How Surface Energy Shapes The Land

Sources of the Energy

Land surfaces everywhere are nearly always uneven or hilly to some extent, even where there are no mountains; expanses of truly flat land, such as dry lake beds, are always surrounded by higher ground. Wherever you look, the land has relief or "non-flatness."

This statement is so obvious that it goes without saying, and like many other such statements, it deserves more attention than it usually gets. Why is it true, and what are its implications? Put briefly, the answer to the first question is that land is raised into hills, ridges, mountains, and volcanoes by the earth's internal energy (as described later, in chapter 15). The raised surfaces are simultaneously worn down by wasting and erosion, which sometimes smooths the relief and sometimes—as when rivers erode deep valleys, for instance—exaggerates it. The energy of these external agents is the topic of this chapter.

Now for the implications of the fact that the land isn't level everywhere. Energy from the earth's interior, in deforming the earth's surface and lifting up mountains, imparts to it potential energy— specifically, gravitational PE (see chapter 2). Recall that gravitational PE is always relative to some chosen base level, usually sea level. This means that any chunk of rock or soil above sea level will move downward if something happens to dislodge it. If it cannot do so now because it is already at the bottom of a valley or hollow, it may be able to in the future when the object's surroundings have been eroded away, leaving it "poised" for a fall. In the few places on earth where the surface is at present below sea level, for example, the valley of the Dead Sea and Death Valley in California, the surrounding slopes have PE relative to the respective valley bottoms.

We now ask how the potential energy of the land is liberated. Or, which comes to the same thing, how the earth's crustal material shifts from higher to lower elevations. Two processes are involved: mass wasting and erosion.

Mass wasting is a general term that includes landslides, rockfalls, earth flows, earth slumps, debris flows, mudflows, and soil creep.1 The downhill movement of snowbanks and sloping snowfields in mountainous country is another, often disregarded, form of mass wasting: everything from sudden avalanches to the slumping of crusted snow to the slow downhill dribbling of "crumbs" of snow is included here. By definition, mass wasting is the downward movement of material caused solely by the pull of gravity.

Erosion, the other process that shifts earth materials from high elevations to low, is transport by flowing water, wind, or glaciers. The need for a medium of transport distinguishes erosion from mass wasting. The two processes resemble each other, however, in that both act on separated chunks of material, sometimes big blocks of rock in the case of mass wasting, more often tiny particles of sand, silt, and clay—"grains" rather than "chunks"—in the case of erosion.

This raises the problem of what breaks rocks from the earth's solid bedrock in the first place and then fractures the detached blocks into smaller and smaller particles. The process is called weathering.


Weathering is the disintegration of surface rock. Wherever bare rock is exposed at the surface, it is always cracked or broken to some extent, sometimes into giant blocks, sometimes into an expanse of sharp-edged rock fragments, sometimes into a layer of crumbs or flakes. The inorganic ingredients of the soil, everything from coarse sand to minute clay particles, are ultimately derived from bedrock by various weathering processes.

Before we go into the details of how rocks disintegrate, it is worth asking what holds them together in the first place. A more inclusive question is, Why doesn't any solid object, be it a rock or a teacup, fall to pieces spontaneously? The object must consist of a collection of atoms and molecules, so why don't the atoms and molecules simply lie there like a pile of dust? What holds them together in definite, recognizable shapes?

These questions are the subject matter of solid-state physics; as with all branches of science, the more you know, the more aware you become that unlimited fields of discovery lie ahead. All we need say here is that what makes solid objects solid is chemical bonds (about which more in chapter 10) and that breaking anything solid, rocks included, entails the rupture of chemical bonds, a process that consumes energy.

Now, briefly, for the details. Rocks disintegrate in two ways: by mechanical weathering and by chemical weathering. In spite of the names, both kinds of weathering entail the breaking of chemical bonds, but only chemical weathering involves chemical reactions in the ordinary sense.

Mechanical weathering at the surface acts on rocks already cracked while they were deep underground. Igneous rocks form when magma (the hot, molten rock at depth) cools and solidifies; the magma shrinks as it crystallizes, and the shrinking produces fissures. Sedimentary rocks, formed when loose sediments become cemented, are also apt to crack; tremendous pressures develop when tectonic plates move against each other, forcing sedimentary strata to bend and fold. This sets up tensions that cause intermolecular chemical bonds to break and fine fissures to develop on the outer sides of the folds.2

Then erosion removes the tremendous weight of material lying on top of the rocks; although they have cracked while deep underground, the fissures have been kept tightly closed by the pressure of the surrounding material. Removal of the overlying sediments allows the rocks to expand: molecular bonds that had been compressed and shortened now lengthen, and they snap if they are stretched too far.3 Some of the fissures enlarge into clean breaks. The whole process is called unloading. Once the fissured rocks are exposed to the air, true mechanical weathering can begin (fig. 9.1).

It happens in a variety of ways, one of which is thermal cracking. When rocks are heated by the sun on a sunny day they expand; then as they cool by radiating heat into a clear sky at night, they contract. The alternate expansion and contraction cause further fracturing.

In cold climates, thermal cracking is augmented by frost cracking: water penetrates exposed fissures and freezes when the temperature falls below the

Figure 9.1. How mechanical weathering starts. (a) Magma from the depths has welled up to form a dome of igneous rock below sedimentary strata; fine cracks have formed as the magma crystallized. (b) Millions of years later. Most of the sedimentary strata have been eroded away; relieved of pressure, the cracks have widened, especially those in the outcrop with no load on them. Frost cracking widens them further.

Figure 9.1. How mechanical weathering starts. (a) Magma from the depths has welled up to form a dome of igneous rock below sedimentary strata; fine cracks have formed as the magma crystallized. (b) Millions of years later. Most of the sedimentary strata have been eroded away; relieved of pressure, the cracks have widened, especially those in the outcrop with no load on them. Frost cracking widens them further.

freezing point. Water in pores in the walls of the fissures also freezes, as does water in pores deeper in the rock, which migrates to the newly formed ice and freezes onto it. The result is an increase in the volume of water trapped in a fissure and freezing there. It expands as it freezes until it cracks the rock.4 Water expands on freezing because the geometrically arranged water molecules in ice crystals occupy more space than they did while the water was liquid.5

Let's consider the energy exchanges in frost cracking. As the water filling a fissure cools, it loses some of its thermal energy by radiation. But not all: the rest is stored as chemical potential energy in the molecular bonds in the ice crystals. This PE is "spent" in stretching and finally rupturing the chemical bonds of the rock. It's worth repeating that when a solid object breaks, what breaks are the chemical bonds holding it together; that's what breaking is.6

Not all mechanical weathering begins with the cracking of bedrock, however. Rocks sometimes grind together with enough force to pulverize each other, yielding fine-grained rock flour. The "rock milling" happens when boulders embedded in the base of a glacier grind against the rocks below. Rock are also crushed and pulverized along geological fault planes.

Now for chemical weathering: it is the disintegration of rock as a result of chemical reactions. Every chemical reaction entails an energy change, sometimes a gain, sometimes a loss. In weathering reactions the change is always a loss: energy is liberated.

In the chemical weathering of rocks, the most frequent reactions are those caused by weak acids attacking and dissolving some of the rocks' component minerals. Weak acids are much commoner than pure water in the natural world. Rain, for example, is always slightly acidic because as it falls it dissolves a fraction of the carbon dioxide in the air, which converts the rain into dilute carbonic acid. Sulfuric acid is produced when sulfide rocks such as pyrite break open (because of either mechanical weathering or mining) and expose fresh surfaces to the air; the exposed sulfur becomes oxidized and dissolves in water to form dilute sulfuric acid. Another source of the acids that attack rocks is living material. A variety of corrosive organic acids are produced by microbes, and also by lichens growing on rock surfaces. Acids also come from the plants and invertebrate animals, both living and dead, that form the organic portion of soil.

The chemical weathering of rocks is greatly promoted if they have been mechanically weathered beforehand; the more fragmented the rock, the more surface there is for acids to work on. In time the products of both forms of weathering become jumbled together as a loose layer overlying solid bedrock; this is called regolith.

The most finely divided products of chemical weathering are, with one exception, divided much more finely than the products of mechanical weathering; the exception is rock flour, which is not produced in large quantities. The smallest, most abundant products of chemical weathering are clay particles, derived from feldspar, the commonest mineral on the earth's surface. The next most abundant, and notably coarser, are quartz crystals—that is, sand grains. Quartz is the most resistant to chemical attack of the common minerals, so it accumulates as a residue when any of the many kinds of rocks that contain it are chemically weathered.7

Most of the products of mechanical weathering are so much coarser than those of chemical weathering that it is not surprising that their subsequent fates are different too. For the most part, the products of mechanical weathering are shifted by mass wasting. Some of the products of chemical weathering are removed by erosion, and some dissolve in water and flow away with it. These processes combined carry weathered rock to lower elevations.

Mass Wasting

Of all the ways mass wasting happens, big landslides are the most spectacular. Whenever a mass of unattached or weakly attached rock chances to accumulate at the top of a steep slope, a landslide impends. The accumulated material, held precariously in place by friction, may be the product of long-continued weathering, or the debris of earlier landslides, or the ejecta of a nearby volcano. The poised mass awaits the conversion of its PE to KE. When something happens to trigger it, the mass starts to slide or fall, and the conversion begins.

The trigger may be a heavy rainstorm. For example, the excessively heavy rain accompanying Hurricane Mitch in October 1998 filled a lake in the crater of a dormant Nicaraguan volcano to overflowing; the escaping water, mixed with volcanic ash, created a mudslide that buried about 2,000 people. Numerous other mudslides caused by Mitch, together with flooding, brought the death toll to more than 11,000.

A landslide believed to be one of the earth's largest in several thousand years was the Frank Slide, which fell in April 1903 near the eastern entrance to the Crowsnest Pass through the Rocky Mountains, in southwestern Alberta.8 Almost half the top of Turtle Mountain collapsed into the valley below, burying much of the little coal mining town of Frank; about 70 people were killed. The slide is thought to have been triggered by frost cracking on the grand scale, caused when large volumes of meltwater from a heavy snowpack poured into fissures on the mountain's summit; the rocks may have been weakened beforehand by coal mining at the foot of the mountain.

The weight of the Frank Slide has been estimated at 9 x 1010 kg, and the distance it fell was approximately 1,000 m. With these numbers we can quickly calculate the amount of potential energy that its fall liberated (see chapter 2). Recall that the number of joules of energy is the mass (in kilograms) times the height of the drop (in meters) times the acceleration due to gravity, which is 9.81 meters per second per second, or 9.81 m s-2. The energy of the slide was therefore

(9 x 1010) x 103 x 9.81 J, or approximately 9 x 1014 J.

This enormous amount of energy was dissipated in about one hundred seconds. What happened to it?

Every time one rock strikes another, both are deflected and diverge in new directions and with altered speeds. Their combined speed, and therefore their combined KE, is reduced because both rocks are dented, broken, or chipped— which uses up some energy. If the collision is gentle these "deformations" may be too slight to be noticeable, whereas a more violent collision causes one or both rocks to shatter. In any case, chemical bonds are altered and heat energy is released, as is obvious when sparks fly.

By the time all the rocks come to a halt at the bottom of the fall, their gravitational PE has been lost because their elevations have been lowered; it could be restored if all the rocks were carried up to their original positions again— entailing much work! At the same time, more energy, including noise energy, has been generated by all the collisions and has been immediately dissipated as waste heat (entropy).

In some landslides the debris comes to an abrupt stop on reaching level ground, piling up into a big mound; in others the debris continues to move forward and doesn't come to rest until it has spread out over a large expanse of lowland. Such slides are known as long-runout slides. When a long-runout slide reaches the bottom of a valley, part of it may even keep on going, climbing some way up the opposite slope.

The Frank Slide is a famous example of a long-runout slide. After a rapid descent of 1,000 m, it "flowed" onward, across the Crowsnest River valley and 130 m up the valley's far side, leaving a sheet of shattered rock 30 m thick spread over the land. The leading edge of the sheet is 4 km from the base of Turtle Mountain; when you stand there, in a "sea" of broken limestone, it seems inconceivable that the debris came all the way from Turtle Mountain, 4 km away in the distance.9

Another well-known long-runout slide is the Elm Slide of 1881. It destroyed much of the Alpine village of Elm in Switzerland, killing 115 people. About 8.2 x 1010 kg of rock slid 600 m down a mountainside, from which it follows that the potential energy lost in the slide was close to 5 x 1014 J, a little more than half that of the Frank Slide. After reaching the foot of the mountain, the slide flowed on for 2 km before stopping; the average slope of the de scent from start to finish was only 17°, much less than the normal angle of rest of piled rocks. As with the Frank Slide, some of the debris flowed uphill.10

The noteworthy character of long-runout slides is that their debris appears to flow like a liquid instead of behaving as you might expect a heavy mass of solid material to behave when it lands abruptly. For years the surprising "flow" of landslide debris was thought to happen when a cushion of air became trapped beneath the falling debris. But the debris of long-runout slides is observable on the moon, where the walls of some large lunar craters formed by meteorite impact have collapsed; trapped cushions of air cannot be the cause on the airless moon. The "fluidization" of dry rock debris is now believed to have the following explanation: In certain conditions, the mass of separate fragments forming the debris act like "molecules in a gas . . . .The entire collection of rocks [behaves] like a dense gas and ... naturally, [flows] like a fluid."11 The phenomenon is called acoustic fluidization.

When slide debris flows uphill at the end of its runout, as happened at Frank and Elm, it is imitating to a small degree the behavior of a glass marble released just inside the rim of a smooth bowl, which rolls to the bottom of the bowl and then nearly to the top on the opposite side. In both cases the potential energy of a mass (slide debris or marble) is converted to kinetic energy as it descends to the foot of a slope (a mountainside or the side of a bowl), after which momentum carries it upslope again, promptly restoring a portion of the PE it has lost. The rest of the energy is dissipated as heat and noise. The only difference between the two cases is that the proportion of PE conserved is far greater, and that of PE dissipated far less, for the marble in a bowl than for the rocks sliding into a valley.

Big landslides are soon over: huge quantities of debris complete their journey in seconds or minutes. Mass wasting in slow motion takes place too when individual rocks fall from a precipice one at a time and accumulate at the bottom; the commonest cause is frost cracking. The angular rocks pile up where they land, forming steep slopes of scree (also known as talus or colluvium) lying at the material's angle of rest, which is typically between 30° and 35°.

Scree slopes are common in mountainous country. The potential energy released while a scree slope builds depends on the mass of rocks in the scree and on the distance they have fallen. It is calculated in the same way as the PE of a sudden landslide; the amount of potential energy is the same whatever the speed of its release.

The slowest, least conspicuous form of mass wasting is creep. It happens on all slopes, however gentle. Obviously, any crumb of soil or particle of rock that chances to be dislodged on sloping ground will automatically move downs-lope, because the pull of gravity prevents it from moving upslope. Creep is the result. Crumbs of soil are dislodged in innumerable ways; they are scattered from the roots of wind-thrown trees; they are shifted by the movements of burrowing animals and pushed by the shoots of growing plants; they are raised and then let down a millimeter or two farther downslope every time the ground beneath them freezes and thaws. Wetting and drying expand and contract the soil just as freezing and thawing do, with the same result. Wind and rain displace both soil crumbs and rock particles.

A particle nudged out of place by moving air or water, however, is the object of erosion as much as of mass wasting. On a small scale, the two processes lose their distinctness.

River Erosion

Erosion is the transport of weathered rock by moving fluids, either water or air. At the present time (geologically speaking) erosion by rivers is the principal form of erosion.12 The questions to be looked into are, How do rivers transport the material produced by weathering? What becomes of the material? And what are the energy exchanges?

The material to be transported consists of clay particles, sand grains, and cobbles, plus a small amount of rock flour that behaves like clay particles. These materials roll into rivers after "creeping" down adjacent valley slopes, or they are washed in by rain, or they come from clods of soil that fall into the water from a river's banks after a rainstorm and disintegrate. These last are recycled particles, as we shall see below.

Once they are in a river, the particles become sediment. The two chief components of the sediment, clay and sand, behave differently because of the markedly different sizes of their particles.13 The fine particles, clay and silt (plus rock flour), are light enough to remain suspended in the water, whose turbulence supports them; they make the water muddy, and they are borne along with the water, whose speed is only slightly reduced.

The coarse particles, mainly sand grains, sink to the bottom because of their weight. If the current is gentle they become one with the riverbed, which more often than not consists of the same material. When the current speeds up, the sand at the bottom is swept along by the moving water as bedload.

A word on the subsequent fate of these sediments before we return to consider the energy of all these activities. River sediments are eventually carried out to sea and settle in layers on the seabed. This may not be the end of their travels: mass wasting goes on underwater as well as above it. Accumulating sediments often build up at the top of a slope on the seabed until they become unstable and collapse. If the particles are already cemented together, they collapse in big blocks in a process called slumping; if the sediments are still loose, they flow like a liquid down submarine slopes, eroding submarine canyons in doing so, and end up as layers on the ocean floor.

In any case, submarine sediments eventually come to rest, and as time passes (millions and tens of millions of years) loose sediments become solidified into sedimentary rocks: clay and silt become shale and siltstone, and sand becomes sandstone. Of the world's land area, 67 percent is covered by sedimentary rocks, of which shale and sandstone are the most abundant.14 Sooner or later the submerged layers (strata) of sedimentary rock are raised above sea level again, as the internal energy of the earth deforms its crust (see chapter 15). The shales and sandstones become available for weathering again, and the resulting clay particles and sand grains (perhaps with some newly formed rock flour added to the mix) find their way into rivers. Another stage in the unending recycling of earth materials is under way.

The Energy in Rivers

We must now consider how a flowing river acquires, and dissipates, energy. Obviously water at high elevations—newly fallen rain, melted snow, and preexisting lakes, for example—has potential energy by virtue of its elevated position, just as rocks on elevated ground have. The energy is converted to kinetic energy when the water flows, or the rocks fall (or roll, or slide) downhill. The potential energy at any point on a river's course has two components, however: elevation energy and pressure energy. The elevation energy exists because of the elevation of the riverbed above sea level; it corresponds with the gravitational PE of a rock on a mountain slope, which could just as well be called "elevation energy."

The pressure energy depends on the water's depth; hydraulic engineers measure it as "head." To understand the distinction between elevation energy and pressure energy, visualize the following situation: suppose the water level in a river were to sink almost to zero at any point on its course; then the pressure energy at that point would be almost zero, and the river's flow would almost stop, regardless of the elevation of the dry riverbed above sea level, which would remain unaltered. This makes it intuitively obvious that the

Figure 9.2. A rock falling from a precipice.(a) Earlier view with the rock at position X. (b) Later view, with the rock at position Y. Were it not for drag, the sum PE + KE would be the same at X as at Y. The open arrows represent the rock's velocity at each position, on which its KE depends.

depth of the water contributes to its total energy. For any site along a river, we may write the equation

PE = elevation energy + pressure energy.

Only a liquid can have pressure energy, not a solid.

Now compare the energy budget of a rock falling from a precipice with that

Water surface


Water surface



Sea level

Figure 9.3. A river flowing in its channel (longitudinal section). The crosshatched strips represent "slices" of water at sites X and Y; were it not for drag, the sum EE + PrE + KE would be the same at both sites (see text for details).

of a "slice" of river flowing down its channel (see figs. 9.2 and 9.3). To avoid vagueness, we must choose arbitrary starting and stopping points to define the interval under consideration; these points are marked X and Y in both figures. We must also stipulate that the rock does not fracture and that the river in figure 9.3 neither gains nor loses water between X and Y.

At any instant, wherever it may be, each "object" (solid rock or liquid "slice") has a PE that depends on where it is at that instant and a KE that depends on its velocity at that instant. For both objects, PE and KE are changing continuously, from one instant to the next.

Were it not for friction (in the general sense, including drag), it would be true to say, for both the rock and the water slice,

This follows from the law of the conservation of energy (see chapter 3). In the real world, where drag is inescapable, the equation becomes

PE at point X + KE at point X = PE at point Y + KE at point Y + w, where w represents energy gone to waste between X and Y because of drag.

PE at point X + KE at point X = PE at point Y + KE at point Y.

In their most concise form, these equations apply equally to the rock and the water slice. Now we put more detail into the equation for flowing water, taking note of the fact that the PE is the sum of elevation energy (EE) and pressure energy (PrE). Then in the ideal case, with no drag, the equation becomes (the word "point" is omitted for brevity)

EE at X + PrE at X + KE at X = EE at Y + PrE at Y + KE at Y.

(Do not confuse PrE with PE.)

This is the law of the conservation of energy as it applies to flowing water.

We can modify the equation for the realistic case (drag operating) in almost the same way, like this:

EE at X + PrE at X + KE at X = EE at Y + PrE at Y + KE at Y + w1 + w2.

Here w, the symbol representing waste energy—nonconserved energy—has been split into two parts, w1 and w2. The first, w1, denotes work done by the flowing water, namely, picking up and transporting a load of sediment; the second, w2, denotes true waste energy, namely heat and noise;15 noise is thought to account for only one part per million of the total energy. Most of the energy goes into shifting bedload, which consists of all particles larger than very fine sand grains, of diameter 0.06 mm.

The lighter particles in bedload are dragged along by the flowing water. They move more slowly than the water, reducing its speed of flow, but they do not come to rest: turbulent eddies support them and keep them moving. The heavier particles, on the other hand, stop and start repeatedly as the strengths of the eddy currents fluctuate. Any particle of bedload that settles on the bottom is soon temporarily entrained (picked up) again: it may be dislodged by a minor eddy, or lifted by the flow of current over it in the same way that an aircraft wing is lifted, and raised into the faster current above the bed. After traveling a short distance, it will be deposited again. In this way the heavier particles hop forward without ever rising appreciably above the riverbed. Because it takes energy to lift the particles, a fraction of the river's kinetic energy is consumed.

The total energy used by all the rivers in the world in transporting sediments can be estimated approximately if we rely on another estimate, according to which erosion, chiefly by rivers, lowers the earth's land surfaces by 8 mm per century.16 The earth's land area is about 149 million square kilometers, and the bulk density of the loose surface material left by weathering is about 1,500 kilograms per cubic meter. The mass of material removed from the surface is therefore about 1.8 x 1013 kg. The average height of the land is about 875 m above sea level. With these data we can compute the potential energy lost, per century, in the same way as we computed the energy of the Frank Slide. The answer is

1.8 x 1013 x 875 x 9.81 = 1.55 x 1017 J per century.

This is energy per unit time, in other words, power. It can be quickly converted to familiar watts (joules per second), or better yet, to watts per hectare, to make it easily imaginable. The answer is about one-third of a watt per hectare. Imagine the energy as light: it would amount to an invisible glow (if that is imaginable) on a dark landscape. The effects of the "glow" are cumulative, however. Over a few million years (not long, in geological terms), the "glow" creates and maintains most of the world's spectacular scenery. The folding and uplifting of the earth's crust by the earth's internal heat merely provides the raw material on which erosion acts.

The rate at which the earth's land surface is being denuded by erosion is vastly greater nowadays than it was before human undertakings like logging, farming, mining, and construction came to be practiced on an industrial scale. It has been estimated that the natural sediment load carried annually by the world's rivers is 1.6 x 1013 kg, while the unnatural load is 1.72 x 1014 kg, more than ten times as much.17 The long-term consequences of accelerated erosion could be as serious as other outcomes of the industrial-technological explosion, such as the increasing concentrations of greenhouse gases in the atmosphere. Time will tell.

Sediment transport accounts for only part of the energy dissipated by a flowing river, the part denoted by w1 in the preceding equation. We have still to consider the part denoted by w2, which is "wasted" or, equivalently, used up (converted to entropy) in overcoming resistance to the river's flow by the bed, the banks, and the sediment load itself. In a word, the flowing water experiences drag and is greatly slowed as a result.

Because of drag, no river can flow faster than its terminal velocity, the velocity at which the force of gravity accelerating it down its channel is exactly balanced by the force of drag restraining it (recall the account in chapter 5 of the terminal velocity of falling rain). The terminal velocity of any particular river depends on the straightness and smoothness of the channel walls. The importance of these factors becomes obvious if you mentally compare the gentle flow of a lowland river with the fierce jet of water spewing out of a penstock (sluice) bringing water from a dam to spin the turbines in a hydroelectric generating station.

Figure 9.4. Beach drift. The open arrows show waves breaking obliquely and carrying sand obliquely upslope; the solid arrows show the direction of the backwash, carrying sand directly downslope. The resultant drift is to the left as seen from the beach.

The terminal velocity of a particular river depends also, of course, on the volume of water it happens to be carrying, which varies from time to time. A typical terminal velocity for a river of average size is in the neighborhood of 5 km/h, while the same river in spate could have a terminal velocity of 6 to 8 km/h. It is difficult to imagine the speed a river would have were it not for drag, but just thinking about it brings an appreciation of the inexorable increase of entropy going on wherever water flows.

At the Beach Again

A river's current peters out when it reaches the sea. Whereas its suspended sediment drifts far from shore before settling to the bottom, the heavy bed-load of sand is dumped much sooner and becomes the raw material for neighboring sand beaches. Its transport along the shoreline requires a further supply of energy.

Some of the needed energy comes from longshore currents, either tide currents or currents driven by the wind. Whatever their origin, currents in shallow water can transport a bedload of sand in the same way a river can.

Sand is transported along the beach itself by beach drift. It happens when ever wind-driven waves roll up a sandy beach obliquely, carrying some sand up with them; when they drain back by the steepest route down the beach, they carry the sand back into the water again a short distance downwind of the point where they picked it up (see fig. 9.4).

Quantities of sand are transported considerable distances by this inefficient process. Each successive wave gives a modicum of PE to its load of sand by carrying it up the beach, but the sand promptly loses the PE by rolling back down again. If the wind direction changes, the sand is carried back the way it came. Eventually the bulk of it is carried along the beach in the direction of the prevailing wind, but the energy lost to friction is enormous. This is obvious if quantities of fine pebbles are mixed with the sand; the roar of pebbles rolling and sliding over each other as they go first up and then down the beach slope is deafening, and noise represents only a minute fraction of the total energy lost, most of it as imperceptible heat. Not surprisingly, the quantity of energy lost is difficult to measure.

Ocean waves cause considerable mass wasting, too. Sea cliffs, formed by wave action, are subject to frequent slumps and landslides, triggered by the action of waves that undercut the cliffs. In this way coastlines are shaped by the energy of the sea, and every landslide liberates potential energy.

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