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Chapter 18

Chapter 5 Glaciers, Deserts, and Wind

5-1    Glaciers:

  1. Glaciers and classification:

    • Ice mass forms over hundreds of years. Snow compact into ice, not melted during the summer.

    • Part of the hydrologic cycle.

    • Based on size and location formed, glaciers classified as: (a) valley or alpine glaciers, at peak of mountains and above snow line, moving downward in valleys; (b) ice sheet cover a bigger area, move in all directions, e.g. Greenland and Antarctica.

  2. How glaciers move?

    • Plastic flow: when overburden increases, plastic deformation will cause particles move to a direction that has less stress. When ice is over 50 m thick, plastic flow appear.

    • Sliding downslope.

    • Zone of fractures at top 50 m, where plastic deformation is minimum. Crevasses form due to tension.

    • Zone of accumulation. Fig. 5.4.

    • Zone of wastage.

    • How to tell the move? Fig 5A.

    • Advancing vs. retreating, depending on the amount produced to the amount wasted. When terminus elevation increases with time, retreating occurs.

  3. Glacial erosion:

    • Plukcing: pick up rocks and embedded into ice.

    • Abrasion: picked rock grind the surface of the valley, produce rock flour.

    • Glacial striation: scratch caused by embedded rocks against wall.

  4. Feature of valley glaciers:

    • U-shape valley, due to sideway erosion.

    • Hanging valley when two glaciers meet. Fig. 5.7 and 5.9.

    • Cirque - bowl-shaped depression.

    • Horns and Arete. Fig. 5.7.

  5. Glacial deposits:

    • Drift: refer to glacial deposits derived from other places. Till: if the deposit is due to melt of glaciers => no sorting. Stratified drift, if deposits were sediment by water melted from glacier = sorted.

    • Erratics: Boulders of other places, differ form bedrock.

    • Moraines: Layer or ridges of till. Could be lateral due to sideway erosion and deposition, medial when two glaciers meet, and terminal at the end.

    • Outwash plains: formed beyond the end moraine, due to melting water. Fig. 5.17.

    • Kettle: small depression due to ice block melting.

    • Drumlins: streamline asymmetric hill, tell the direction of ice movement.

    • Eskers: small ridges from the deposit fo stream water within glaciers.

  6. Glaciers of the past:

    • Nearest time: pleistocene epoch ~ 2 million years ago. Commonly known as Ice Age.

  7. Cause of glaciation: 

    • Variation in Earth's orbital. Fig 5.23A. Eccentricity.

    • Change in obliquity, affect the direction of Sun's ray.

    • Wobbling of Earth's axis. Fig. 5.23 A-C.

5-2    Deserts:

  1. Deserts: precipitation < evaporation. Therefore, dry region. Common in 30 dgree N and 30 degree S latitude.

  2. Weathering: due to lack of water, chemical weathering secondary. Thus, physical weathering major. Large contrast in daily temperature make rocks easy break apart.

  3. Water: water table is deep. Evaporation fast. Thus, less water. But ephemeral stream do exist, due to sudden and heavy precipitation. Often flash flood. Erosion by water is heavy due to lack of vegetation.

  4. Wind erosion: By deflection and saltation. The former refers to blow off by suspension in air while the latter rolling at surface.

    • Desert pavement: coarse rocks can't be moved by wind. Thus, surface is made of larger rock crop can be polished by wind dust.

  5. Wind deposits:

    • Loess: made of wind blown silts. Particles in air for long time, travel to a greater distance.

    • Sand dunes: Particles too large to be carried far away. Particles suspended no more than several meters high. Limited to small mound due to topographic feature.

    • Direction of wind blow can be determined.

    • Cross bedding with high angle very common. Fig. 5.32.

  6. types of dunes:

    1. Barchan: crescent shape indicating wind direction.

    2. Transverse: Fig. 5.33B., flat surface, wind in fixed direction.

    3. Barchanoid: combination of the two above.

  7. Evolution of desert: Basin and range provinces: NV, UT, AZ, and NM. Happened in the interior drainage system. Water doesn't drain to sea. Base level is high, easy to deposit.

    • Uplift of mountains

    • Alluvial fans develop

    • Playa lake: internal lake that dry fast, very high salt content.

    • Most of all in arid climate

Homework:

  • Read chapter summary on p.150.

  • Use your own word to explain the key terms on page 151.

  • Answer the review questions on page 152.