Geology Lab

7 6 P a r t O n e / G e o l o g y

Desert Landscapes Arid (desert) and semiarid (steppe) climates cover about 30 percent of Earth's land area (Figure 5.2). At first glance, many desert landscapes with their angular hills and steep canyon walls may appear to have been shaped by processes other than those that are respon- sible for landforms in regions with an abundance of water. Flowever, as striking as the contrasts rnay be, running water is still the dominant agent responsible for most of the erosional work in deserts. Wind ero- sion, although more significant in dry areas than else- where, is only of secondary importance.

The distinct effects that running water has on humid and d.y areas are the result of the same processes operating under different climatic condi- tions. Precipitation in the dry climates is minimal, often sporadic, and frequently comes in the form of torrential downpours that last only a short time. Con- sequently, in desert areas flash floods occur, and few streams or rivers reach the sea because the water often evaporates and infiltrates into the ground.

Evolution of a Mountainous Desert Landscape Mountainous desert landscapes have developed in re- sponse to a variety of geologic processes. A classic re- gion for studying the effects of running water in dry areas is the western United States. Throughout much of this Basin and Range region, which includes southeast- ern Californta, Nevada, western Utah, southern Ore- gon, southern Arizona and New Mexico, the erosion of

mountain ranges and subsequent deposition of sedi- ment in adjoining basins have produced a landscape characterizedby selueral unique landforms (Figure 5.3).

In a large area of the Basin and Range region of the western United States fault-block mountains have formed as large blocks of Earth's crust have been forced upward (Figure 5.3A). The infrequent and inter- mittent precipitation in this desert region typically re- sults in streams that carry their eroded material from the mountains into interior basins. Alluvial fans and bajadas often form as streams deposit sediment on the less steep slopes at the base of the mountains (Figure 5.38). On rare occasions when streams flow across the alluvial fans, a shallow playa lake may develop near the center of a basin.

Continuing erosion in the mountains and deposi tion in the basins may eventually fillthe basin and only isolated peaks, called inselbergs, surrounded by gently sloping sediment, remain. As the front of the mountain is worn blck by erosion, a broad, sloping bedrock sur- face called a pediment, covered by a thin layer of sedi- ment, often forms at its base (Figure 5.3C). In the final stages, even the inselbergs will disappeag and all that remains is a nearly flat, sediment-covered surface un- derlain by the erosional remnants of mountains.

Use Figurc 5.2 to answer questions 1 and 2.

1. Where are the desert and steppe regions of North America located?

Desert:

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Figurc 5.3 Stages of landscape evolution in a block-faulted, mountainous desert such as the Basin and Range region of the West. A, Early stage;8. lvliddle stage; C. Late stage.

2. Using an "X" to mark your selection(s), indicate which of the following statements are commonly held misconceptions concerning the world's dry lands.

The world's dry lands are always hot.

Desert landscapes are almost complete- ly covered with sand dunes.

The dry regions of the world encompass about 30 percent of Earth's land surface.

Dry lands are practically all lifeless.

Figure 5.4 is a portion of the Antelope Peak, Ari- zorLa, topographic map that illustrates many of the fea- tures of the mountainous desert landscapes found in the western United States. Use the map and accompa- nying stereogram of the area (Figure 5.5) to answer questions 3-13. You may find the diagrams in Figure 5.3 helpful.

3. On the map, outline the area that is illustrated in the stereogram.

Use a stereoscope to examine the stereogram, Figure 5.5.

The vegetation in the area is (dense, sparse), and there are (few, many) dry stream courses. Circle your answers.

By examining the map, determine the total relief of the map area.

Total relief : ft

6. (Continuously flowing, Intermittent) streams dominate the area shown on the map.Circle your answer.

On the map, of the two lines, A or B (A, B), fol- lows the steepest slope. Circle your answer.

By drawing arrows on the tn"pr indicate the di- rections that intermittent streams will flow as they leave the mountains.

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9. Where on the map is the most tikely place that surface water may accumulate? Label the area "possible

lake."

L0. Identify the features indicated on the map at the following letters and briefly describe how each formed.

Letter C:

Letter D:

The area at letter A on the map is a bedrock surface covered by a thin layer of sediment.

L1. The feature labeled A is called a(n)

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12. Bfiefly describe how the Antelope Peak area may have looked millions of years ago.

L3. Assume that erosion continues in the area with- out interruption. How might the area look mil- lions of years from now?

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14. The topographic map (fig.14.16) and aerial photographs (fig. 14.17) are of Kame Springs, California. They are is located west of the Salton Sea and several dunes are developed on a generally smooth plain. Carefully examine the aerial photographs and compare them with the topographic map.

a. 1ffhat type of sand dunes are developed in this area?

b. What is the averege height of the dune crest$ (in feet)?

c. What is the average dune width (in feet)?

d. What is the prevailing wind directiop

e. Does there appear to be any type of vegetative control on the dune formation?

f. In which direction will the dune migrate? North, South, East or West?

g. These dunes are actively migrating downwind. Careful monitoring has shown that individual dunes have moved about200 meters in a $even year period. If these rates of migration continue, about how long will it take for the dune in the center of the map (iust south of the 100 ft contour line) to cross the road to the water torryer at Clancy?

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239

Lab9.docx

TIMELINE OF EVENTS FOR CONSTRUCTING OREGON

PHASE 1 FOUNDATION — Rocks many miles thick underlie the entire state and range from 400 million to 50 million years old.

Generalized Geologic Units:

1. Exotic Terranes: foundation blocks of Oregon

2. Batholiths and Plutons: mortar for the foundation

3. Early Sediments: Oregon’s first coast

 

PHASE 2 BRICKS AND MORTAR — Volcanic and sedimentary rocks thousands of feet thick cover most of the foundation. These rocks are generally between 60 million and 2 million years old.

Generalized Geologic Units:

4. Siletz Terrane: last exotic arrival

5. Early Volcanic Arc: Oregon’s tropical volcanoes

6. Coast Range Sediments: 50 million years of mud

7. Coast Range Volcanoes: Oregon’s first hot spot

8. Columbia River Basalt: the Yellowstone hot spot arrives in a flood of fire

9. Rift Volcanoes: aftermath of the Yellowstone hot spot

 

PHASE 3 PLASTER AND PAINT — This is the familiar land that we live on. Rocks hundreds of feet thick began forming 15 million years ago and continue to be shaped today.

Generalized Geologic Units:

10. Ancient Waterways: home of Oregon's first salmon

11. Rattlesnake Tuff: Oregon’s largest known eruption

12. High Cascade Volcanoes: land of fire and ice

13. High Desert Volcanoes: sleeping giants of eastern Oregon

14. Lakes, Rivers, and Dunes: painting the landscape

15. Pluvial Lakes: Oregon’s inland seas

16. Glacial Deposits: runaway global cooling

17. Ice Age Floods: Oregon's best soils lifted from eastern Washington

18. Mazama Deposits: a jewel born of destruction

19. Unstable Oregon: land of 10,000 landslides

20. Cascadia Subduction Zone Earthquakes: the big one(s)

1. Which two terranes were the first to accrete to form Oregon?

2. Where did these two terranes form and what evidence do we have?

3. What geologic processes were predominating during the Cenozoic?

4. Where did the Columbia River basalts originate and what land area did they cover?

5. What are some of the geologic regions formed from Oregon running over the Yellowstone hotspot?

6. What caused the High Lava Plains and Basin and Range Provence?

7. What type of rock erupted from the early High Cascades?

8. What happened in the early Tertiary period in Oregon?

9. What was the cause of the Clarno and Challis volcanic eruptions?

10. How has Oregon’s climate changed throughout its geologic history?

11. Describe how Oregon’s climate and geology will change in the future.

On the accompanying map, clearly mark and label the approximate location and extent (for instance, a mountain range is not symbolized by one dot) of the following features and cities. Use different colored pencils to mark the different features. Please use blue for rivers, brown for mountains, red dots for cities, and green for other features.

Rivers and Waterways

1. Columbia River

2. Strait of Juan de Fuca

3. Willamette River

4. Yakima River

5. Puget Sound

6. Skagit River

7. Snake River

Mountains

8. Olympic Mountains

9. Cascade Range

10. Mt. St. Helens

11. Mt. Rainier

12. Mt. Baker

13. Klamath Mountains

14. Willapa Hills

15. Okanogan Highland

16. Wallowa Mountains

17. Oregon Coast Range

Other Geographic Features and

Places

18. Columbia Plateau

19. The Palouse Hills

20. San Juan Islands

21. Vancouver Island

22. Grand Coulee (not just the dam)

23. Crater Lake

24. Snake River Plain

25. Glacier National Park

26. North Cascades National Park

27. Gray’s Harbor

28. Newberry Volcano

29. Methow Valley

30. Hell’s Canyon

Cities (WA, unless otherwise stated)

A. Seattle

B. Portland, OR

C. Yakima

D. Spokane

E. Olympia

F. Bellingham

G. Wenatchee

H. Richland/Pasco/Kennewick (Tri-Cities)

I. Tacoma

J. Orting

K. Forks

L. Republic

M. Boise, ID

N. Missoula, MT

O. Eugene, OR

P. Salem, OR

Q. John Day, OR

R. Joseph, OR

S. Cache Creek, BC

T. Vancouver, BC