Lesson 9 - Mass Wasting
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To do:
- Check the schedule for this week's reading & upcoming assignments
- Read the lecture and assigned reading in the text
- Participate in discussions
- Take the Week 9 Quiz
By the end of this lesson you should be able to:
- Assess how insolation drives the major physical weathering processes
- Relate major weathering processes to different climates
- Identify evidence of glacial weathering
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We are only three lessons from the end of the quarter, and we are covering a lot of material in this lesson. So to make things easier on you, I have structured this lecture to guide you through the major points of this week's reading. If you find that you are behind on the reading from previous lessons, take the light lecture load this lesson to catch up on your reading.
Weathering Processes
Weathering is the process by which surface and subsurface rocks disintegrate, dissolve, or are broken down. Rocks at or near the earth's surface are exposed to physical and chemical weathering processes. Weathering is known as an exogenic process because it (like all exogenic systems) is powered by insolation, which drives all systems above the surface of the earth, including the atmosphere, weather, and ice which are also influenced by the earth's gravity.
Soil is formed by weathering-Define the following two words and write a brief description of how soil is formed:
- bedrock
- sediment
- Soil is formed when...
Physical Weathering takes place when rock is broken, but no chemical changes take place. Water is a major cause of physical weathering. Recall from lecture 5 that water actually expands when it becomes solid (freezes). Can you recall the four types of physical weathering?
Chemical Weathering takes place when the air or water interacts with the crystals in a rock to change their chemical composition. This can take the form of oxidation (by the air) or dissolving (by water and other chemicals in the water such as sulfuric acid from acid rain).
Mass Movement -- Weathering by Gravity and Water
In lesson 5 you read about sink holes, one form of weathering by water and gravity. Sink holes form what is known as Karst Topography, which has the effect of making the landscape appear pockmarked. Karst topography results from the chemical weathering of limestone underground, and the action of gravity forcing the overlying earth to fill in the void. You might want to look for examples of topographic maps of karst topography in the central United States (one example of water-gravity weathering) or other places in the world.
Another, more common example of weathering by water and gravity is mass movement or landslides. Mass movement occurs when soil is saturated with water, which makes it heavy, and the force of gravity overcomes the resistance of the slope.
Finally, the alluvial fan, is an example of weathering and movement of material by water and gravity. Alluvial fans occur in desert landscapes which see very little water year-round. Alluvial fans form at the mouths of mountain canyons that lead to valleys. When rains come to the usually dry mountains, they form brief, torrential rivers which flow out of the canyons and into the valleys carrying with them rocks and debris. The larger objects are deposited close to the canyon mouth, but the torrent of water flowing out of the canyon usually has enough momentum to carry small particles several miles out from the canyon mouth. The materials are deposited in a 'fan' formation, leading to the name. Try to find a picture of an alluvial fan on the internet or your library.
Eolian Processes -- Weathering by Wind
Eolian processes are erosion, transport and deposition by wind. We will briefly discuss each. First, however, let's make an important clarification about the ability of wind to move material: the smaller material is (and the lighter weight it is) the easier it is for the wind to move. Small particles (sand, dust, soil) are moved by the wind very easily. Large particles (pebbles, cobbles, rocks) are not moved by the wind easily.
Eolian Erosion: Summarize how desert pavement is formed:
Desert pavement is formed by deflation, or the movement by the wind of all the small particles, leaving only the larger pebbles and gravel.
Eolian landforms: Sand Dunes Large areas of our earth are covered with sand. The Grand Erg Oriental in the central Sahara Desert in Africa covers an area the size of Nebraska and is estimated to be 4000 feet deep. Sand dunes are always in motion. They migrate in the predominant wind direction, usually forming long ridges. The windward slope is shallow, and sand grains are driven up slope, until they tumble down the steep leeward slope.
Glacial Processes -- Weathering by Ice
The majority of the earth's fresh water is frozen in large sheets of ice known as glaciers. The majority of glaciers on the earth are at present located on Greenland and Antarctica. Glaciers form by the continual accumulation of snow that recrystalizes under its own weight into ice. In the past, as much as 30% of the earth's land surface was covered with glaciers, and these enormous sheets of ice have had a major effect on the earth's landscape.
Glaciers are not stationary. Rather, they flow with gravity from a high point (usually high in the mountains) to a lower point (a valley or the ocean). While their movement is very slow (several centimeters to several meters per year) their effect as they grind over the land is enormous.
Macro Effects Glaciers carve out large areas of land where they pass. These glacial valleys have a characteristic U shaped valleys (compared to the V shaped valleys that result from rivers). Yosemite, the Fjords of Norway and Swiss Alpine Valleys are all examples of glacial valleys. Glacial valleys often have spectacular steep walls and dramatic water falls which occur where a river was shaved in half by the advancing glacier.
When glaciers retreat (or melt), they often leave behind massive scars on the landscape. Moraines are hills of sediment that were pushed out of the way by a glacier. Lateral Moraines form along the sides of glaciers, while Terminal Moraines form at the furthest extent of the glacier.
Micro Effects As glaciers move across the land, bits of rock and debris get stuck in the ice at the bottom of the glacier and are carried along as the glacier advances. The enormous weight of the glacier can force these bits of debris to scrape and polish bedrock that the glacier passes over.
In addition when the glacier melts, it will leave these rocks (sometimes as large as a house) behind, often many kilometers from where they were picked up. Areas which at one time were covered with a glacier often have large boulders in the middle of grassy fields. If you have ever driven through the Sierra Nevada, you may have noticed large boulders sitting atop a bare rock face. These were not placed by Herculean pranksters, but rather by a melting glacier.
Review learning outcomes.
Please complete the Assignments and Exams section for each lesson before proceeding to the next lesson.
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