary process, a young soil tends to be thinner and less developed than an old soil. The rate of soil formation varies greatly with environment. 
• Vegetation type: Different kinds of plants extract or add different nutrients and quantities of organic matter to a soil. Also, some plants have deeper root systems than others and help prevent soil from washing away.

Soil Classification 

Soil scientists worldwide have struggled mightily to develop a rational scheme for classifying soils. Not all schemes utilize the same criteria, and even today there is not worldwide agreement on which works best. In the United States, a country that includes many climates at mid-latitudes, many soil scientists use the U.S. Comprehensive Soil Classification System, which distinguishes among 12 orders of soil based on the physical characteristics and environment of soil formation. Canadians use a different scheme focusing only on soils that develop north of the 40th parallel. The Canadian  scheme works well for cooler, high-latitude climates. As we've noted, rainfall and vegetation play a key role in determining the type of soil that forms. For example, in deserts, where there is very little rainfall and sparse vegetation, an aridisol forms. (In older classifications, these were known as “pedocal” soils.) Aridisols have no O-horizon (because there is so little organic material), and the A-horizon is thin. Soluble minerals, specifically calcite, that would be washed away entirely if there were more rainfall, instead accumulate in the B-horizon. In fact, capillary action may bring calcite up from deeper down as water evaporates at the ground surface. The calcite locally cements clasts together in the B-horizon to form a rock-like mass called caliche or calcrete. In temperate environments, an alfisol forms this soil has an O-horizon, and because of moderate amounts of rainfall, materials leached from the A-horizon accumulate in the B-horizon. (In older classifications, these were known as “pedalfer” soils.) In a tropical climate, oxisols develop. Here, so much rainfall percolates down into the ground that all reactive minerals in the soil undergo chemical weathering, producing ions and clay that flush downward. This process leaves an A-horizon that contains substantial amounts of stable iron-oxide, aluminium-oxide, and aluminium-hydroxide residues. The resulting soil tends to be brick-red and is traditionally called laterite.

U.S. Department of Agriculture map of soil types around the world.

Soil Erosion 

As we have seen, soils take time to form, so soils capable of supporting crops or forests are a natural resource worthy of protection. However, agriculture, overgrazing, and clear cutting have led to the destruction of soil. Crops rapidly remove nutrients from soil, so if they are not replaced, the soil will not contain sufficient nutrients to maintain plant life. When the natural plant cover disappears, the surface of the soil becomes exposed to wind and water. Actions such as the impact of falling raindrops or the rasping of a plow break up the soil at the surface, with the result that it can wash away in water or blow away as dust. When this happens, soil erosion, the removal of soil by running water or by wind, takes place. In some localities, erosion carries away almost six tons of soil from an acre of land per year. Human activities can increase rates of soil erosion by 10 to 100 times, so that it far exceeds the rate of soil formation. Droughts exacerbate the situation. For example, during the 1930s a succession of droughts killed off so much vegetation in the American plains that wind stripped the land of soil and caused devastating dust storms. Large numbers of people were forced to migrate away from the Dust Bowl of Oklahoma and adjacent areas. The consequences of rainforest destruction have particularly profound effects on soil. In an established rain forest, lush growth provides sufficient organic debris so that trees can grow. But if the forest is logged, or cleared for a griculture, the humus rapidly  disappears, leaving laterite that contains few nutrients. Crop plants consume whatever nutrients remain so rapidly that the soil becomes infertile after only a year or two, useless for agriculture and unsuitable for regrowth of rainforest trees.

Types of weathering

Types of weathering

kacimo samy   11:38 ص

Weathering

 The texture of granite changes as it weathers.

Weathering refers to the combination of processes that break up and corrode solid rock, eventually transforming it into sediment. Geologists refer to rock that has not undergone weathering as unweathered or “fresh” (figure above). Rock exposed at the Earth’s surface sooner or later crumbles away because of weathering. Just as a plumber can unclog a drain by using physical force (with a plumber’s snake) or by causing a chemical reaction (with a dose of liquid drain opener), nature can attack rocks via two types of weathering: physical and chemical. 

Types of weathering

Physical Weathering 

Clasts are classified by grain diameter.

Physical weathering, sometimes referred to as mechanical weathering, breaks intact rock into unconnected clasts (grains or chunks), collectively called debris or detritus. Each size range of clasts has a name (table above). Many different phenomena contribute to physical weathering, as described below. 

Jointing 

Rocks buried deep in the Earth’s crust endure enormous pressure due to the weight of overlying rock or overburden. Rocks at depth are also warmer than rocks nearer the surface because of the Earth’s geothermal gradient. Over long periods of time, moving water, air, and ice at the Earth’s surface grind away and remove overburden, a process called erosion, so rock formerly at depth rises closer to the Earth’s surface. As a result, the pressure squeezing this rock decreases, and the rock  also becomes cooler. A change in pressure and temperature causes rock to change shape slightly. Such changes cause hard rock to break. Natural cracks that form in rocks due to removal of overburden or due to cooling are known as joints.

Joints (natural cracks) break bedrock into blocks and sheets, which can tumble down a slope.

Almost all rock outcrops contain joints. Some joints are fairly planar, some curving, and some irregular. For example, large granite plutons may split into onion-like sheets along joints that lie parallel to the mountain face; this process is called exfoliation. Sedimentary rock beds, however, may break into rectangular blocks bounded by joints on the sides and bed (layer) surfaces above and below (a in figure above). Regardless of their orientation, the formation of joints turns formerly intact bedrock into separate blocks. Eventually, these blocks topple from the outcrop at which they formed. After a while, they may collect in an apron of talus, the rock rubble at the base of a slope (b in figure above).

Wedging is one type of physical (mechanical) weathering.

Frost wedging

Freezing water bursts pipes and shatters bottles because water expands when it freezes and pushes the walls of the container apart. The same phenomenon happens in rock. When the water trapped in a joint freezes, it forces the joint open and may cause the joint to grow. Such frost wedging helps break blocks free from intact bedrock (a in figure above). 

Salt wedging

In arid climates, dissolved salt in groundwater precipitates and grows as crystals in open pore spaces in rocks. This process, called salt wedging, pushes apart the surrounding grains and weakens the rock so that when exposed to wind and rain, the rock disintegrates into separate grains. The same phenomenon happens along the coast, where salt spray percolates into rock and then dries (b in figure above). 

Root wedging

Have you ever noticed how the roots of an old tree can break up a side walk? As roots grow, they apply pressure to their surroundings, and can push joints open in a process known as root wedging (c in figure above). 

Thermal expansion

When the heat of an intense forest fire bakes a rock, the outer layer of the rock expands. On cooling, the layer contracts. This change creates forces in the rock sufficient to make the outer part of the rock break off in sheet-like pieces. Recent research suggests that the intense heat of the Sun’s rays sweeping across dark rocks in a desert may cause the rocks to fracture into thin slices.

Animal attack

Animal life also contributes to physical weathering: burrowing creatures, from earthworms to gophers, push open cracks and move rock fragments. And in the past century, humans have become perhaps the most energetic agent of physical weathering on the planet. When we excavate quarries, foundations, mines, or roadbeds by digging and blasting, we shatter and displace rock that might otherwise have remained intact for millions of years more.

Chemical Weathering 

Up to now, we've taken the plumber’s-snake approach to breaking up rock. Now let’s look at the liquid-drain-opener approach. Chemical weathering refers to the many chemical reactions that alter or destroy minerals when rock comes in contact with water solutions and/or air. Common reactions involved in chemical weathering include the following:

Dissolution is one type of chemical weathering.

• Dissolution: Chemical weathering during which minerals dissolve into water is called dissolution. Dissolution primarily affects salts and carbonate minerals (a and b in figure above), but even quartz dissolves slightly.
• Hydrolysis: During hydrolysis, water chemically reacts with minerals and breaks them down (lysis means loosen in Greek) to form other minerals. For example, hydrolysis reactions in feldspar produce clay. 
• Oxidation: Oxidation reactions in rocks transform iron bearing minerals (such as biotite and pyrite) into a rusty brown mixture of various iron-oxide and iron-hydroxide minerals. In effect, iron-bearing rocks can “rust.” 
• Hydration: Hydration, the absorption of water into the crystal structure of minerals, causes some minerals, such as certain types of clay, to expand. Such expansion weakens rock.

Not all minerals undergo chemical weathering at the same rates. Some weather in a matter of months or years, whereas others remain unweathered for millions of years. For example, when a granite (which contains quartz, mica, and feldspar) undergoes chemical weathering, most of its minerals except  quartz transform to clay. Until fairly recently, geoscientists tended to think of chemical weathering as a strictly inorganic chemical reaction, occurring entirely independently of life forms. But researchers now realize that organisms play a major role in the chemical-weathering process. For example, the roots of plants, fungi, and lichens secrete organic acids that help dissolve minerals in rocks; these organisms extract nutrients from the minerals. Microbes, such as bacteria, are amazing in that they literally eat minerals for lunch. Bacteria pluck off molecules from minerals and use the energy from the molecules’ chemical bonds to supply their own life force.

Physical and Chemical Weathering  Working Together 

So far we've looked at the processes of chemical and physical weathering separately, but in the real world they happen together, aiding one another in disintegrating rock to form sediment.

Physical and chemical processes work together during the weathering process.

Physical weathering speeds up chemical weathering. To understand why, keep in mind that chemical-weathering reactions take place at the surface of a material. Thus, the overall rate at which chemical weathering occurs depends on the ratio of surface area to volume the greater the surface area, the faster the volume as a whole can chemically weather. When jointing (physical weathering) breaks a large block of rock into smaller pieces, the surface area increases, so chemical weathering happens faster (a in figure above).

Similarly, chemical weathering speeds up physical weathering by dissolving away grains or cements that hold a rock together, transforming hard minerals (like feldspar) into soft minerals (like clay) and causing minerals to absorb water and expand. These phenomena make rock weaker, so it can disintegrate more easily (b in figure above).

Differential weathering.

Weathering happens faster at edges, and even faster at the corners of broken blocks. This is because weathering attacks a flat face from only one direction, an edge from two directions, and a corner from three directions. Thus, with time, edges of blocks become blunt and corners become rounded 9a in figure above). 

In rocks such as granite, which do not contain layering that can affect weathering rates, rectangular blocks transform into a spheroidal shape (b in figure above).
When different rocks in an outcrop undergo weathering at different rates, we say that the outcrop has undergone “differential weathering.” As a result of this process, cliffs composed of a variety of rock layers take on a stair-step or sawtooth shape (c in figure above). You can easily see the consequences of differential weathering if you walk through a graveyard. The inscriptions on some headstones are sharp and clear, whereas those on other stones have become blunted or have even disappeared (d in figure above). That’s because the minerals in these different stones have different resistances to weathering. Granite, an igneous rock with a high quartz content, retains inscriptions the longest. But marble, a metamorphic rock composed of calcite, dissolves away relatively rapidly in acidic rain.

Credits: Stephen Marshak (Essentials of Geology)

Stratigraphy: Making sense of chaos

Stratigraphy: Making sense of chaos

kacimo samy   10:42 م

What is Stratigraphy?

Stratigraphy- The branch of geology that seeks to understand the geometric relationships between different rock layers (called strata), and to interpret the history represented by these rock layers.

Public Domain Image by the US Dept. of Interior.

Contact- A boundary that separates different strata or rock units.

Steno's Laws of Stratigraphy

Image from J. P. Trap: berømte danske mænd og kvinder, 1868

Nicholas Steno (1638-1686) was a Danish-born pioneer of geology, and is considered to be the father of stratigraphy.

Nicholas Steno's observations of rocks layers suggested that geology is not totally chaotic.  Rather, the rock layers preserve a chronological record of Earth history and past life.

He developed three fundamental principles of stratigraphy, now known as Steno's Laws:

1) Law of Original Horizontality– Beds of sediment deposited in water form as horizontal (or nearly horizontal) layers due to gravitational settling.

2) Law of Superposition– In undisturbed strata, the oldest layer lies at the bottom and the youngest layer lies at the top.

3) Law of Lateral Continuity– Horizontal strata extend laterally until they thin to zero thickness (pinch out) at the edge of their basin of deposition.

Other Important Principles of Stratigraphy

4) Law of Cross-Cutting Relationships– An event that cuts across existing rock is younger than that disturbed rock.  This law was developed by Charles Lyell (1797-1875).

5) Principle of Inclusion– Fragments of rock that are contained (or included) within a host rock are older than the host rock.

Unconformities

Unconformity – A surface that represents a very significant gap in the geologic rock record (due to erosion or long periods of non-deposition).

There are 3 main types of unconformities:

1) Disconformity – A contact representing missing rock between sedimentary layers that are parallel to each other.  Since disconformities are parallel to bedding planes, they are difficult to see in nature.

2) Angular Unconformity – A contact in which younger strata overlie an erosional surface on tilted or folded rock layers.  This type of unconformity is easy to identify in nature.

Image provided by FCIT. Original image from Textbook of Geology by Sir Archibald Geikie (1893).

3) Nonconformity – A contact in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock.

4) Paraconformity- A contact between parallel layers formed by extended periods of non-deposition (as opposed to being formed by erosion).  These are sometimes called "pseudo unconformities").

Unconformities VS Bedding Planes

Unconformities represent huge gaps in time!  The nonconformity between the Vishnu Schist and overlying sedimentary layers in the Grand Canyon represents 1.3 billion years of missing rock record.

Bedding planes, or planes separating adjacent sedimentary layers, also represent gaps in the rock record but on a much smaller scale than an unconformity.

Relative Age Dating

Relative age dating is a way to use geometric relationships between rock bodies to determine the sequence of geologic events in an area.  Relative dating is different from absolute dating in which specific dates are assigned to geologic events (we will discuss absolute dating techniques later).

Relative dating is based on the five principles of stratigraphy discussed above.

Historical Perspective on the Origin of Rocks: Werner's Concept of Neptunism

Abraham Werner (1749-1817), a German geologist, proposed that Earth’s crust originated in ocean water through the process of precipitation.  This idea became known as Neptunism, in reference to the Roman God of the sea.

Werner classified rocks into 4 categories, as shown in the diagram below:

Figure by RJR

1. Primitive rock (red)– Granite and metamorphic rock were precipitated from oceans.

2. Transition rock (light brown)– Next, fossil-rich sedimentary rocks were precipitated.  These rocks are tilted due to deposition on the non-horizontal surfaces of primitive rocks.  This aspect of Werner's model was useful for explaining the origin of tilted sedimentary rocks.

3. Secondary rock (dark brown)– Flat lying sedimentary rocks were eventually precipitated.  The secondary rocks were thought to include interlayered basalts, which Werner thought formed by combustion of buried coal layers.

4. Tertiary (or alluvial) rock (yellow)– Finally, after the ocean receded, recent erosion and deposition created a thin veneer of overlying sediment.

Today we know that Werner's basic assumption that granite precipitated from seawater is incorrect.  We also know that basalt is not the product of coal combustion.

Nevertheless, Werner's concept of Neptunism was influential because:

1) Werener was right that some sedimentary rocks, such as limestones, do precipitate from ocean water.

2) Werner was not a catastrophist and did not need to make his interpretation of rock layers consistent with scriptual teachings.

3) Werner’s relative age assignments represents an early attempt to determine Earth's sequential history.

Historical Perspective on the Origin of Rocks: Hutton's Concept of Plutonism

The Scottish geologist James Hutton (1726-1797) argued that granite and basalt by solidification within the earth (as opposed to precipitating in from oceanwater).  This idea is known as Plutonism, in reference to the God of the deep underworld.

This concept of plutonism was supported by basalt melting/cooling experiments Sir James Hall conducted in 1792.  These experiments showed that the basalts form by the solidification of liquid magma.

Hutton viewed tilted strata as having been initially deposited horizontally, and then were subsequently deformed (tilted and folded) by the forces of Earth's internal heat engine.  He would argue that these forces gave rise to mountains.

Furthermore, he suggested that the mountains eroded to produce the sedimentary rocks we find in the rock record.

Hutton viewed the earth continually recycling itself with a balance between destruction and rejuvenation.  Mountains are created, eroded, and reformed.

Hutton’s ideas were not well received by people in the early 1800’s because he was a poor writer, and because his science was anti-catastrophic and did not support the scriptures.

How Sinkholes like Guatemala happen

How Sinkholes like Guatemala happen

kacimo samy   6:36 ص

This is How Sahara Desert Was Made

This is How Sahara Desert Was Made

kacimo samy   3:10 م

Weathering and erosion

Weathering and erosion

kacimo samy   10:40 ص

Weathering processes

Rock that is close to the land surface is subject to physical and chemical modification by a number of different weathering processes. These processes generally start with water percolating down into joints formed by stress release as the rock comes close to the surface, and are most intense at the surface and in the soil profile. Weathering is the breakdown and alteration of bedrock by mechanical and chemical processes that create a regolith (layer of loose material), which is then available for transport away from the site.

Physical weathering

These are processes that break the solid rock into pieces and may separate the different minerals without involving any chemical reactions. The most important agents in this process are as follows.

Freeze-thaw action

Water entering cracks in rock expands upon freezing, forcing the cracks to widen; this process is also known as frost shattering and it is extremely effective in

areas that regularly fluctuate around 0 degree Celsius, such as high mountains in temperate climates and in polar regions.

Salt growth

Seawater or other water containing dissolved salts may also penetrate into cracks, especially in coastal areas. Upon evaporation of the water, salt crystals form and their growth generates localised, but significant, forces that can further open cracks in the rock.

Temperature changes

Changes in temperature probably play a role in the physical breakdown of rock. Rapid changes in temperature occur in some desert areas where the temperature can fluctuate by several tens of degrees Celsius between day and night; if different minerals expand and contract at differentrates, the internal forces created could cause the rock to split. This process is referred to as exfoliation, as thin layers break off the surface of the rock.

Chemical weathering

These processes involve changes to the minerals that make up a rock. The reactions that can take place are as follows.

Solution

Most rock-forming silicate minerals have very low solubility in pure water at the temperatures at the

Earth’s surface and so most rock types are not susceptible to rapid solution. It is only under conditions of strongly alkaline waters that silica becomes moderately soluble. Carbonate minerals are moderately soluble, especially if the groundwater (water passing through bedrock close to the surface) is acidic. Most soluble are evaporite minerals such as halite (sodium chloride) and gypsum, which locally can form an important component of sedimentary bedrock.

Hydrolysis

Hydrolysis reactions depend upon the dissociation of H2O into H+ and OH ions that occurs when there is an acidifying agent present. Natural acids that are important in promoting hydrolysis include carbonic acid (formed by the solution of carbon dioxide in water) and humic acids, a range of acids formed by the bacterial breakdown of organic matter in soils. Many silicates undergo hydrolysis reactions, for example the formation of kaolinite (a clay mineral) from orthoclase (a feldspar) by reaction with water.

Oxidation

The most widespread evidence of oxidation is the formation of iron oxides and hydroxides from minerals containing iron. The distinctive red-orange rust colour of ferric iron oxides may be seen in many rocks exposed at the surface, even though the amount of iron present may be very small.

The products of weathering

Material produced by weathering and erosion of material exposed on continental landmasses is referred to as terrigenous (meaning derived from land). Weathered material on the surface is an important component of the regolith that occurs on top of the bedrock in most places. Terrigenous clastic detritus comprises minerals weathered out of bedrock, lithic fragments and new minerals formed by weathering processes. Rock-forming minerals can be categorised in terms of their stability in the surface environment. Stable minerals such as quartz are relatively unaffected by chemical weathering processes and physical weathering simply separates the quartz crystals from each other and from other minerals in the rock. Micas and orthoclase feldspars are relatively resistant to these processes, whereas plagioclase feldspars, amphiboles, pyroxenes and olivines all react very readily under surface conditions and are only rarely carried away from the site of weathering in an unaltered state. The most important products of the chemical weathering of silicates are clay minerals. A wide range of clay minerals form as a result of the breakdown of different bedrock minerals under different chemical conditions; the most common are kaolinite, illite, chlorite and montmorillonite. Oxides of aluminium (bauxite) and iron (mainly haematite) also form under conditions of extreme chemical weathering.

In places where chemical weathering is subdued, lithic fragments may form an important component of the detritus generated by physical processes. The nature of these fragments will directly reflect the bedrock type and can include any lithology found at the Earth’s surface. Some lithologies do not last very long as fragments: rocks made of evaporite minerals are readily dissolved and other lithologies are very fragile making them susceptible to break-up. Detritus composed of basaltic lithic fragments can form around volcanoes and broken up limestone can make up an important clastic component of some shallow marine environments.

Soil development

Soil formation is an important stage in the transformation of bedrock and regolith into detritus available for transport and deposition. In situ (in place) physical and chemical weathering of bedrock creates a soil that may be further modified by biogenic processes. The roots of plants penetrating into bedrock can enhance break-up of the underlying rock and the accumulation of vegetation (humus) leads to a change in the chemistry of the surface waters as humic acids form. Soil profiles become thicker through time as bedrock is broken up and organic matter accumulates, but a soil is also subject to erosion. Movement under gravity and by the action of flowing water may remove part or all of a soil profile. These erosion processes may be acute on slopes and important on flatter-lying ground where gullying may occur. The soil becomes disaggregated and contributes detritus to rivers. In temperate and humid tropical environments most of the sediment carried in rivers is likely to have been part of a soil profile at some stage. Continental depositional environments are also sites of soil formation, especially the floodplains of rivers. These soils may become buried by overlying layers of sediment and are preserved in the stratigraphic record as fossil soils.

Erosion and transport

Weathering is the in situ breakdown of bedrock and erosion is the removal of regolith material. Loose material on the land surface may be transported downslope under gravity, it may be washed by water, blown away by wind, scoured by ice or moved by a combination of these processes. Falls, slides and slumps are responsible for moving vast quantities of material downslope in mountain areas but they do not move detritus very far, only down to the floor of the valleys. The transport of detritus over greater distances normally involves water, although ice and wind also play an important role in some environments.

Erosion and transport under gravity

On steep slopes in mountainous areas and along cliffs movements downslope under gravity are commonly the first stages in the erosion and transport of weathered material.

Downslope movement

There is a spectrum of processes of movement of material downslope. A landslide is a coherent mass of bedrock that has moved downslope without significantly breaking up in the process. Many thousands of cubic metres of rock can be translated downhill retaining the internal structure and stratigraphy of the unit. If the rock breaks up during its movement it is a rock fall, which accumulates as a chaotic mass of material at the base of the slope. These movements of material under gravity alone may be triggered by an earthquake, by undercutting at the base of the slope, or by other mechanisms, such as water logging of a potentially unstable slope by a heavy rainfall. Movement downslope may also occur when the regolith is lubricated by water and there is soil creep. This is a much slower process than falls and slides and may not be perceptible unless a hillside is monitored over a number of years. A process that may be considered to be intermediate between creep movement and slides is slumping. Slumps are instantaneous events like slides but the material is plastic due to saturation by water and it deforms during movement downslope. With sufficient water a slump may break up into a debris flow.

Scree and talus cones

In mountain areas weathered detritus falls as grains, pebbles and boulders down mountainsides to accumulate near the bottom of the slope. These accumulations ofscree are often reworked by water, ice and wind but sometimes remain preserved as talus cones, i.e. concentrations of debris at the base of gullies. These deposits are characteristically made up of angular to very angular clasts because transport distances are very short, typically only a few hundred metres, so there is little opportunity for the edges of the clasts to become abraded. A small amount of sorting and stratification may result from percolating water flushing smaller particles down through the pile of sediment, but generally scree deposits are poorly sorted and crudely stratified. Bedding is therefore difficult to see in talus deposits but where it can be seen the layers are close to the angle of rest of loose aggregate material. Talus deposits are distinct from alluvial fans because water does not play a role in the transport and deposition.

Erosion and transport by water

Erosion by water on hillsides is initially as a sheet wash, i.e. unconfined surface run-off down a slope following rain. This overground flow may pick up loose debris from the surface and erode the regolith. The quantity of water involved and its carrying capacity depends not only on the amount of rainfall but also the characteristics of the surface: water runs faster down a steep slope, vegetation tends to reduce flow and trap debris and a porous substrate results in infiltration of the surface water. Surface run-off is therefore most effective at carrying detritus during flash-flood events on steep, impermeable slopes in sparsely vegetated arid regions. Vegetation cover and thicker, permeable soils in temperate and tropical climates tend to reduce the transport capacity of surface run-off. Sheet wash becomes concentrated into rills and gullies that confine the flow and as these gullies coalesce into channels the headwaters of streams and rivers are established. Rivers erode into regolith and bedrock as the turbulent flow scours at the floor and margins of the channel, weakening them until pieces fall off into the stream. Flow over soluble bedrock such as limestone also gradually removes material in solution. Eroded material may be carried away in the stream flow as bedload, in suspension, or in solution; the confluence of streams forms larger rivers, which may feed alluvial fans, fluvial environments of deposition, lakes or seas.

Erosion and transport by wind

Winds are the result of atmospheric pressure differences that are partly due to global temperature distributions, and also local variations in pressure due to the temperature of water masses that move with ocean currents, heat absorbed by land masses and cold air over high glaciated mountain regions. A complex and shifting pattern of regions of high pressure (anticyclones) and low pressure (depressions) regions generates winds all over the surface of the Earth. Winds experienced at the present day range up to storm force winds of 100 km to hurricanes that are twice that velocity. Winds are capable of picking up loose clay, silt and sand-sized debris from the land surface. Wind erosion is most effective where the land surface is not bound by plants and hence it is prevalent where vegetation is sparse, in cold regions, such as near the poles and in high mountains, and dry deserts. Dry floodplains of rivers, sandy beaches and exposed sand banks in rivers in any climate setting may also be susceptible to wind erosion. Eroded fine material (up to sand grade) can be carried over distances of hundreds or thousands of kilometres by the wind. The size of material carried is related to the strength (velocity) of the air current.

Erosion and transport by ice

Glaciers in temperate mountain regions make a very significant contribution to the erosion and transport of bedrock and regolith. The rate of erosion is between two and ten times greater in glaciated mountain areas than in comparable unglaciated regions. In contrast, glaciers and ice sheets in polar regions tend to inhibit the erosion of material because the ice is frozen to the bedrock: movement of the ice in these polar ‘cold-based’ glaciers is mainly by shearing within the ice body. In temperate (warm based) glaciers, erosion of the bedrock by ice occurs by two processes, abrasion and plucking. Glacial abrasion occurs by the frictional action of blocks of material embedded in the ice (‘tools’) on the bedrock. These tools cut grooves, glacial striae, in the bedrock a few millimetres deep and elongate parallel to the direction of ice movement: striae can hence be used to determine the pathways of ice flow long after the ice has melted. The scouring process creates rock flour, clay and silt-sized debris that is incorporated into the ice. Glacial plucking is most common where a glacier flows over an obstacle. On the up-flow side of the obstacle abrasion occurs but on the down-flow side the ice dislodges blocks that range from centimetres to metres across. The blocks plucked by the ice and subsequently incorporated into the glacier are often loosened by subglacial freeze-thaw action. The landforms created by this combination of glacial abrasion and plucking are called roche moutone'e, apparently because they resemble sheep from a (very) great distance.