Geol 33 Eolian Landforms Lecture

Geol 33 Environmental Geomorphology

J Bret Bennington

Eolian Landforms

Eolian landforms are those created by wind. These landforms include ventifacts and dunes, which are associated with shoreline and desert environments. Although most people equate sand dunes with deserts, only 25% to 30% of desert regions are covered by sand deposits, which are usually concentrated in great "sand seas" produced by an abundant supply of sediment.

Uneven Heating of the Globe

Winds are caused by the fact that the Earth is heated unevenly by the Sun. The Earth is a sphere revolving around the sun in the same plane as its equator. Because the surface of the Earth is perpendicular to the path of the sunâs rays at the equator but parallel to the sunâs rays at the poles, the equator receives the greatest amount of energy per unit area, with energy dropping off toward the poles. Therefore, the Earth is heated unevenly and their is a general temperature gradient from warm at the equator to cold at the poles.

Global Circulation

If the Earth were not rotating and if it were covered with water, then the uneven heating would produce a simple pattern of circulation in the troposphere. Warm air would rise at the equator and flow toward the poles, where cooling it would descend. This circulation would create a permanent region of low pressure around the equator and permanent regions of high pressure at the poles.

Because the Earth is rotating this simple circulation pattern is disrupted. As air flows north and south away from the equator it is deflected eastward by the coriolis effect. Likewise, as it flows toward the equator from the poles the air is deflected westward by the coriolis effect.

As air travels east or west it has time to either cool or to gain heat from the surface. This causes the air to descend or rise before it can travel from equator to pole. Because of this, instead of one large circulation cell in each hemisphere, there are three smaller circulation cells.

The Coriolis Effect

The coriolis effect is often referred to as the Îcoriolis forceâ because it appears to be a force that pushes air flowing north - south onto an east - west path. However, nothing is really pushing the air. Instead, the deflection of the flowing air is due to the fact that it is moving over a rotating sphere (the Earth).

What causes the coriolis effect is the fact that the surface of the rotating Earth is moving faster at the equator than at the poles. This is easy to prove. The circumference of the Earth at the equator is about 24,000 miles. It takes about 24 hours for the Earth to complete one rotation, so the surface of the Earth at the equator is moving at about 1000 mph. Now, letâs imagine that we are standing about 1 mile from the North Pole. If we draw a circle around the North Pole with a 1 mile radius the circumference of that circle (and the Earth at that point) would be 3.14 x 1 x 1 or about 3 miles. Standing on that circle one mile from the pole it will still take 24 hours to complete one revolution, but you will only have traveled a total of three miles. This means that one mile from the North Pole the surface of the Earth is moving at about .125 mph, or about 11 ft per minute. Pretty slow, eh? Basically, from the equator to the poles the speed of rotation of the Earthâs surface steadily decreases from 1000 mph to zero at the pole.

Now, let us follow the path of a mass of air that begins to flow south from the North Pole. The begins its journey moving eastward with the rotation of the surface at a very slow speed. However, as it moves southward the Earth beneath the air mass moves faster and faster. The surface of the Earth will move increasingly far eastward relative to the air mass. However, to someone standing on the ground moving with the Earth, the effect is to see the air mass moving westward as you pull ahead of it. To you, the air appears to be blown to the west.

Contrariwise, an air mass rising and moving north or south from the equator begins with an eastern velocity of 1000 mph. As it moves poleward the ground beneath it slows and the air mass gains in an eastward direction, appearing to be blowing to the east.

General rule: air moving toward the equator will be deflected west, air moving toward the poles will be deflected east.

Global Wind Patterns

We are now ready to understand the general patterns of winds and circulation in the lower atmosphere. These patterns also explain the general climate of the modern Earth.

Warm, moist air rises at the equator and flows toward the poles at high altitude. As it rises it expands and cools (adiabatic cooling). The cooler air can hold less moisture. Clouds form and it rains. There is a zone around the equator characterized by low pressure, warm air, and high rainfall. We call this zone the tropics.
At about 30¡ latitude the air flowing from the equator has cooled sufficiently that it descends to the surface. As it descends it contracts and heats up (adiabatic warming). This warm, descending dry air creates high pressure at the surface and evaporates surface moisture. Clouds rarely form and it rarely rains. The zone around 30¡ N and S latitude is characterized by deserts.
Where the air is descending at 30¡ latitude it flows along the surface both north and south. Flowing back to the equator it is deflected to the west to create the surface trade winds (so named because sailing ships relied on them to travel from the old world to the new world). Air flowing toward the poles is deflected eastward to create the surface westerlies (named because they flow out of the west).
Where the surface winds converge at the equator they stall, causing the doldrums. Where they diverge at 30¡ winds are weak and shifting. 30¡ is called the horse latitudes (sailing crews that became stuck there often had to resort to eating the horses on board to survive).
Air flowing along the surface toward the poles warms and rises at about 60¡ to create a high latitude low pressure zone. Some of this air flows toward the equator, the rest toward the poles where it descends to create dry, high pressure conditions (in terms of humidity, the South Pole is one of the driest places on Earth).

Local Winds

Uneven heating of the Earth's surface also generates local pressure differences that create surface winds. In a desert, local winds fluctuate on a daily cycle. As the land heats up through the day, surface winds rise to 15 - 20 mph, peaking near the middle of the day and dying down as the sun sets.

Erosional Eolian Processes and Features

Abrasion

Abrasion occurs from the impact of sedimentary particles accelerated by the wind. The most effective agents of abrasion are sand grains. Rocks that are exposed to the sand blasting of prevailing winds become pitted, grooved, and polished. Also, facets can be worn into the face of the rock oriented into the prevailing wind. These features are collectively referred to as ventifacts.

Larger hill-sized features sculpted by the wind are called yardangs. These features are similar to drumlins in form, with a steep side facing into the wind that tapers toward the lee. Yardangs form from the wind erosion of hills or rock outcrops. They are common in deserts around the world (except for Australia) and have been noted on the surface of Mars.

Deflation

Deflation is the blowing-away of loose material to form a sediment-free pavement or circular depressions in loose sediment called deflation hollows.

Depositional Eolian Processes and Features

Dunes

Most dune deposits are associated either with shoreline barrier islands or with vast, sandy deserts called sand seas or ergs (in the desert of the Sahara of North Africa). The largest sand sea in the Western Hemisphere is the Sand Hills region of Nebraska. The Sand Hills formed earlier in the Holocene and cover some 57,000 km² in northwest Nebraska. Currently, that region is relatively humid, and the dunes have been immobilized by a cover of vegetation.

Sand seas form wherever wind transport of sand is interrupted by a reduction in wind energy, causing deposition of the sand. Reductions in wind energy occur due to physical barriers to airflow, such as plateaus or mountains, or to a change in climate zone that affects wind speed. Humidity is also an important factor because moisture and vegetation can stabilize sand deposits and prevent them from becoming wind-blown. Many sand seas appear to alternate periods of movement with periods of stability under the control of changing climate. For example, the sand dunes of the Sand Hills are currently immobile, whereas the sands of the Sahara desert are encroaching farther north each year, because of the current prolonged period of aridity in northern Africa.