Week 8
Deserts
and
Glaciers
Week 8
Deserts
and
Glaciers
Introduction
The Earth has experienced wide variations in climate throughout geologic history, and at almost any time in that history experienced wide climate fluctuations from place to place. At this time if you were in the Space Shuttle looking at the Earth you would see that deserts cover roughly 10 % of the land surface of the Earth and glaciers cover an additional 10 %. The current percentages of glacial and desert land coverage are in no way unique in geologic history, however we are greatly concerned that the growth of deserts and the loss of glaciers may occur due to our impact on the Earth’s short term climate variation.
Deserts
Much of the surface of the Earth is covered with arid (<25 cm/yr. rainfall) and semi arid land (25-50 cm/yr. rainfall). It is clear from the diagram below that the global distribution of arid and semi arid lands on the Earth is mostly located within a belt along the equator, stretching from 30S to 30N in latitude. The distribution of deserts around the equator is the result of climatic conditions created by global wind circulation patterns.
Global Wind Patterns
Air tends to flow from areas of high pressure to areas of lower pressure. Much like most processes of the Earth, the circulation of air occurs in an effort to establish an equilibrium in which the air achieves the lowest possible energy state. The circulation of air on the surface of the Earth must be examined in the vertical and horizontal directions in order to understand how global wind patterns occur and deserts form.
The warm moist air at the equator is heated and becomes less dense than the air surrounding it. Equatorial heating is so intense because the angle of inclination of the sun at the equator is 90 degrees. Because of this density difference the air must rise in order to achieve a more stable, equilibrium state. As the air rises, it moves to the north and south, as shown above. After it has become equilibrated at higher elevation it begins to cool and becomes denser than the air around it, in part because of the water vapor it contains. It then begins to flow downward toward the Earth. The rising and falling of air in these patterns creates vertical wind convection cells called Hadley cells. Hadley cells are responsible for much of the arid lands we observe north and south of the Equator, because all of the warm moist air is sucked out of this region and redistributed to the north and south.
When air moves north from the equator it begins to drift to the east because the Earth rotates in and eastward direction. The air in not attached to the Earth, and so it is easily deflected. Air moving south from north of the equator towards the equator is deflected to the west by the Coriolis effect, as shown in above. The patterns for the Southern Hemisphere are reversed from those in the Northern Hemisphere. The net effect of these wind patterns is a set of wind currents moving air away from the mid-latitudes towards the equator, called the Northeasterly Trade Winds and Southwesterly Trade Winds. North and south of the mid-latitudes air moves away towards the temperate zones between 30 and 60 latitude north and south. These wind patterns are called the Westerlies. The result of the vertical and horizontal wind patterns is the stripping of moist air away from the latitudes between the equator and 30N and 30S.
Causes of Deserts
All deserts form because there is little rainfall in the area and the air is dry. There are several ways in which an area can become a desert.
The first is simply to be located in the mid-latitude area in which there is little moisture in the air, as discussed earlier. These types of deserts are called climate zone deserts, and he distribution of climate zone deserts is shown below.
Deserts may also form in the center of large land masses. The wind moving onto the continent often loses the moisture it contained as it moved off the ocean by precipitation over the land mass. As the air moves inward less and less moisture is present in the air, because of the precipitation. This type of desert, shown below, is called a continental isolation desert.
The third type of desert is formed when air moves from the ocean onto the land in a mountainous area, as shown in below. When the moist wind is blown onto the land from the ocean, along a mountainous shoreline, it is forced upward where it cools and expands. The cooling leads to precipitation because the colder air cannot hold as much water vapor as the warm air and it loses the excess through precipitation. If the mountain range is high, most of the water vapor is lost by precipitation along the flanks of the mountain. The now cold dry air is relatively dense as it crosses to the other side of the mountain and it flows downward towards the ground again where it is compressed and heated again. This type of desert is called a rain shadow desert.
In the exercises below you will be asked to determine the cause of several deserts.
Weathering, Erosion, and Transport by Wind
Wind, as an agent of weathering and erosion is not as effective as water for several reasons. Wind is of course not particularly chemically active and so it is not an agent of chemical weathering. Wind erodes the surface of the Earth by a process called deflation. Deflation is the winnowing of the finer materials on the surface by the wind; eventually resulting in a surface that is paved with coarse cobbles and sediment, as shown below.
A deflated desert floor is shown below. Note the lack of sand visible on the surface and the coarse cobbles that dominate the desert floor. Notice that rocks weathered and eroded by wind are either polished or grooved.
Erosion by wind of outcrops tends to be concentrated in areas in which the rock layers are less resistant to weathering. This usually means rock layers that are not as well cemented as others, or layers that contain softer minerals that are less resistant to abrasion and fracture. The Sphinx, outside of Cairo, Egypt, is a good example of wind erosion. In the roughly years 4500 years since its creation the base of the Sphinx has undergone significant wind erosion (except during a brief period of flooding during the Nabtian Pluvial event around 2000-1000 BC) along the faces of the base that are roughly parallel to the wind direction, as shown below. The missing nose of the Sphinx is not due to wind erosion. The legend is that Napoleon Bonaparte had his troops use the Sphinx for target practice, to the detriment of the Sphinx’s fine profile!
Below you can examine a landscape produced by wind erosion. What characteristics do you see that make it clear that this is a wind erosion landscape?
The transport power of wind is deceptive. Although it takes nearly 20 km/hr wind velocity to move significant amounts of sediment, the carrying power of wind rises exponentially as the wind velocity continues to increase, as shown below. Another way of demonstrating the transport ability of the wind is the fact that a cubic kilometer of wind in a dust storm can move up to 1000 tons of fine grained sediment. In late November of 1933 a wind storm struck Oklahoma with such force that it carried dust as far as New England and turned the snow in the Midwest and New England brown.
Deposition and Dunes
Wind deposition is similar to that of water except the structures left behind are somewhat different. Wind shifts from transport to deposition when the energy (velocity) of the wind drops below that which can sustain the sediment load it is carrying. The sediment load is either suspended in the wind current as suspended load or rolls and bounces along the ground as the saltation load. The most common depositional structure that wind leaves behind is the dune. Typically dunes have a low slope face, called the windward face, that faces the wind direction and a steeper face, called the slip face, which points downwind, as shown below. Dunes move by wind eroding sand from the windward face by pushing it and bouncing the sand along the windward face until it reaches the top of the dune. It is deposited along the slip face, which in effect moves the slip face forward in the direction of the wind, as shown below.
There are four major types of dunes. Barchan dunes are crescent-shaped dunes that are concave downwind, as shown below. Barchan dunes form in places where there is limited sand and a constant wind direction. The parabolic dune is a crescent shaped dune that is concave upwind and forms in areas in which there is some vegetation and a good supply of sand. The longitudinal dune is a linear dune that is parallel to the direction of the wind and forms in areas in which the wind direction is not constant and the supply of sand is moderate to good. Longitudinal dunes can be kilometers in length, and the formation of them is not entirely understood. They are formed in areas in which there is a desert pavement and variable winds. Transverse dunes are linear dunes that are perpendicular to the direction of the wind and are not as long as longitudinal dunes. Transverse dunes form in areas with abundant sand and little vegetation.
In the exercise below you are asked to identify the major types of dunes.
The Desert Virtual Field Trip
In the virtual field
trip you will visit a desert in Death Valley, California. To move around in
the VR pano, simply hold the cursor in the image, hold down the mouse, and move
the mouse. Does the desert follow look like what you imagined? What form are
the sand deposits and dunes? Click the link below.
Glaciers
Glaciers form when the climate in a region, or on a worldwide basis, changes enough that the accumulation of snow is very much greater than the loss of it due to melting or evaporation, over a long period of time. Glaciers are an excellent example of an equilibrium process on the Earth. Glaciers tend to accumulate snow near their sources or centers and lose snow though melting at their edges or termini.
- When the rate of melting is equal to the rate of accumulation the glacier neither grows nor recedes, and is in an equilibrium state.
- If the rate of melting exceeds the rate of accumulation, the glacier begins to recede, indicating a movement towards a new equilibrium at which the rates will be balanced again.
- Conversely if the rate of accumulation is greater than the rate of melting, the glacier grows.
The growth or recession of glaciers on a worldwide basis has a significant effect on the rest of the Earth’s surface. Currently the glaciers and polar ice caps of the world contain about 44 billion cubic kilometers of the Earth’s water, roughly 3% of the total water on the planet and 85% of the total fresh water.
Glaciers and Glacial Ice
As snow accumulates it gets buried under additional snow and is compacted. Fresh snow as it accumulates on the grounds contains about 5% water and 95% air. The first state of compaction is granular snow or ice that contains about 50% water and 50% air. As further burial occurs the granular ice becomes firn, which is about 25% air and 75% water. The final stage of compaction is glacial ice, which is almost like a rock with closely packed grains of ice and 5-20% air in the form of air bubbles distributed throughout the ice. Obviously as the snow goes through the transformation from snow flakes to glacial ice it becomes tremendously compacted and loses about 80-90% of its initial volume.
Glacial Formation
There are two factors that lead to the creation and growth of glaciers; low temperatures and adequate supplies of snow. For enough snow to accumulate to form a glacier ground temperatures must be low enough that snow remains on the ground year round. This does not necessarily mean that the air temperature is below the freezing point of water continuously, but it must remain below freezing for most of the time. Currently glaciers are more common in the upper latitudes because the temperature is lower on a continual basis than at the lower latitudes, due to the fact that the inclination of the Sun is low and the amount of solar energy reaching the ground is less than it is at lower latitudes.
The temperature variation as a function of elevation is shown for a hypothetical mountain in the central plains area of the U.S. the temperature varies from 23 o C at sea level to –10 o C at 15,000 feet. The vegetation style changes from deciduous (leaf bearing, oak), to coniferous (cone bearing, pine) and disappears at about 8,000 ft. The snow line occurs at about 8500 ft. Clearly above about 8500 ft. a glacier could be sustained at this location, if there was enough accumulation of snow.
The accumulation of snow is the other important factor in the formation and growth of glaciers. Often cold climates tend also to be dry, making the formation of a glacier unlikely. Even mountainous areas near sources of moist air are not always populated by glaciers. If moist air moves off the ocean onto the shore and encounters a large mountain snow accumulation will occur on the windward side, however the moisture is mostly gone by the time the air reaches the leeward side of the mountain. Glaciers will likely form only on the windward side of the mountain. A good example of this effect is the contrast between the eastern and western sides of the Cascade Mountains in Oregon.
Ablation and Accumulation
Glaciers grow or recede as a function of the balance between accumulation and ablation of snow and ice. Accumulation of snow occurs at the upper portions of the glacier. After accumulation the snow is buried and begins to flow down slope towards the terminus of the glacier.
The glacial budget is the total of accumulation and ablation for a glacier. The glacial budget of Antarctica between 1945 and 1995 decreased due to a warming of the ocean around West Antarctica by about 3 oC, resulting in a loss of more than half of the floating ice shelf portion of the region, including a single iceberg more than 75 km long. Although the shrinkage appears to have stabilized, the loss of glacial ice in this region remains a concern because it may be a precursor to a more catastrophic worldwide ablation event.
Glacial Movement
Glaciers behave almost like the rock in the mantle of the Earth, a very viscous fluid that flows slowly down slope. Glaciers move by basal slip along the base of the glacier and by plastic flow within it. Basal slip is the movement of ice along the interface between the glacier and the bedrock by slippage along a layer of water. The layer of water is caused by the pressure exerted on the ice along the base of glacier, which causes a small amount of melting of the glacier. It is the same effect that makes ice skating possible. Plastic flow occurs as the pressure of the snow and ice at the top of the glacier causes the ice at depth to deform and move down slope in an effort to reduce the pressure on it. On a microscopic level the individual grains in the ice change their shape in order to accommodate the pressure.
Types of Glaciers
There are two types of glaciers; alpine glaciers (also called valley glaciers) and continental glaciers. Continental glaciers tend to occur in high latitude regions where temperatures are extremely low, and alpine glaciers occur almost anywhere in the world. Alpine glaciers are much smaller than continental glaciers. Greenland is covered by a continental glacier that is up to 9500 ft, in elevation covering an area of about 13 million square kilometers.
Alpine glaciers form in river valleys in mountainous regions, and in many ways resemble river systems. Alpine glacier form high in mountain valleys and often extend many kilometers down the valley. Snow accumulates in the upper parts of river and tributary valley. Often a single mountain will numerous tributaries separated by sharp narrow ridges called aretes emanating from the highest peak in the area, called the horn, as shown below. At the source of the alpine glacier, below the horn is a bowl-shaped valley called a cirque. The main valleys alpine glaciers are thicker and erode deeper into the river valley than the tributaries. This leads to the tributaries being suspended above the main valley in hanging valleys. The aretes between the tributary valleys are cut off by the ice in the main valley and are called truncated spurs. The features described here are erosional features that illustrate the strength of glaciers as agents of erosion.
Virtual Field Trip to a Glacier
In the virtual field trip you will visit a glacier in the Teton Mountains of Montana. To move around in the VR pano, simply hold the cursor in the image, hold down the mouse, and move the mouse.
The terrain around this glacier can be viewed and manipulated in the object movie link below. Simply hold your mouse down and rotate or tilt the terrain model.
In the next virtual field trips, in the links below, you will visit an area over which an Alpine glacier passed. Can you see the erosional features?
Glacial Features
In the exercise linked below you will be asked to identify characteristic glacial features.
Continental glaciers are much larger than alpine glaciers and are not constrained by existing topographic features like alpine glaciers. The two largest continental glaciers are in Greenland and Antarctica and they cover virtually the entire continent. Continental glaciers reach thickness of almost 10,000 feet and completely flatten the topographic features over which they flow, due to the tremendous pressure the mass of ice exerts on the bedrock beneath it. Continental glaciers move more slowly than alpine glaciers because they do not flow down slopes like alpine glaciers but rather flow outward from the central course area. Most of the midwestern part of North America is due to the erosion of the topographic features by a continental glacier that covered about half of North America during the Pleistocene Ice Age. The polar regions contain large continental glaciers called ice caps. Ice caps are not usually anchored on bedrock but are actually huge floating ice masses.
Glacial Depositional Features
Alpine glaciers leave a complex array of depositional structure behind when they recede. Glacial sediments are called drift. The poorly sorted sediment that is deposited directly by the ice is called till. The most pervasive depositional feature of alpine and continental glacier is the moraine. Moraines are deposits of sediment that are carried along at the base, sides and front of the glacier. Glacial moraine sediment is characterized by poorly sorted material ranging from rock flour up to coarse cobbles as large as a washing machine. The ground moraine is carried along between the bottom of the glacier and the bedrock over which it flows. The ground moraine is partially trapped within the lower part of the glacier and acts as a huge abrasive blanket that scours the bedrock below the glacier as it passes over it. Terminal moraines are ridges of poorly sorted sediment and debris that are pushed along the ground at the end of the glacier as it advances. Terminal moraines can be more than one hundred feet high and can extend for many kilometers. Recessional moraines are smaller moraines that form at the terminus of a receding glacier. Recessional moraines are formed by brief periods of advance during the recession of a glacier. Both continental and alpine glaciers display ground, lateral, terminal and recessional moraines. Alpine glaciers also display lateral moraines which are similar to ground moraines in terms of their grain size distribution and the fact that they are partially embedded in the glacier, but are located at the sides of the glacier. Medial moraines are another feature of alpine glaciers that form when a tributary of an alpine glacier coalesces with the main valley trapping the lateral moraines of the tributaries within the main valley of the glacier.
Significant amounts of water are carried beneath glaciers and additional melt water flows in front of glaciers even if they are not receding, and the water carries with it some of the finer sediment contained by the glacier. Much of the water-borne sediment is deposited out in front of the terminal and recessional moraines along the outwash plain. The sediment is composed of well sorted sand-sized particles that are deposited by the braided streams of melt water that flow from the glacier. Another depositional feature formed by melt water is the esker. Eskers are small sinuous ridges of well sorted sediment that are deposited in tunnels beneath the glaciers by the melt water that flows through the tunnels. Boulders as large as houses are often found in areas which were once covered by glaciers. The boulders, called erratics were carried along on top of the glacier and deposited when the glacier melted and receded. One unusual depositional feature usually associated with continental glaciers is the drumlin. Drumlins are elliptical mounds of glacial till that are aligned parallel to the movement direction of glacial movement. One end of the drumlin is steeply inclined and the other is gently sloped. It is thought that the steep side is pointing in the direction of the glacial movement.
Much has been written about global warming and the recession of glaciers. In the exercise below you will be asked to look at the data regarding glaciers and global warming and come to a conclusion about whether global warming is having an effect on glaciers worldwide.
