Victoria Geology Maps and

Victoria Geology Maps and

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Victoria Geology Maps and

1. Tuff cones – Menin Buttes (chapter 7, page 129)

Working with air photos: To help you get started using air photos, refer to Figures 7.15 and 7.16 and answer the following:

7. Which of the Menin Buttes is shown in Figure 7.15? How do you know? (A butte is an isolated, steep-sided hill with a flat or irregular top.)

Figure 7.15

7. Draw an arrow pointing north on Figure 7.15.

Figure 7.16 (not on scale)

7. Outline on Figure 7.16 the area covered by the right-hand aerial photograph in Figure 7.15.
8. Which direction was the sunlight coming from in Figure 7.15? How do you know?
9. REFER TO THE MAPS IN YOUR TEXTBOOK Use the map in Figure 7.16 to determine the approximate scale of Figure 7.15. Show your work. Circle or describe the features on the map and the photo that you used for your measurements.

This can be determined by measuring the distance between two points that are recognizable on both the aerial photograph and the map. For example, the distance between the narrow part of the Butte Market Lake Canal just before it enters the Snake River and the tip of the point where a tributary joins Henrys Fork (just south of the word Fork on the map) is 11.3 centimeters on the photos and 12.8 centimeters on the map.

Consider that the aerial image and the map ultimately show and represent the same ground distance.

1. Cinder cones are normally made of lapilli- and block-sized basaltic scoria. The Menin Buttes are tuff cones, which are like cinder cones, but are made of ash- and lapilli-sized basaltic pyro clasts. The Menin Buttes formed when rising basaltic magma hit abundant groundwater in the gravel along the Snake River, erupted explosively, and thereby fragmented into smaller pyro clasts than is normal with basalt. Which way was the wind blowing when the Menin Buttes erupted? Why do you say this?
2. Examine Figure 7 .15 and carefully look at the contours on Figure 7.16. Would you say that these volcanic cones are steep-sided or broad and flat? Is the rim of the crater generally sharp or is it broad and wide?
3. Recall that air photos make objects appear three or four times taller than they are. To get a more accurate view of the relief of the Menin Buttes, make a topographic profile on the graph below following the line A-A’ in Figure 7.16 of your TEXTBOOK. Use the 100-ft elevation increments of the index contours (from your TEXTBOOK), and be sure to locate the tops of ridges and the bottom of the crater.

It is important to use the graph paper provided in your textbook to make a topographic profile with NO VERTICAL EXAGGERATION. You can achieve this by hand if you keep the horizontal and vertical scale the same: one inch = 2000 feet

Add here the A-A’ topographic profile

1. How wide is this tuff cone at its base (in miles)? How tall is it (in feet)? Show your work.
2. Given their size, height, and basaltic composition; do you think that tuff cones such as these represent a substantial volcanic hazard? Are they likely to produce vast lava flows, scalding pyroclastic flows that travel 10 or 20 miles from the vent, roof ­crushing air-fall deposits, or devastating lahars? Why?

ANSWER IS GIVEN: Tuff cones are unlikely to represent a serious volcanic hazard. They are too small to produce large lava flows, they are too short and the wrong composition to produce pyroclastic flows of more than local significance, they are the wrong composition to produce large volumes of ash, and they are unlikely to produce lahars because they are too short to be capped by glaciers and too porous (being made of pyroclastic debris) to hold much of a lake in the crater. The typical hazard is posed by hydrothermal explosions.

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