Lab 9: Eolian and Arid Region Landforms

Transcription

1 Name: Lab 9: Eolian and Arid Region Landforms The objective of this lab is to familiarize yourself with a few basic desert and eolian landforms. Answer the following problems completely. You may need to utilize the lecture text to supplement your answers. Part One: Analysis using a topographic map and Google Earth. Questions using the Ashby, NE quadrangle. 1. What type of eolian features are the long, linear ridges? 2. Offer an explanation for the rough surfaces of the ridges. 3. What is wind direction in this area? How did you determine this? 4. What is the proper geomorphic term applied to the numerous closed depressions in this area? How did they form? 5. Characterize the relationship between topography, water table position, and drainage pattern. Given that the vast majority of the lakes are natural, how do you explain their presence in an area so dominated by eolian processes?

2 Questions using the Guadalupe Peak, TX quadrangle (15 ). 1. What are the landforms at the base of the steep cliffs in the northwest portion of the map? Compare and contrast their formation with the formation of deltas. 2. What is the geomorphic name of the surface that is formed where the features in question #1 coalesce? 3. What geologic feature is responsible for the presence of the steep cliffs east of the features identified in question #1? 4. Salt Lake has historically been an important source of salt. a) What type of geomorphic feature is this lake? b) How/why does the lake become saline? 5. Name and briefly describe 2 other landforms found on this map that are typical of regions with an arid climate.

3 Questions using the Antelope Peak, AZ quadrangle. 1. Locate each of the features listed below and identify them as either a(n) alluvial fan, arroyo, bajada, inselberg, pediment, or wash. a. Sec 18 T7S R3E: b. Sec 34 T6S R2E: c. Sec 28 T5S R3E: d. Sec 19 & 30 T6S R3E and Sec 25 & 36 T6S R2E: e. SE1/4 Sec 2 T7S R3E : f. SE1/4 Sec 2 T6S R1E: 2. Why do you suppose there are very few alluvial fans on this landscape? 3. Compare and contrast the stage of landscape development evident on this map with that visible on the Guadalupe Peak, TX map. Questions using the Furnace Creek, CA quadrangle 1. Determine the average slope of the alluvial fan emerging from Trail Canyon, in degrees. Measure from the mouth of the canyon to the base of the fan at the Devil s Speedway. Show your work (and remember the measurement tool or the path/elevation profile tool in Google Earth)! 2. What is the difference between an alluvial fan and a pediment?

4 Part Two: Analysis using stereographic photographs Questions using Stereogram 116 (Inselbergen). 1. Notice the 'island mountains' surrounded by a low relief surface. Explain how the 'island mountains' and the surrounding surface form. 2. Citing evidence visible on the photograph, discuss the relative importance of eolian and fluvial processes in creating the visible landforms. Why does one dominate the other? 3. What role does vegetation appear to play here? Questions using Stereogram 125 (Stovepipe Wells). 1. Study the differences in tone on the alluvial fans. Given the arid climate, explain why the surface tones vary so dramatically and suggest the processes that are responsible for the obvious contrasts. 2. What type of dune is present in the upper left portion of the photograph? Assume that the sand source is to the upper left of the dunes. 3. What type of dune is present in the lower left portion of the photograph?

5 4. How does prevailing wind direction change across the photograph (assume north is toward the top of the page)? Questions using Stereogram 509 (Tularosa Basin). 1. This photograph is from White Sand National Monument. Identify the dune types in the following portions of the photograph: a. Lower right corner: b. Lower left corner: c. Upper half: 2. What is the prevailing wind direction across the photograph (assume north is toward the top of the page)? 3. Contrast the relationship here between topography, water table position, and drainage pattern with your observations from the Ashby, NE quadrangle in Part 1. Part Three: Eolian concepts. 1. Explain the processes that allow sand and snow fences to control blowing or drifting sand/snow.

6 Part Four: Use of Entrainment Equations As might be expected, the ability of wind to erode sediment is constrained by the ability of typical wind velocities to mobilize sediment. On a typical windy day in Fort Collins, you observe wind speeds at 5 feet above the ground surface that attain sustained velocities of 25 mph. The following equation describes the change in wind velocity with the height above the ground: V z = 5.75 V * log z where z = the height of interest (above the ground k surface), V z = velocity at a given height above the surface, V * = surface friction speed or drag velocity, and k = thickness of zero velocity boundary layer at ground surface. Using the figure below and the above equation, calculate the maximum particle size that this observed wind (25 mph at 5 ft above the ground) can mobilize and transport through saltation. Assume that k is 1/30 the diameter of the surface grain size. Show all calculations and keep in mind that you may need to iteratively solve the problem (excel may be useful) (note that sand ranges in size from mm to 2 mm). Pages of your text will be very helpful. Fluid threshold Impact threshold