In This Chapter
Sculpting the planet
Making soil for plants and food
Living with flood plains and sea shores
G radational force, which wears down the Earth’s crust and is the subject of this chapter, is the opposite of tectonic force, which builds up the Earth’s crust, and was discussed in Chapter 6. Indeed, you could say a competition of sorts is going on between those two powerful and opposing sets of forces which respectively wear down and build up Earth’s crust, and thus create and alter the natural landforms that give character to Earth’s surface. Gradational forces may not have the cataclysmic pizzazz of earthquakes and volcanoes, but their results, as shown in Figure 7-1, may be truly grand.
Listening to the reactions of people who are seeing the Grand Canyon in person for the first time is always extremely interesting. Some gasp. Some say, “I had no idea!” Others ask, “What’s for lunch?” Okay, so not everybody is mightily impressed.
But they ought to be! The beauty is spectacular, and the scale is grand. In Grand Canyon National Park, the featured attraction is a mile deep and 10 to 18 miles across, rim to rim, depending on where you measure from. By way of explanation, carved is a verb you see a lot — as in “The Colorado River carved the Grand Canyon.” Phooey. The river didn’t carve diddly. Instead, it carried away every last ounce of rock and soil that once occupied the space that is now the Canyon.
Now take a look at the Appalachian Mountains, even though it may seem like a complete change of subject. The Canyon “goes down” while the Appalachians “go up,” but not very grandly. Indeed, while they sport a decent peak or two, nobody is going to call them “The Grand Mountains.” But they used to be. As noted in Chapter 6, geologists estimate some of the Appalachians were once 30,000 feet high. That’s higher than Mount Everest, the highest mountain on Earth. But those once-mighty mountains got worn down and carried away, mainly by runoff from precipitation, making molehills out of mountains.
And so the Grand Canyon and the Appalachians — two very different landforms — turn out to have something fundamentally in common. A process of removal has shaped both of them. That is, both have been shaped by gradational force.
Getting Carried Away
Just how does gradational force work, as for example when it turns a 30,000 foot high mountain of yesteryear into a 3,000 foot high mountain of today? Basically, it’s a two-part process. Gradational force is part of nature, which has at its disposal mechanisms (described in the next section) that can break great big rocks into tiny bits of rocks that are easily transportable. After that, another set of mechanisms picks up the tiny pieces and carries them away. These two sets of mechanisms are known respectively as weathering and mass wasting. It may take them hundreds or thousands or even millions of years to turn a mountain into a molehill, but nature is in no rush. Indeed, it has all the time in the world.
Weathering the Earth
Weathering refers to the natural processes that break rock into smaller and smaller pieces. Weathering can be broken down into two types, mechanical and chemical.
Mechanical weathering is the disintegration of rock and other solid earth material by physical means. Here are the major mechanisms used to accomplish this:
Frost action. Some rocks allow water to occupy space between particles. If the water freezes, the resulting ice crystals may exert outward pressure that cracks the rock.
Hot-cold fluctuation. In some locations, rock may be exposed to extreme temperature fluctuations. Over time, alternating heat and cold causes expansion and contraction, respectively, and may result in some parts of the rock breaking off.
Root action. Fine, delicate roots may find their way into cracks or spaces within rock. As the roots grow, it may exert pressure sufficient to break the rock.
Abrasion. Chips may break off when a hard object scrapes or rubs against rocks. During the Ice Ages, gigantic glaciers pulverized and ground up untold tons of bedrock, converting it to pebbles, gravel, and soil particles that are still with us. This action continues with today’s glaciers, but on a much-reduced scale. Similarly, scraping or rubbing occurs when one piece of rock strikes another. This may happen as a rock rolls downhill, hitting other rocks as it goes; or when high winds send sand particles smashing into other rock particles; or when a swiftly flowing river causes one rock to hit another.
Chemical weathering is the disintegration of earth material by chemical means. Here are the major mechanisms used to accomplish this:
Rusting away (oxidation). Lots of rock contain bits of iron. When oxygen combines with the iron, rust develops and destabilizes the rock, possibly leading to its break-up.
Dissolution. Direct and prolonged contact between water and certain rock minerals may cause the latter to decompose and contribute to the ultimate break-up of rock.
Carbonation. Atmospheric carbon dioxide may dissolve in rainwater and create a weak carbonic acid. Long-term, this may also contribute to dissolution of certain elements in rock and contribute to its ultimate break-up.
By itself, weathering does not create a Grand Canyon or turn a mountain into a molehill. Instead, by converting large immobile pieces of rock into small transportable ones, it makes possible the movement of surface materials in ways that create or alter landforms.
On a totally different matter, weathering is fundamental to the creation of soil, which is discussed in some detail in the “Getting down and dirty: Soil” sidebar. Suffice it to say here that if you think gradation and weathering are relevant only to earth-science geeks, then guess again. It’s critically important to you; without it, there would be no soil, and therefore next-to-no vegetation, and therefore next-to-no food.
Getting down and dirty: Soil
One of the principal products of weathering is soil, which by definition is a collection of earth particles that are no more than 2 millimeters in diameter. Soil provides nutrients to plants, without which they simply would not thrive. The key to this is the process called osmosis, which is the transfer of nutrients from soil to plants through root membranes. When precipitation seeps into soil, it mixes with mineral and organic matter and becomes a kind of a “nutrient soup” that coats soil particles. Plants come into contact with soil by means of their roots, which take in the “soup” by osmosis, and thus receive nutrients from the very substance in which they grow.
A fertile soil is one that makes lots of nutrients available to plants. An infertile, or poor, soil is one that does the opposite. Soil fertility varies geographically and is one of several important elements that explains why the population of a particular country may be better nourished than people who live elsewhere. Factors that determine soil fertility include the following:
Parent material. This is the bedrock from which soil is derived. It can be extremely hard, like granite; or fairly soft, like limestone; or in between. Generally, soft rock is best because it weathers easily and produces much more soil than harder parent material. Thus, the nature of the bedrock that underlies an area may have much to do with the fertility of the soil.
Particle size. Soil particles can be large (sand), tiny (clay), or in between (silt). Sandy soils tend to be infertile because the “nutrient soup” tends to seep through rather than be held and made available to plants. Clay soils tend to be infertile because particles are spaced tightly together, making it difficult for the “soup” to seep in or roots to penetrate. Silty soils, in contrast, tend to be very productive because they both admit and hold good amounts of “soup,” as well as roots. The bottom line, therefore, is that moderate-size particles generally make the best soil.
Climatic conditions. Soil fertility tends to be best where the climate is not too hot, not too cold, not too wet, or not too dry. High heat speeds up organic decay, basically wasting a high volume of nutrients before soil can make them available to plants. Low temperatures slow down decay, and again render nutrients unavailable to plants. Very wet conditions flush away nutrients in soil (a process called leaching), while very dry climates produce very little “soup” to surround soil particles. In contrast, climatic moderation (such as occurs in the middle latitudes) encourages nutrient accumulation and retention in soil, and thereby enhances fertility.
Profile depth. This is the vertical distance from surface to bedrock — “thickness,” in other words. Thick is preferable for the simple reason that more soil is present. As is the case with other factors, thickness varies geographically. Soft-parent material may be responsible for locally thick soils, and so, too, the process of deposition, as when a silt-laden river overflows its banks and adds a new layer of sediments to the flooded countryside (also, see “Deposition” in this chapter).
Plant cover. Organic matter contributes greatly to the quality of topsoil. Thus, plant cover is an important determinant of the geography of soil fertility. Generally, grasslands are best because their fine roots and root hairs readily decompose and are in the soil to begin with. Leaf-fall from trees has good nutrient potential, but these are deposited on top of the ground and therefore are dependent on climatic factors (rainfall and temperature) to mix with topsoil.
Mass wasting is the movement of particles that are products of weathering. Thus, it’s the aspect of gradation that is most directly concerned with creation and alteration of landforms and has two discrete components:
Erosion: This is the removal of particulate matter (pieces of soil and rock) from a particular location. Thinking back to the Grand Canyon, running water removed — or eroded — material from the space now occupied by the canyon. Of course, those eroded pieces didn’t just disappear. Instead, they went someplace else.
Deposition: This is the putting down (or coming to rest) of eroded materials. Returning once more to the Grand Canyon, all the eroded material that was carried away by the Colorado River was eventually deposited either down-river or settled in the Gulf of California, into which the Colorado empties.
The difference between erosion and deposition is rather like that between pick-up and delivery. Material is taken from one place and put in a different place.
Changing the Landscape
Beauty, as they say, is in the eye of the beholder. You can find it in art museums throughout the land and, as far as geography is concerned, in the land itself. Sculptures are made by removing material that once surrounded the final forms. Compositions are achieved by bringing together materials that were formerly separate. In a similar manner, erosion and deposition (see previous section) are creative processes. Like standard works of art, the results may inspire us or, frankly, leave us unmoved. Either way, the creative power of gradation produces works that we live on and, in the case of soil, cannot live without.
In the real world, nature has four means, or agents, at its disposal to quite literally carry out erosion and deposition: gravity transfer, flowing water, glaciers, and wind. Although a glacier is a form of flowing water, the gradational actions of solids (ice) and liquids is sufficiently different to merit separate treatment.
Staying grounded: Gravity transfer
Gravity is constantly “pulling down” on surface material. If not somehow restrained, therefore, particulate matter ranging in size from soil to boulders may move downslope in an act of gravity transfer. This process can be awesome, as in the case of a landslide. Much more common, however, are the decidedly unspectacular minute movements (soil creep) of small particles, and the occasional pebble and rock that roll a bit downhill. Given enough time, however, the cumulative effect of gravity transfer may be really noticeable. Uplands are eroded and reduced, while deposition creates new landforms, as when rock materials accumulate at the bases of mountains or cliffs to form talus cones, as shown in Figure 7-2.
Going with the flow: Water
Flowing water is far and away the principal agent of erosion and deposition. The extent to which it can rearrange the landscape is largely dependent on three things that vary widely.
The amount of water. This is a no-brainer. The larger a river or wave, the greater its ability to move surface materials.
Velocity. The faster water travels, the greater its ability to pick up and move surface material. On land, gradient (the steepness of a slope) is a major determinant of speed. The steeper the inclination, the faster water travels. Similarly, storms at sea greatly increase the size and velocity of waves, and therefore greatly increase their impact on coasts.
Surface cover. Generally, the more open or bare a landscape, the greater is the likelihood of its alteration by flowing water. Vegetation inhibits erosion by slowing down flow speed and by generating root systems that hold soil in place. Bare ground, in contrast, is rather at the mercy of water and velocity. Thus, one of the great geographical ironies is that flowing water is the principal agent of landscape change in dry areas, where bare ground is very common. Rain and streams may be scanty in arid and semi-arid regions, but it doesn’t take lots of flowing waters to make major impressions on the land in those areas. Witness the Grand Canyon, for instance.
Any volume of water that interacts with earth’s surface can produce mass wasting. That includes small-scale phenomena like raindrops and rivulets. Thus, run-off on exposed soil in an agricultural field or even a backyard garden may erode soil and result in gullying that produces very miniature versions (an inch or so deep) of the Grand Canyon. From the perspective of geography, however, it is large-scale phenomena, like rivers, waves, and ocean currents, that are of greatest interest.
Rivers and streams
The characteristics of individual rivers and streams vary greatly. In very general terms, however, one may think of a river system as originating in highlands, gathering the waters of tributaries as it snakes through foothills, and then flowing through a low-lying coastal plain as it approaches the seas (illustrated in Figure 7-3). The nature and effects of mass wasting tend to be very different in each of these settings. Here is what happens in each setting:
Highlands setting: By their nature, highlands are high above sea level. Steep gradients are most likely to occur there, resulting in rapidly flowing streams that erode their beds and carry away weathered material with comparative ease. This process creates valleys. In highlands, V-shaped valleys are fairly commonplace, and a sure sign that erosion rather than deposition is playing the largest (in fact, almost exclusive) role in changing the landscape.
Foothills setting: In foothills, river gradients tend to be much less steep, resulting in decreased velocity and, therefore, decreased erosion of streambeds. In fact, erosion of valley walls may exceed erosion of the riverbed. As a result, the profiles of foothills valleys may assume the shape of a somewhat flat-bottomed V — or rather \_\ — with much of their floors being occupied by land rather than river.
While the fringing slopes — that is, the lines of the V — provide evidence of the continued presence and power of erosion, the appearance of a flat and comparatively (versus the highlands) expansive valley floor is testimony that deposition has been an active player in shaping the landscape. Not uncommonly, the width of these valley floors is sufficient to accommodate modest-size towns and substantial agricultural activity.
Coastal plain setting: In the coastal plain, rivers are at their peak volume by virtue of so many tributaries having added their waters to the combined flow. Velocity is fairly slow, however, since by definition coast plains are low-lying and flat. Here the individual rivers flow through broad floodplains bound by diminutive valley walls. The rivers themselves are bound by even less diminutive natural levees that consist of sediments deposited during past episodes when the river overtopped its banks.
Indeed, “floodplains” aren’t so-named for nothing. Because the landscape is so flat and expansive, flooding tends to affect a very wide area. Such events are beneficial to the extent that silt and other sediments in floodwaters are deposited and enrich the soils. On the other hand, flat and productive land attracts people and enterprise, and thus virtually guarantees that floods have a major impact on life, property, and infrastructure (see sidebar).
When rivers empty into the sea, they disgorge eroded sediments that have been carried along. Where coastal waters are fairly calm, these particles may be deposited at the rivers’ mouths, and progressively accumulate and form a delta, as shown in Figure 7-4. This landform is so-named because many of them — most notably that of the Nile River — assume a roughly triangular shape reminiscent of the Greek letter delta (∆).
The floodplain: Land of promise and pitfalls
Floodplain refers to flat lands beside rivers that are prone to flood when the waters overflow their banks. Their extent may vary from a few feet to many miles on either side of the watercourse. The latter is common in the case of large rivers flowing through coastal plains.
Floodplains often are characterized by rich, fertile soils (alluvium) that have been deposited by past floods. For thousands of years, people have been attracted to these areas due to their favorable prospects for good harvests. Indeed, the attraction is so great that several alluvial regions — including the lower Nile, Ganges, Huang, and Yangtze (Chang) Rivers — have long supported very high human-population densities. And therein lies a major predicament: The alluvial lands that attract and nourish so many, may also be the scenes of terrible flooding and uncountable drownings. For that reason, the Huang (Yellow) River, which provides irrigation for millions of acres of cropland, is also called “China’s Sorrow.”
In modern times, engineering has sought to control (and ideally eliminate) flooding by building high artificial levees (banks) intended to make rivers stay put. While these defenses generally work, they occasionally fail in the face of record high water, as happened spectacularly and tragically in the cases of the Mississippi and Missouri Rivers during the summer of 1993. Indeed, critics claim that, for two reasons, these bulwarks merely guarantee that the inevitable flood will be exceptionally and unnecessarily disastrous. First, they say, the human earthworks (levees) create a false sense of security that encourages construction and settlement in patently hazardous areas. Second, artificially high levees negate the natural “sponge effect” of long, wide floodplains, and therefore magnify the effects of the inevitable flood.
Low-cost federal flood insurance is an interesting variable in relation to the floodplain. It clearly and compassionately helps to relieve the pain and suffering of people who “lose everything” in times of flooding. But critics claim its promise of compensating individual losses also encourages settlement of hazardous areas, and thus may serve to increase the very suffering it seeks to relieve.
Waves and currents
Coastal areas bordering oceans and seas witness significant gradational activity. Every wave that strikes land potentially performs mechanical weathering, however minutely. Every drop of wave water that soaks land may contribute to chemical weathering. And, of course, every wave and coastal current, if strong enough, can erode sand and other coastal particles and deposit them somewhere else. As a result, coastal zones are among those parts of Earth that are most prone to change by natural means.
Cliffs characterize many coasts, and are a sure sign of erosion. Each wave mi-nutely helps to weather and erode the base of those landforms, undermining the entire cliff. Eventually, a portion of the cliff collapses and the shoreline retreats, that is, erodes inland. Figure 7-5 shows an example of a cliff formation. The rate of erosion varies from one cliff to the next depending on its composition. For example, a cliff composed of a mix of soil, gravel, and rock retreats far more rapidly than one composed of solid rock.
“Million-dollar views” may be had from cliffs, and that often results in prized pieces of property. The fact that a cliff is an erosional feature, however, virtually guarantees loss of adjacent real estate. Property owners who build houses close to drop-offs sometimes try to artificially stabilize the cliff to prevent further erosion. While such efforts usually are effective against minor storms, they stand little chance against repeated major ones.
Coastal areas that consist of sandy beaches and dunes are among the easiest landforms for nature to erode because they consist of small particles. The number of pieces of sand that make up a beach is beyond comprehension; but as far as mass wasting is concerned, particle sizes matter more than particle numbers. Take a nice sandy strand consisting of several giga-trillions of sand particles, let a severe storm pound away at it for several hours, and the result may be a greatly diminished beach.
The implications of this for property owners are severe and have been recognized for some time. A well-known parable compares a house built on rock to one built on sand. After a storm, the former remains standing while the latter has been washed away, which indicates that people thousands of years ago understood the basics of coastal erosion.
That knowledge has not, however, deterred people from building on sand. In the United States the number of people who live along the coast has soared in recent decades. Part of this is simply due to general population growth, as a result of which coastal cities have expanded up and down their respective shorelines. Probably of greater importance, however, in explaining the extent of coastal development is the American love affair with the beach, and the growing number of people who possess the financial wherewithal to purchase vacation or retirement homes by the water. Indeed, the combination of disposable income plus competitive bidding has served to make coastal real estate among the highest priced to be found in non-urban settings.
Nature, however, is not impressed by price tags. In several instances severe storms have eroded dunes from under houses, literally leaving them high and dry, such as in Figure 7-6. In response, several coastal communities have spent large sums of money to replenish beaches by dredging or pumping in offshore sand. At best, however, these are short-term cures with paltry prospects for long-term success, given the inevitability of future storms aided and abetted by rising sea levels (see Chapter 8).
Of course, all the material that is eroded along beaches and cliff fronts goes somewhere. Much may be carried out to sea and come to rest on the ocean bottom. Sand, for example, may accumulate in the near-shore environment, forming shallow rises called sand bars. Other materials may create or add to spits (small points of land that extend outward into the water), or long nar-row barrier islands that roughly parallel the coast. The latter are particularly important elements of the East Coast of the United States. New York’s Fire Island, the Jersey shore, North Carolina’s Outer Banks, South Carolina’s Hilton Head, Cape Canaveral, Miami Beach — barrier islands are numerous and have seen significant development in recent decades. Each, however, represents a huge collection of small particles that can be eroded far more readily than they are deposited (see the Applied Geography sidebar).
The chill factor: Glaciers
A glacier is a large, moving mass of ice on land. It originates when more snow falls in winter than melts in summer. If this is repeated for many years, then the annual surplus of snow compacts under its own weight and forms ice.
The massive weight of glaciers is capable of grinding up even the hardest rocks into soil particles. Since glaciers move, they are a combination earth crusher-bulldozer that perform weathering, erosion, and deposition all in one. The precise nature and results of these activities depends on whether the ice in question is a mountain glacier or a continental glacier.
As the name suggests, mountain (or alpine) glaciers originate in snow that falls in mountainous areas. When large ice masses form, the slopes facilitate their downhill movement, and thus their power to erode and transform the landscape. Far up-slope, the erosional power of so much ice “gnaws away” at the mountain, turning rounded tops into pointed horns and nondescript ridges into jagged crests called arêtes.
Applied Geography: Insuring against erosion
People who buy land and build on it generally expect their property to be there tomorrow. But just in case, they buy insurance. Some policies that cover coastal real estate are issued by private companies and others by branches of government. Either way, and rather often, erosion takes its toll, so policy owners end up cashing in.
If you are thinking, “I don’t own coastal property, so this doesn’t concern me,” then I beg to differ. Every successful insurance claim drives up policy costs, so, in that sense, everybody pays to compensate people for eroded coastal property. In recent years, however, many have begun to question the wisdom of such insurance. Key to this is growing public awareness of basic physical geography, particularly erosion as it relates to beaches. In some states and regions, the results have been moratoriums (or talk of them) on beachfront housing, or on insurance policies that protect them. In so doing, the general public and their elected representatives have been applying a basic concept that geographers have appreciated for some time, namely, that the works of humans ultimately stand little chance of permanency in that most dynamic of natural environments, the coast.
Main valley floors are widened and deepened, changing from V-shaped valleys into larger U-shaped glacial troughs. Higher up, side valleys are also carved out. Less ice flows through them than the main valley, so less down-cutting occurs. Once the glaciers recede, the former side valleys are left “hanging above” the main valley, and are thus referred to as hanging valleys. In the mountainous coastlands of Norway, New Zealand, Chile, Western Canada, and Alaska, glacial troughs have filled with seawater, resulting in steep-sided ocean inlets called fiords.
Glaciated mountains have a spectacularly rugged look about them. Perhaps not surprisingly, therefore, such areas tend to be major magnets of travel and tourism. Also, one must remember that these landforms were created by erosion, which leads one to ask where all the removed material went. The answer is that it was deposited downslope, forming or mixing in with soil or, in the case of fiords, ocean bottom.
Continental glaciers build up over large land masses in general, as opposed to mountains in particular. At the height of the last Ice Age, some 20,000 years ago, they covered substantial portions of North America, as well as fair portions of Eurasia. The present ice caps of Greenland and Antarctica (see Chapter 8), which are more than 2-miles thick in some places, offer insight into continental glacier scale and movement.
In North America, massive ice sheets (as seen in Figure 7-7) slowly built up over many years in what is now Canada. Eventually, the unimaginable weight and volume caused the ice to “ooze” outward in response to the pressure, at which point, the ice technically became a glacier. As the ice sheet advanced, its immense weight weathered the underlying Earth and gorged out (eroded) low-lying landforms through which it passed. Thus were formed, for example, the beds of the Great Lakes, the Finger Lakes (New York State), and numerous other future water bodies. Much of Canada was scoured by that process, which explains the relative lack of soil today over parts of that country, as well as the presence of thousands of lakes.
Eventually, of course, all of the eroded material being carried or pushed along by the ice got deposited, particularly as climates warmed and the ice sheets waned. In some places, these deposits of rock, sand, and other debris (called glacial till) merely coated the surface. In others, however, major accumulations occurred, resulting in landforms called moraines. Long Island and Cape Cod are noteworthy examples. These were created by the bulldozing effect of the ice sheets and the transporting and deposition of debris beneath them.
Generally, as the ice receded across the United States, it left behind a blanket of stones, gravel, and soil. Melt water issued from the retreating glaciers in innumerable streams that carried and deposited fine soil particles, which further transformed the post-glacial landscape. Today, thousands of years later, much of those materials underlie America’s agricultural heartland and are important factors in explaining its productivity.
Making a deposit: Wind
Given sufficient velocity, wind can pick up soil particles and carry them long distances before they are deposited. This was dramatically demonstrated in the 1930s when portions of the American High Plains endured the Dust Bowl phenomenon. Wind-related erosion and deposition occurs most commonly in arid and semi-arid areas where ample bare ground is exposed to the atmosphere. In such venues, dust and sand “storms” of varying intensity are not unusual. Sand dunes are a classic landform that results from the inevitable deposition.
In a few non-arid parts of the world, substantial (that is, meters deep) deposits of wind-blown silt, called loess, are present. Because these are very fertile soils, loessal areas are among the most agriculturally productive to be found anywhere. Large areas of the American Midwest are covered by loess, as illustrated by Figure 7-8. Some of it probably blew in from arid areas. The common belief is that most loess, wherever they are found, originated long ago in dry glacial till that was exposed to wind after the ice sheet retreated.