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Chapter 8: Mass Movements, Wind, and Glaciers

Chapter Worksheet

Ch. 8.1 Mass Movements

Alters Earth's surface over time due to gravity moving sediment and rocks downslope.

Mass Movements

Mass movement- the downslope movement of soil and weathered rock from the force of gravity. Climate has a major effect on the weathering in an area so climatic conditions determine the extent of mass movement.

All mass movements occur on slopes and range from sudden slides, falls, flows, and barely detectable.

Factors That Influence Mass Movements

First, a material's weight can work to pull it downslope. Second, materials resistance to sliding or flowing (friction, cohesion, anchored). Third, a trigger. Fourth, is water. Saturated water is heavier and acts as a lubricant.

Mass movement occurs when the forces pulling material downslope are stronger than the material's resistance to sliding, flowing, or falling.

Types of Mass Movements: View Image

Mass movements move different types of materials in various ways.

Creep- The slow, steady, downhill flow of loose, weathered Earth materials, especially soils. Can only be a few centimeters a year. Leaning poles, etc. are indicators.

Solifluction occurs in regions of permafrost. Melting permafrost forms water that creates a mud-like liquid that saturates the surface layer of soil. It is unable to move downward due to ice.

Flows- Earth materials flow as a thick liquid. As slow as a few cm per year or 100km per hour. Earth flows are moderately slow.

Mudflows- are swiftly moving mixtures of mud and water. Mudflows can be triggered by earthquakes or similar vibrations and are common in volcanic regions where heat melts snow on nearby slopes with fine sediment and little vegetation.

Lahar is a mudflow of melted snow and volcanic ash.

Mudflows are also common in sloped, semiarid regions that experience intense, short-lived rainstorms. These areas have frequent fires so little vegetation to anchor soil.

Slides- when a relatively thin block of soil, rock, and debris separates from the underlying bedrock. The material rapidly slides downslope as one block with little internal mixing. The mass eventually stops and becomes a pile at the bottom of the slope. Common on steep slopes, especially when soils are saturated.

Rockslide occur when a sheet of rock moves downhill on a sliding surface. Often triggered by earthquakes.

Slumps

When a mass of material in a landslide moves along a curved surface. Material at the top moves downhill, and slightly inward, while the material at the bottom moves outward. Occurs in areas with thick soils on moderate-to-steep slopes.

Common after rains when water lubricates the materials of underlying layers, the additional weight of the water pulls material downhill.

Can be triggered by earthquakes. Leave crescent-shaped scars on slopes.

Avalanches

Landslides of thick accumulations of snow. The sun melts surface snow which refreezes at night into icy crust. When new snow accumulates and slides downslope. Occur at slope angles between 30º and 45º.

Rockfalls

Rocks are loosened by physical weathering processes (freezing and thawing, plant growth). Produce a cone shaped pile of debris at the base of the slope called a talus. Common with steep slopes and human-made rock walls such as road cuts.

Mass Movements Affect People

Construction of buildings, roads, and other structures can make slopes unstable. Also, poor maintenance of septic systems that leak can trigger slides.

Reducing the Risks

  1. Humans can minimize destruction by not building structures on or near the base of steep and unstable slopes.
  2. Trenches can be dug to divert water.
  3. Steep slopes can covered with steel nets or fences to control landslides and rockfalls.
  4. Retaining walls at the base of weak walls.
  5. Monitor mass movements, and educate people. USGS data base.

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Ch. 8.2 Wind

Wind modifies landscapes in all areas of the world by transporting sediment.

Wind Erosion and Transport

Rapidly moving air can pick up and carry sediment in the same way water does. However, it can not carry particles as large as the ones transported by moving water, with the exception of extreme storms.

  1. Wind transports materials in different ways.
  2. Sand can roll on ground.
  3. Suspension: strong winds keep particles airborne for long distances.
  4. Saltation: a bouncing motion of larger particles. Accounts for most sand transport by wind.

Wind transport and erosion primarily occur in arid areas with little vegetation (deserts, seashores, some lakeshores).

    Deflation

    Deflation- the lowering of the land surface from wind's removal of surface particles. Creates shallow depressions known as deflation blowouts. As deflation removes fine particles, it leaves behind course gravel and pebbles; known as desert pavement.

    Abrasion

    Abrasion- when particles such as sand rub against the surface of rocks or other materials. The quartz in sand can erode many things and creates unique structures. Ventifacts- rocks that have been shaped by windblown sediments.

Wind Deposition

When wind speed slows down, windblown sand and materials cannot stay airborne and drop out of the air stream to form deposits on the ground.

    Dunes

    Sand particles accumulate where an object blocks the forward movement of the particles. Sand will continue to be deposited as long as the wind blows in one general direction to form a dune. All dunes have a characteristic profile. Windward side is the gentle slope on the side the wind blows. Leeward side is the steeper side that is protected from the wind.

    Dune shape is determined by the conditions in which it forms:

    1. Availability of sand
    2. Wind velocity
    3. Wind direction
    4. Amount of vegetation

    Dune Migration

    As long as winds continue to blow, dunes will migrate. Prevailing winds move sand from the windward side to its leeward side causing the dune to move slowly over time.

    Loess

    Wind can carry fine, lightweight particles such as silt and clay in great quantities for great distances. Many parts of Earth's surface are covered by thick layers of yellow-brown windblown silt know as loess.

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Ch.8.3 Glaciers

Glaciers modify landscapes by eroding and depositing rocks.

Moving Masses of Ice

Glacier- a large mass of moving ice. Currently covers 10% of Earth's surface; at poles and high elevations. During last ice age (2.6 mya-10,000) ice covered about 30% of Earth.

In areas that have temps at 0ºC year round keep snow from completely melting and snow accumulates in an area called a snowfield. Snow increases as years pass and develops into a glacier. The weight of the top layers of snow exerts downward pressure to force the accumulated snow below to recrystallize into ice. Glaciers are classified as valley glaciers or continental glaciers.

Valley Glaciers

Valley Glaciers- glaciers that form in valleys in high, mountainous areas. When the ice mass becomes heavy enough it will begin to flow as a solid piece. Usually when the ice exceeds 40m in thickness. As it moves crevasses can form.

The speed of the glacial flow depends on slope of the valley floor, the temperature and thickness of the ice, and the shape of the valley walls. With a valley glacier, the sides and bottom move slower than the middle due to friction. As the glacier flows it changes V-shaped stream valleys into U-shaped glacial valleys.

Continental Glaciers

Continental glaciers- glaciers that cover broad, continent-sized areas. Form where snow accumulates over many years. Thickest at the center. Today, are confined to Greenland and Antarctica.

Glacial Movement

Both glacier types move outward when snow gathers at the zone of accumulation. With valley glaciers the zone of accumulation is above the snow line while continental glaciers zone of accumulation is at the center. Receding occurs when melting is faster than the zone of accumulation builds up snow and ice.

Glacial Erosion

Their size, weight, and density make glaciers powerful erosional agents. Plucking; when a valley glacier breaks off pieces of rock as it moves. Minerals scratch parallel lines into the bedrock. Small scratches are called striations, large scratches are called grooves; both indicate direction of flow.

  1. Cirques- deep bowl-shaped depressions from erosion.
  2. Arete- a sharp, steep ridge formed where two cirques on opposite sides of a valley meet.
  3. Horn- when there are glaciers on three or more sides of a mountaintop; erosion creates a steep, pyramid-shaped peak.
  4. Hanging valley- created when a small glacier joins a larger primary glacier at a higher elevation. When both glaciers recedes it leaves a valley that hangs above a U-shaped valley.

Glacial Deposition

Glacial till is the unsorted rock, gravel, sand, and clay that glaciers carry. Formed from abrasion of the glacier. Moraines- unsorted ridges of till.

  1. Terminal moraine: found along the edge where the retreating glacier melts.
  2. Lateral moraine: located parallel to the direction of a valley glacier flow.

Outwash

Sediment left behind by receding glacier. Sediment carried by the water of the melting glacier so is sorted by particle size. Outwash plain- Area at the leading edge of glacier.

Image

Drumlins, Eskers, and Kames

  • Drumlins- when glaciers move till from older moraines into elongated landforms. Steeper slope face the direction which the glacier came.
  • Eskers- winding ridges of layered sediments from streams flowing under melting glaciers.
  • Kames- a mound of layered sediment formed when till gets washed into depressions or openings in the melting ice.

Glacial Lakes

When large blocks of ice break off of a continental glacier and surrounding area is covered by sediment. When the ice melts it leaves a depression called a kettle hole. Kettle- when kettle hole fills with runoff and precipitation to form lakes.

Cirques can have lakes, terminal moraines can block off a valley which fills with water to form lakes.

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