Well, hello, and welcome again. Today's program is number 26 covering Lesson 22. That's the lesson on wind, dust and deserts, Chapter 19 in the text, and yes, you will note that we are covering Chapter 19 before Chapter 18 getting a little bit out of sequence again in the text. Deserts are interesting and important geologic features. Not only do they cover about 30 percent of all the land surface on earth,but no other climatic region of any type covers such a large area on the earth. And deserts, like all geological entities, help us to understand the processes and features that have been active in the past; in fact, by understanding these processes and features of deserts today we can help to unravel geologic history, to recognize ancient desert environments, and also understand past climatic changes.
The desert environment is very different from more humid areas and water plays a most significant role even though it's present only in small amounts. So, before we actually start today's lesson, let me remind you of the assignment, that's Chapter 19 in the text, pages 433-451. It's Lesson 22 in the study guide, and as usual, don't forget to follow the study plan in the study guide, and when you're done,go back to the learning objectives and make sure that you've learned each one. So, be sure to look at the learning objectives in the study guide before you actually start the lesson. Well, let's take a little overview of the desert. What I'd like to do in today's program is not spend so much time on details but sort of take a little sampler of several of the various features of deserts to see if we can't help to understand the environment.
First of all, what exactly is a desert? All of us, I think, have a sense of what we mean by the desert, but geologically speaking, a desert is simply any region where the precipitation is less than 10 inches per year. This is about 25 centimeters per year. One of the things about deserts is that a small amount of rainfall is usually unreliable, so averages are misleading. That means that whereas in some places the rainfall may come in pretty continuous amounts, in the desert the rain is usually episodic and often torrential, with long dry periods in between. That's why one of the reasons we associate the desert with dryness, and, in fact, in some deserts, in Peru, for example, there are regions where no rainfall has ever been recorded in the desert at all. Another characteristic we usually associate with desert is high temperature,but deserts aren't necessarily hot.; in fact,much of the polar region, the continent of Antarctica, for example, qualifies as a desert since the precipitation actuallyis less than 10 centimeters per year.
We usually think of deserts as being dry, having dry air, but that's not necessarily true either; in fact, right here on Oahu Waikiki actually qualifies as a desert based on the rainfall alone. It's sort of on the borderline of having the 10 centimeters per year, and there, of course, the air is relatively humid. 00:05:39:06 We have other examples of deserts right here in Hawaii; for example, on the island of Hawaii there's the Ka'u Desert, and on western Molokai, the entire western part of the island of Molokai actually qualifies as a desert, and on the high slopes of most of the Hawaiian volcanoes that are more than about 8,000 feet, like the tops of Haleakala, Maunakea, Maunaloa, and Hawalalai would qualify as deserts. We also generally think of deserts as having no vegetation. This is not quite true either; in fact, most deserts do actually have a barren look, but if you look closely you see that the desert actually contains many species of generally salt-tolerant plants. The plants may have extensive and deep root systems, so that they can tap what little water is available and get down deep to reach the deep water table and generally have small leaves to minimize evaporation from the surface during the times when there isn't any water around. These plants may look like dried dead sticks most of the year, but during a brief rainfall they turn green and flower and go through an entire life cycle of reproduction in a very short period of time.
In Hawaii we have some examples of specialized desert plants, the famous silverswords of Maui and the Big Island, for example, are good examples of specialized endemic species which have adjusted to the desert environment, but what about the geographical distribution of deserts? Where do we find deserts? It turns out that there are four main factors that influence whether a desert forms in a particular area. And as it turns out, many actual deserts worldwide may combine two or more of these factors or features, so the first place, on a global scale, are the subtropics, around 30 degrees north and 30 degrees south latitude. Here, global wind patterns produce sinking air in these regions, and the sinking air is compressed and heated and so its relative humidity is lowered. Many of the world's largest deserts fall into this category, the Sonoran Desert in southwestern United States, the Sahara and Kalihari in Africa, and most of the continent of Australia all fall into these subtropical desert categories. Just by contrast, rising air near the equator and also at mid-latitudes produces cooling and condensation, so there we find more humid climates or more humid atmospheres. We also find deserts in rain shadows of mountain ranges. This is because air which flows over the mountain ranges also rises, and as it rises it dumps its moisture on the windward slopes. In Nevada and northern Arizona, for example we find deserts which are rainshadowed desert from the Sierra Nevada and here in Hawaii we find deserts on the leeward side of the islands, on the leeward sides of the volcanic mountains; in other words, we also find deserts, as you might expect, far from the oceans.
Here the air that's being transported with its humidity from the ocean simply loses the humidity before it has travelled far enough to reach these isolated regions. The Gobi Desert, for example, in south central Asia is a good example of a desert that's far from the ocean. And, I might mention, there's also a rainshadow effect in the Gobi which comes from the monsoons which flow over the Himalayas at certain times of the year. Finally, we find deserts associated with cold ocean currents. This usually occurs along the Pacific coast of all the continents or on the west coast of the continents, especially in tropical and subtropical regions. The central desert of California in the central valley of California, for example, is one of these types. So, let's look at some of the characteristics of deserts now. What is it that characterizes the desert environment from other types of environments? One of these is the presence or the occurrence of flash floods. Flash floods are simply local floods of high discharge and short duration, a lot of water in a short amount of time, and the water may attain very high velocities because of the narrow stream channels.
Generally, flash floods are more common in arid regions than in humid regions, although here in Hawaii we have sort of an exception. In fact, how often have we heard over the radio whenever a storm comes up that flash flood warnings are in effect for the islands of Oahu and Kauai, for example. Here the flash floods are due to heavy rains and small drainage areas so that because the drainage areas are small, it simply doesn't take very long for the water to reach the lower parts of the streams and also most of the streams in Hawaii are quite small, but despite the heavy rainfall, except during flooding because of the high infiltration a lot of water soaks into the ground, but the phrase "when it rains, it pours," is no more applicable than it is in the desert; In fact, in the typical continental type desert it's not uncommon to see rainfall amounting to five inches per hour. It may not last an hour, but the rate of accumulation is five inches per hour, and the heavy rain, of course, cannot readily infiltrate the dry soils. When the soils become dry, the clay particles become packed, so the ratio of runoff to infiltration is extremely high. That means that the water can't soak into the ground, so it has to run off. It's worthy to note here that the flash flood's a very effective agent of erosion; in fact, more erosion may be done in a few minutes in flash flood than in years of normal flow in the steams which are normally dry anyway. The streams during flash floods are heavily laden with sediment, and the stream may pass through a cycle from a dry streambed to flood stage to a braided stream and back to a dry streambed again in days or, in some cases, even hour as the flood rises and crests and then subsides. Many of the streams in a flash flood are so laden with sediment that they can easily erode enough material to become mudflows and do severe damage at the bottom of the stream channels. We also note that in desert streams that mechanical weathering dominates over chemical weathering, not just in the streams, within the desert region as a whole.
Now, don't get me wrong, chemical weathering is still present. We learned in the section on weathering that both of these processes operate at all times but in different types of climates one process may dominate over another. Chemical weathering is present, but less important because of the scarcity of water overall, and one of the effects of this is a rock like limestone which is normally quite soluble in water and is actually quite susceptible to chemical weathering in temperate climates or in moist climates becomes resistant rock in the deserts or in dry climates. This means that limestone stands as ridges and cliffs very much like sandstone conglomerate. The distinction between the weatherability of these two materials changes radically in the absence of water.
Drainage is another characteristic of desert streams. Usually to begin with the streams in a desert are typically what we call intermittent streams. That just means they're dry much of the time. The streams in a desert tend to disappear; in other words, there are many disappearing streams. There may be streams flowing into the desert region with large discharge from neighboring mountainous regions, but because of the nature of the desert soil and the high temperatures, much of the water may evaporate or soak into the soil once it reaches the actual desert floor, so the water simply evaporates. There are exceptions to this, the Colorado River in western United States and the Nile in Africa are examples of streams which do flow through the desert because they have sufficient discharge from their origin in the mountains to sustain the flow of water through the deserts, but in both of those rivers we note that the discharge is significantly smaller at the mouths than at the headwaters. Another characteristic of deserts is what we might call interior drainage. In deserts streams often drain into landlocked basins; that is, where the streams cannot flow out. Many streams, or most streams as you recall, actually wind up flowing into the sea, but in desert regions they may not reach the ocean, so the surface of this enclosed basin acts as a base level for the erosion of these streams. Base level, you may remember, is the lowest level at which a stream can erode. Because the adjacent basins within the same desert region may be filled to different levels, there may be many different local base levels which affect the erosion of various streams. We hope this is also a rarer case where we find that rising base level may be associated with the filling of a particular basin by sediments. This is something that's rare in human climates; we seldom find a rising base level in human climates because the stream is constantly downcutting instead.
We also find in streams a fairly deep water table. The ground water in the desert may actually be quite abundant, but virtually inaccessible. The water table, in many cases, may be hundreds of feet deep, although in special cases as we'll see later springs and oasis of various types may tap the water table providing surface water. Okay, landscape is another characteristic feature of the desert; I should say the difference in the desert landscape and the more humid landscape. In general, desert features are more angular. This is partly because of less chemical weathering, but also partly because there is a drier soil cover so there is not as much soil creep. Also, the coarse and rocky soils lack humus and they lack the wet clay which could contribute to mass wasting, and the sparse plants in the desert don't hold the fine grain particles together, so we tend to find the products of mechanical weathering accumulating in talus slopes, etc. The desert slopes are not only more angular, but also generally steeper, and as we'll see in the video, the slope of a particular hillside is controlled partly although not entirely by the resistance of the rock, so we find very steep cliffs with extensive talus aprons at the bottom. There are several types of unique landforms that only occur in deserts. One of these are called Badlands; the Badlands of North Dakota are a prime example of this. Badlands are simply regions usually on flat lying sedimentary rocks, where dendritic drainage patterns have developed with steep slopes, very little vegetation, and extensive erosion. Alluvial fans are another feature of deserts. Alluvial fans build from the deposition of sediments as streams coming from the mountains enter these large flat valleys that represent the desert floor. The alluvial fans themselves may be like big, dry brilliant streams and many of them show a system of distributaries much as a delta; in fact, you might think of alluvial fans as dry deltas where a river empties into not into the ocean but into a flat floor. The alluvial fan itself may be further eroded during flooding and may extend itself outward, and many desert regions show an apron of coalescing alluvial fans. When this happens it forms a base of pediment along the bottom of the mountains; it's called a "bajada."
Another feature of deserts is the presence of playas and playa lakes. Even when the stream enters the flat floor of the desert basin and deposits its coarse grain sediment in the alluvial fan, the fine grain sediment may be carried further into the desert floor to be eventually deposited in a depression in the desert basin itself. In many cases, water may collect temporarily to form a shallow playa lake, so a playa lake is simply water that's accumulated in a depression in the interior drainage of a lake, and when it's dry, when the water evaporates or soaks in, it's called a playa lake. The characteristic of the playa lake, of course, is a fine muddy surface with many mud cracks, and there may be a crusty white layer of salt. The crusty white layer usually consists of minerals like haokailate and gypsum and other evaporative minerals. One last feature before we watch the video. This is the presence of mesas and buttes. are capped by layers of ordinarily resistant rock, and mass wasting processes are generally responsible for eroding the mesas and buttes, eroding the cliff backwards so that the area of the top of the mesa simply decreases over time, so I hope this is gives little bit of a background so that when you watch the video, you can see some of these features. The video does show us many things that we can't show here in the studio, so with this in mind, let's sit back and watch the video. A valley hasn't always looked like this. Within the last 100,000 years a lake 600 feet deep covered this valley floor. The surrounding shoreline was grassland, and these hillslopes were covered with pine trees. The mild, relatively humid climate was enjoyed by such diverse animals as bison, antelope, horses, pelicans, and flamingos, and all this changed about 11,000 years ago when the Earth's climate began to warm, and the glaciers of the last Ice Age melted back in retreat. Today the lake is gone. It's been replaced by this salt encrusted dry lake bed. The lake disappeared; so did the grasses, and the forest retreated up the mountain slope. A very different community of plants and animals now inhabit this region. In short, all of the characteristics of a typical desert, but Death Valley is not alone. You see this same dramatic change in environment in deserts all over the world, and this change is the direct result of geologic processes that operate on a global scale. Located primarily along the Tropics of Cancer and Capricorn, deserts cover almost one quarter of the Earth's surface.
Most of us have a very limited, somewhat stereotypical picture in mind when we think of what a desert looks like, but the fact is that deserts can take many, sometimes unexpected shapes and forms. Well, if you asked most people, they would probably say that the desert is a dry area with no vegetation, but this isn't a very good definition for several reasons. One, there are many dry areas with no vegetation on the surface of the Earth that you wouldn't think of as deserts; Antarctica, as an example. There's almost zero precipitation at the South Pole. We can look at Point Barrow, Alaska, which receives only four, five inches of rain a year and has very little vegetation; yet the soil there is water sodden, and there are little ponds and lakes, and it's hardly what you would think of as a desert, and on the other hand, some deserts have a great deal of vegetation. The Mojave Desert of Eastern California has vast stands of the giant yuccas, the Joshua trees. To develop a simple definition of a desert, we can find a common denominator in each of these harsh desert landscapes. From the polar regions of Antarctica and Northern Alaska to the vegetated sands of the Mojave Desert. Deserts are regions with infrequent precipitation averaging less than 25 centimeters per year. In most deserts, evaporation rates are high enough to quickly remove this moisture. To understand the origin of the Earth's hot subtropical deserts, the largest on the planet, we must first consider the Equator.
Like the deserts themselves, the Equator is a hot place, but it is also very humid with torrential rain any time of year and steamy tropical rainforests. Why are the tropics wet and the subtropics dry? As the Earth orbits the sun from season to season, the sun always shines directly overhead somewhere in the tropical latitudes; therefore, the sun's rays strike the Equator more directly than that rest of the globe. As the air heats us, water evaporates. The warm, wet air rises, and as it rises it expands and cools. The vapor it contains condenses into clouds, many of which release rain. The cool air is now dry, but more and more warm air is rising up beneath it. This displaces the cool air away from the Equator pushing it north and south toward the subtropics. Because of Earth's spheroidal shape air currents begin to crowd together as they move into higher latitudes. This causes the air to grow dense and heavy, so that it descends earthward about 30 degrees north and south of the Equator. The air is compressively heated upon reaching low altitudes. The result is a mass of warm, dry air, few clouds, and low humidity. This encourages evaporation. The result is the stark parched landscape of most of the world's deserts. A few deserts occur outside these subtropical latitudes in the rain shadows of mountains such as California's Sierra Nevada. The rain shadow effect works as follows: Warm, moist air moves east across the Pacific. It hits the coast and is forced upwards to get over the mountains. As it rises, it expands and cools, and its moisture turns into clouds, rain, and sometimes snow. This leaves the air dry. As this cool dry air moves down on the leeward side of the mountains, it is compressed and heats up again. The air, now warm as well as dry, sucks up what little moisture may be available form the land below creating deserts as it continues its journey east of the Sierra Nevada. Many of the world's most prominent deserts, well- known deserts are the result of this rain shadow effect.
The Mojave in the United States, for example, the Gobi Desert in Central Asia. Western South America has a similar situation. We have a chain of mountains down the west coast, the Andes, which serve as a barrier forming this rain shadow. Another factor that plays an indirect role in the formation of deserts is plate tectonics. The position of the continents whether they be in the polar regions, or the equatorial regions, or the subtropical regions, of course, is a function of plate tectonics. As an example, Africa 250 million years ago in permian times was much farther south toward near the South Pole, so what we call the Kalihari Desert now was glaciated at that time, and that wasn't very long ago; that was only about five percent of geologic time. In the Southwestern United States, too, there's evidence of a once widespread desert that existed 200 million years ago. Fossil dunes are preserved in the upper wall of the Grand Canyon and in the sandstones of Zion National Park. The buried surfaces of the shifting sand dunes appearing as crisscrossing sets of beds. Their large size and coloration from the oxidation of iron show that they formed on dry land. Since that time, plant and animal fossils indicate that this region became moist and forested, but in recent geologic time conditions have become dryer in response to plate motions, mountain building, and the development of rain shadows. Some lands are deprived of moisture simply because they lie a great distance from the ocean, which is the primary source of moisture for rainfall. In Western China, the Gobi and the Taklamahan are both locked deep inside a land mass. Moist air masses precipitate most of their water before reaching these regions. The rain shadow effect of surrounding mountains is also a factor. A few deserts exist where cold marine water comes into contact with warm air next to a coastline. Such conditions prevail along the coasts of Northern Chile and Southwest Africa. Along the north coast of Chile and the coast of Southwest Africa, there are deserts which extend all the way to the sea, and these deserts are quite unusual because they owe their existence to cold offshore marine currents. Now, in many coastal latitudes the air coming off the ocean is full of moisture which has evaporated from the sea given ordinary sea surface temperatures, but these cold marine currents chill the overlying air and so reduce its capacity for holding moisture. The air that blows inland as a result is very dry, and the result of that are these coastal deserts. So there are several ways by which deserts can come into being: Subtropical descent of equatorial air currents, rain shadow effects, great distance of land mass from the sea, cold coastal currents in warm latitudes, and in polar regions, the inability of cold air masses to hold much moisture. Most of these extreme desert environment contain many unique land forms which despite the infrequency of rainfall are often shaped by running water. This seeming paradox can be explained by the fact that desert rainstorms, while sporadic, are generally intense creating flash floods. These brief but violent episodes are high erosive, quickly transporting enormous quantities of sediment, and over time carving canyons. These floods also cause sediment to accumulate at the base of mountains in cone shaped deposits called "alluvial fans."
The phenomenon of flash flooding in the desert raises the question of drainage. Where does all the water go? Drainage in deserts is characterized by internal drainage, and by this I mean it has a drainage pattern that isn't connected to the regional drainage pattern. Only the largest rivers in the world: the Nile in North Africa, and the Niger in West Africa, and the Colorado in the Southwestern United States persist as they flow through deserts. For the most part, any river or stream flowing into a desert will sink into the soil and disappear or else collect in a pond or a salt lake. Generally, deserts streams disappear in desert because of the high rate of evaporation and also because of the unconsolidated nature of the sand and sediment of the floor. A good example, I think, is the Mojave Desert in California where there's only one stream, that's the Mojave River involved in the drainage pattern. It rises on the edge of the Mojave in the San Bernardino Mountains, flows out into the Mojave, but for the most part is underground. There's only three places where in a normal year it surfaces. In wet years like 1969, '78, '80, and '83, it was above ground most of the way; in fact, it flowed over into Soda Lake and became and honest to goodness lake. Although it plays the dominant role, running water is not the only geologic agent that shakes the desert landscape. Throughout most of the year, the desert surface is dry. This allows the wind to pick up grains of sand and silt and move them. Sand is relatively heavy. It's not carried far by the wind. It tends to be deposited as dunes close to the source.
On the other hand, silt is much lighter, a finer grain. In a single storm, silt can be carried across entire continents or ocean basins. The airborne desert dust blown by the wind consists mainly of particles of rock and mineral grains, but as the wind stream continues around the globe, it also carries with it tiny fragments of plants and animals, ash from coal fired electrical plants, other industrial detritus and occasionally glassy volcanic ash. This flotsam and jetsam is shuffled and shifted through the atmosphere storm after storm, until there's hardly a square meter of the Earth's surface that does not contain material blown in at some time from somewhere else. Some of this windblown dust can be a blessing. The soil of the Midwestern United States owes much of its fertility to what geologists call "loess." "Loess" is made up of fine particles of silt and clay that have drifted in over the millennia from baron lands uncovered by melting pleistocene ice.
In Eastern China deposits of "loess" have reached remarkable proportions hundreds of meters thick. From time immemorial the Chinese have carved cave dwellings out of this soft but surprisingly cohesive material. Windblown sand too heavy to blow across oceans and continents bounces and skips along the ground until it is caught by an obstacle in its path. Here it begins to accumulate forming an even larger trap for additional sand. Wind plucks sand from the windward side of the dune blowing it across the crest where it settles on the quiet leeward side, so in time the entire dune shifts downwind grain by grain, possibly migrating kilometers from its point of origin. A major source of desert sand is desert playas, lake beds from more humid times which have long been dry. The same winds that build up dunes may also hollow out depressions in the land surface. These dish shaped hollows are called "blow outs." Sometimes wind can be channeled in narrow streams. In parts of the Sahara Desert this is happening, for example, between long rows of parallel dunes. Where this wind is concentrated against the Earth's surface it can blow away vegetation and through time bit by bit carry away the loose soil and sediment present at the Earth's surface as well. Under certain extreme circumstances, so much material may be removed that the top of the water able is exposed, and this provides for creation of an oasis. These aren't common in most parts of the world but do occur in some of the larger deserts. More commonly seen than blowouts is what's known as "desert pavement." As the wind blows away silt and particles of dust, eventually only heavy chips of rock and gravel are left behind. Over thousands of years this rocky waste accumulates as a layer of stones resembling a pavement which protects the land from further erosion, but desert pavement is extremely fragile and easily damaged. Many outcrops as well as desert stones are also covered with desert varnish, a thin shiny coating on the rocks that increases with age. The varnish is composed of dark manganese oxide and clays. One explanation for the formation of this feature involves weathering, evaporation, and precipitation. This varnish forms over rock surfaces throughout the desert as the result of acid weathering, chemical weathering during periods of rainfall or heavy moisture, winter moisture.
Obviously this water can't travel very far before it evaporates because of the dry desert conditions, and so it precipitates a residue of dissolved mineral constituents as a thin film across the rocky surface over which it's flowing, hence the buildup of time of these manganese oxides. Windblown clay grains adhering to rock surfaces may assist varnish formation by soaking up moisture from adjoining soil. Microbes could also play a role in producing varnish through complex biological processes. Desert varnish can be 2,000 years in the making providing a writing surface for rock inscriptions from ancient cultures. The desert like any environment on Earth is the result of a critical balance of geologic conditions. Climate, topography, and plate tectonics interact to determine whether an area will be, say, a desert, or a forest, or a prairie grassland. Human activity, however, is capable of disrupting this natural balance triggering a chain of events that can cause deserts to invade a nondesert region. This process called "desertification" can be frightening rapid in its consequences staggering. Consider a fertile spot like this. Its most obvious characteristic is its color, green. The green color arises because the vegetation absorbs the sun's energy in all wavelights except that of the color "green." This absorption of energy means there is less heat available to warm the overlying air, so the air is cooler and more likely to produce rain. The chance of rain is further increased by the fact that the vegetation is an important source of water vapor released to the air through leaves. If great amounts of vegetation are destroyed as land is developed or overgrazed, the bare earth may be exposed reflecting heat back into the atmosphere. If there are no trees or plants to store heat and release moisture, the air gets warmer and dryer, and there's less chance of rain. With no tree or plant cover for anchorage, the topsoil can erode rapidly discouraging new plants from taking root. If the region lies in a semi- arid climate or near the margin of a desert, this destruction of soil and vegetation may convert it into new desert. This change may be permanent for all practical purposes for once desertification starts it takes a costly effort to stop. In the United States the most dramatic example of desertification occurred on the Great Plains.
There had always been a very delicate ecological balance in this region between the very slight rainfall and the fragile plant life. When rancher starting overgrazing the land, and farmers began overworking the soil, the situation became extremely dangerous. All it took was the great drought of the 1930s to turn the Plains into a raging dustbowl. Fortunately, thanks to conservation efforts and a series of wet years in the 1940s the wheat lands of the Great Plains were eventually saved from becoming a desert. It remains to be seen whether the result of more recent and even more catastrophic desertification in Africa can ever be reversed. This is the Sahil, a semi- arid region to the south of the Sahara Desert. In the 1960s a series of abnormally rainy years encouraged farmers to expand their herds and grazing lands. Then in the early 1970s there was a terrible drought. The plant life of the region was virtually wiped out. Some 40 percent of the cattle died. By the 1980s continuing drought had completely denuded the soil creating choking dust storms and migrating dune fields. The desert was creeping into formerly verdant areas at an average rate of 10 to 15 meters a day destroying the livelihoods of over 20 million people. More than a hundred thousand starved to death. The suffering was particularly acute in Ethiopia and the Sudan. The sad irony is that modern technology helped to magnify the disaster. Prior to the drought, deep water wells had been drilled in the Sahil providing abundant new sources of water for livestock and humans. This stimulated an excessively large migration into the area, so when the drought struck there was even greater devastation. Numerous international programs are currently underway to teach people how to graze their cattle and grow their crops, so as to avoid similar disasters in the future. There are a number of techniques that are being employed to prevent desertification of water conservation techniques, yet one of the primary causes of desertification is the depletion of ground water reserves and also intelligent farming techniques.
For example, if you move into an area deforest it, remove the vegetation, and then plow it. Particular in an area like the subtropics where you have the prevailing winds you can lose your topsoil, so intelligent farming techniques, for example, those that don't remove all of the trees, those that don't necessarily plow the ground or plow it deeply are being tested in Africa, for example, and have been quite successful. The yield the first several years of farming is less but on the long run you don't destroy the soil; you don't remove the topsoil; you don't pollute the streams from pesticides and fertilizers, so these are various hope. While human activity can influence the expansion of arid lands on a short term basis, more powerful geological forces are at work to change the shapes of deserts over long periods of time. Global climate changes, still poorly understood, have caused the edges of the Earth's deserts to shift by hundreds of kilometers over the past few million years. Eighteen thousand years ago, for example, the deserts of Africa were much smaller than they are today, but while the size of the world's deserts has fluctuated throughout history, one factors has remained constant. Deserts have always been regarded as hostile, extreme environments. As a result, we have tended to overlook the great beauty, wonder, and potential value of these unique regions. There's more to the desert than geologic processes. Deserts have a surreal quality that has captured the human imagination throughout the ages. Perhaps this is because deserts of places of extreme, of contrast. This stark seemingly lifeless expanse of bare rock and sand is actually home to a rich assemblage of life forms. The blinding white of dune sand and basalt stands in sharp contrast to rock surfaces blackened by desert varnish. The searing hot desert day is followed by the refreshingly cool, sometimes frigid desert night, but in spite of the fact that the desert is a harsh environment, it's also a fragile one; it deserves our respect, but in return, we and those who follow us can continue to enjoy beckoning vistas, its clean and rugged beauty, and its diverse assemblage of life forms and land forms until the climate pendulum swings back and transforms the desert to lake, or grassland, or forest once again. Funding for this program was provided by the Annenberg C.P.B. Project.
Well, once again, I think this video really shows us things that we probably can't see without traveling extensively, and I think it also points out something that we might not notice if they weren't pointed out to us. I want to encourage you at this point also to be sure to pay attention to the diagrams in the textbook because they'll not only show you some of these features where you can study them, but they will also show you the development and the evolution of some of the features as well.
Well, I want to note also that it's important to understand the role of water in the formation of the desert landscape. Although running water and streams is very infrequent and intermittent, it accounts for the majority of the major erosional features, and even though it takes place during these brief but very intense episodes of flooding. Running water is still the most important agent of erosion in the desert as far as creating the desert landscape, but wind also plays a role, not as important as water in the long run, but you might say that wind sort of fine tunes the landscape that's created by the running water in the first place, so we can look at the role of wind, and we already understand how water affects the movement of particles and how water acts as an erosive agent. It's not too surprising that moving air behaves similar to moving water with several differences though. For one thing, moving air doesn't usually run in channels, not in stream channels, and wind consists of many gusts and eddies of quite different speed. The changes in density of air are responsible for the wind, and these changes in density are generally caused by warming and cooling, so whereas water is under the influence of gravity directly; wind is under the influence of pressure changes by convectional heating.
Okay, so this is the driving force for air movement, and we often find wind speeds in the desert exceeding 60 miles per hour even in the absence of storms. If you recall that one of the things that characterizes the desert is the temperature changes may be extreme on a daily basis and also seasonally. On a typical day in a desert in this time of year the temperature may go from near freezing at night to above 100 degrees fahrenheit in the daytime, so this rapid change in the air temperature has a very strong effect on producing wind. We also want to note here that dry sediment is much more easily eroded by wind than wet sediment, so for this reason, of course, wind is much more effective in deserts than it is in humid regions, but still less effective over all than running water. Let's turn our attention now to the behavior of wind that bore sediment. Dust storms may suspend silt and clay for long periods of time. These dust storms are especially prevalent during extreme droughts when the ground is dry for long periods of time, and I want to note here that there's much greater erosion by wind if the soil is disturbed by animals or vehicles. Once the soil becomes windborne like water these wind can support the material easier than can erode it in the first place. The wind may carry the dust thousands of feet upwards and may carry it hundreds of miles horizontally; in fact, dust from the Sahara Desert is common in sediments in the western Atlantic Ocean and also in the Southeastern United States, and we have many examples of severe dust storms; in fact, in the 1930s in the central part of the United States, dust storms from the "Dust Bowl" as it was called frequently blocked out the mid-day sun and made the'e daytime almost as dark as night. In the process much fertile soil was lost. I want to note here the effect of this was increased by mismanagement of the soil and the loss of vegetation through poor crop management as well. It's also worthy to note here that volcanic ash may also be carried great distances by the wind, and in fact, may be carried all the way into the stratosphere to cause decreased solar radiation and colorful sunsets.
The year of 1816 was called the "year without a summer" because a series of volcanic eruptions put so much dust into the air that there virtually was no summer; crops were ruined in New England and in Europe, and so forth. Okay, sand grains being larger are harder to pick up and harder to move by the wind, so sand movement in the desert usually move by saltation. Remember, "saltation" means a series of jumps and bounds over the surface similar to what happens in a stream bed, but even so high winds may create sand storms, and the movement of this sand in the wind can sandblast smooth surfaces on hard rocks and other structures such as telephone poles, for example. Many telephone poles have to be replaced fairly often in the desert because the bases are simply eroded away by the sandblasting effect like sanding with sandpaper, but it's important to note here that the sand movement stays fairly close to the ground, and as a result, may form fairly interesting features in rock outcrops. There's a nice example of this on Figure 19.15 on page 443 in the textbook, one of these toadstool type rock formations. Another interesting result of this erosion or abrasion by sand is the presence is what geologists now call "ventifacts." "Ventifacts" are simply rocks with flat wind- abraded surfaces that often have the appearance of being manufactured by people; in fact, they once were thought to be tools, and they were originally called "artifacts" until we use the word "ventifacts" now. "Vent" means formed by the wind like in the word "ventilate."
Okay, so the overall removal of material sand, silt, and clay from the surface is called "deflation," and it's worthy of noting here that the process of deflation may lower the surface substantially, in other words erode the surface if fine grained sediment is thick enough. For example, in Egypt there's a region called the quatra depression which has been eroded by deflation more than 300 feet below sea level simply by the blowing away of the particles by the wind. In general, depression of a land surface from wind erosion is called a "blowout," and the blowouts of this type are common in the Great Plains, and under extreme conditions may actually be a deep enough depression to reach the water table to form an oasis. Another example of deflation or another product of deflation is often called "desert pavement" or sometimes "pebble armor." This is where the wind removes the finer grained particles leaving behind the coarser grained pebbles and boulders flattening the surfaces from sand abrasion and leaving behind the thin layer of closely packed gravel which resembles shingles or the armor on an armadillo, and of course, once these desert pavement particles have been collected together, it protects the underlying fine sediments from further wind erosion because the wind can't get down to reach them. Another important type of wind action produces a feature called "loess." "Loess" is a windblown clay and fine silt-sized particles that accumulates for various reasons up near desert regions. It's rather unusual sediments because it consists mainly of unweathered grains of quartz and feldspar; in other words, it's products of mechanical or glacial weathering. It also may contain other minerals, but the quartz and feldspar grains are generally weakly consolidated by calcareous cement. It's sort of like a lithification without a compaction. The loess often has a very high porosity; it may be as much as 60 percent, and it may blanket hills and valleys downwind of sources of fine sediments like glaciers and desert regions. Loess has the ability to form nearly vertical cliffs. This is due to a compilation of the angularity of the sediment particles and also the weak cementation. Loess covers much of the Great Plains of the United States and much of mid latitude Europe where it was blown from glacial outwash deposits, and we'll study the glacial period in an upcoming lesson. It's distributed over large areas, and it provides a fertile and productive agricultural soil which was original grasslands, and it's worth noting that the loess deposits are the source of most of the world's grains in agriculture. Loess also covers much of the interior of China where it's been blown from the Gobi Desert; in fact, it's been used for thousands of years for cave dwellings that are cut into the loess cliffs. It's noteworthy here that an earthquake in 1820 caused a collapse in many of these cave dwellings, which buried an estimated 100,000 people alive in these loess caves.
Okay, the feature that most people associate with deserts are actually some of the least common, and those are "sand dunes." Basically "sand dunes" are simply mounds of loose sand which are piled up by the wind. They develop in sandy areas, especially in areas where there are strong winds in a constant direction. There are patches scattered throughout the Southwestern United States, along the shores of Lake Superior, and in the Sahara, and even here on Hawaii. We also find them inland of many beaches: The Great Lakes, the Pacific and Atlantic coasts, a whole bunch of them on west Molokai from strong northeast tradewinds. Well, I don't want to go into all of the details of the sands dunes. The text covers this fairly well, but basically we find different types of dunes, different shapes of dunes forming, depending upon the strength of the wind, the constancy of the wind direction, and the amount of sand available. We might note here that the sand dune as a whole moves slowly grain by grain in the downwind direction, much more slowly than the speed of the wind itself. A typical speed for sand dune movement may be 30 to 50 feet per year, and again, this is not very fast but over the year over a long period of time can result in significant movement as people who have built their houses near sand dunes can attest when the houses are overtaken by the dunes in a few years. I might also note that sand dunes can be stabilized by being covered with vegetation, and that disruption of this vegetation, for example, dune buggies and motorcycles can cause the dune to start mobilizing once again.
Okay, there are several features of dunes that you might want to note. These have to do with cross bedding, which we've already examined in an earlier lesson, and the fact that the windward side of the dune is generally gentle; whereas, the leeward side of the dune has a relatively steep face. Well, I'll leave you to the textbook to see the classifications of the various kinds of dunes, but I want to note a very important aspect of deserts, which I think requires our attention. This is called in geological terms "desertification" or the increasing number of deserts or the growth of the deserts over a large region. Basically it's an invasion of the desert into a nondesert or populated region. This is a global problem. It's difficult to reverse, and it seems to be related somehow to global warming. There are many symptoms involved with this process of desertification. The symptoms include the lowering of the water table regionally, or increasing saltiness of water and topsoil making them useless for people to use, reduction of surface water supplies as the streams disappear, high soil erosion and destruction of the vegetation. It's not clear exactly what are the causes of desertification, but it's clear that it has something to do with the changing global climates. It may be related to deforestation. It may be related to overuse of land by agriculture and livestock, excessive withdrawal of groundwater, population increase, expanded agricultural production. Whatever the reason, it's a serious problem, and it's one that we need to talk about. It's not limited by the way to underdeveloped countries. In the United States alone it's affected about 10 percent of the land area. Well, that's about it for today's program. I would just note that deserts are interesting and important aspects of our planet. They allow us to see how processes operate where water is scarce. We can apply this understanding to other planets, for example, like Mars. We can apply uniformitarianism to interpret Earth's history. We can apply that history to understand climate changes. We can also try to interpret these changes in present deserts in terms of the global climate changes that might occur in the future, so if you have a chance to visit or fly over a desert region, look for these features. Well, next time we'll study ice and glaciers, Lesson 20, Chapter 18, so study hard, enjoy our planet, and I'll see you next time.