Name: _______________________________________________ Section:_______________________
part
1: Cascade Range Volcanos
1. Explain briefly how a caldera forms, and how a caldera is different from other types of volcanic craters.
Eruption empties the magma chamber beneath the mountain, which then collapses to leave a crater
2. Use the topographic map in Figure 18.3 (
3. Determine the total depth of the crater in Figure 18.3 (i.e. the distance from the top of the rim to the bottom of the lake) by following the steps below.
a. Approximately how
deep is the water in
highest
elevation on crater rim (above sea level) elevation of
water surface (above sea level)
b. Determine the local relief from the water level in
depth local relief
Examine the contour lines carefully to determine where the
ridge defining the top of the crater is located. Then look for the benchmark designating the
highest point along the crater rim.
What is the highest elevation of
crater rim? 8156' (
What is the elevation of the water surface? 6176'
What is the local relief from the water level to the top of the crater? 1980'
c. Using the answers to parts (a) and (b), what is the total depth of the crater, in feet? 3980'
4. Use the topographic map of
a. The diameter of the crater from rim to rim in
an east-west direction, across the center of the lava dome, in miles. 1.25 miles
b. The total depth of the crater from the
highest point on the crater rim to the bottom of the crater, in feet. ~2065’
5. Use the
topographic map of
a. The diameter of the crater from rim to rim in an east-west direction, in feet.
1350’
b. The total depth of the crater from the highest point on the crater rim to the bottom of the crater, in feet.
~412’
6.
Crater Width Crater Depth
a. Comparing
the measurements of
It is a caldera, as the volume of ejecta
was large and the central portion of the original mountain partially collapsed
into the evacuated magma chamber
b. What accounts for the large difference in size of the craters at
7. Determine the slope angle for
a. Select an index contour line that approximates the base of the mountain and determine its elevation.
Approximate base elevation (feet) 4600’
b. The highest elevation of the crater rim is 8365 feet and is marked with an “x”. Use a ruler to draw a line from this point downhill to the southwest to the contour line you selected to approximate the base of the mountain. Measure the length of this line in feet.
Distance from rim to base (feet) 9082’
c. Calculate the gradient in feet/foot. .414556
d. The inverse sine of this number will tell you the slope in degrees.
Slope in degrees 24.5º
8. Determine the slope angle for
Highest elevation (ft) 8182’
Base elevation (ft) 7000’
Distance (ft) 3200’
Gradient (feet/foot) .369375
Slope (degrees) 21.7º
9. Compare your slope measurements (in degrees)
for
a. What type(s) of volcanoes have slopes similar
to yours? Composite or cinder cone
b. Based on all of the measurements taken so
far, what type of volcanoes are these, shield volcanoes, composite volcanoes,
or cinder cones?
10. a. What type of landform is Red Cone (shown on
the
b. Where are two other examples of this type of landform on the
topographic map (Figure 18.3? DESERT CONE,
c. What type of rocks are these cones
composed of (Figure 18.6)? pumice, rhyolite
d. Based on the geologic map in Figure 18.6, do you think these cones
formed before or after the eruption that formed
11. Examine the map of pumice deposits associated
with
a. Based on the isolines showing the thickness of pumice fall, what
was the prevailing wind direction at the time
b. Examine the topography to the south and
southeast of
c. What might be the reason for this difference in topography? Use Figures 18.6 (Geologic Map) and 18.7
(Pumice Map) to help answer this question, and think about what the topography
to the north and to the south of Crater Lake might have been like prior to the
eruption that created the lake. Burial by pumice ash
12. Figure 18.8 shows the ash fallout from the 1980
eruption of
a. Based on the patterns shown on the map, what
was the prevailing wind direction at the time of the eruption? Westerly in Washington, but northerly in the
b. How does the thickness of ash fall from
c. Examine the stereo photos in Figure 18.9. Describe the topography to the north of the
blast zone (north of the light grey area). flatter
d. What do you think this landscape to the north of
13. Figure 18.10 shows the tectonic setting of the
Cascade Mountain Range. What type of
plate boundary is causing the volcanic activity in this region? Convergent (subduction zone upwarp)
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Mt St. Helens,
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figure 18.3 [
FIGURE 18.3 USGS Topographic map
Original was1:62,500;
this is a 3/4 reduction
contour interval = 50 feet
FIGURE 18.4 Mt. St. Helens topographic map
FIGURE 18.4 USGS Topographic map
Original was1:48,000;
this is a 2/3 reduction
contour interval = 40 feet
FIGURE 18.5
FIGURE 18.5 USGS Topographic map
Original was 1:24,000;
this is a 1/2 reduction
contour interval = 20 feet
Figure 18.6
Generalized Geologic Map of
Figure 18.7 Map
Showing Distribution Pumice Deposits from
FIGURE 18.8 Mt.
FIGURE 18.9 Stereopair
of
figure 18.10
Map showing the relationship of the major
Cascade volcanoes to plate interaction offshore of western
Name: _______________________________________________ Section:_______________________
part
2: landscape alteration: Ship Rock, New Mexico
Use the topographic map of Ship Rock, NM (Figure 18.11) to answer the questions below.
1. What type of volcanic landform is Ship Rock? VOLCANIC NECK
2. a. On the topographic map, find two long ridges, one extending to the south-southeast and one to the west of Ship Rock. Highlight these two ridges on the topographic map.
b. If you walked along the top of either of these ridges, would the top be smooth or irregular? How did you figure this out? Irregular; the upper contours are not single enclosures
c. What type of landform are these two ridges examples of? EXHUMED VOLCANIC DIKES (MORE RESISTANT THAN HOST ROCK)
3. Examine the stereo-pair of Ship Rock. On these photographs, north is to the left. Rotate the topographic map (Figure 18.11) so that north is to the left.
a. How many dikes can be seen radiating outwards from Ship Rock on the air photos? FIVE
b. Examine the topographic map carefully and locate all the dikes which can be seen on the air photos. Use the shape of the contour lines to help locate these dikes. Highlight these remaining dikes on the topographic map.
4. a. On the air photos, just below and to the right (southwest) of Ship Rock, a cinder cone is evident. Highlight this cinder cone on the topographic map.
b. How do the contour lines show the cinder cone? What shape do the contour lines have? CONCENTRIC CIRCLES
c. There are three other distinct cinder cones in the photos. Locate and highlight them on the topographic map.
5. What is the approximate local relief of Ship Rock?
a. Highest elevation 7178'
b. Lowest elevation (approximate): 5600'
c. Local relief 1578'
6. a. What is the major process which has modified the landscape following the end of volcanic activity, producing the features evident today?
FLUVIAL EROSION
b. Why has this process not erased all traces of the volcano that once existed here? SOLIDIFIED LAVA IS RESISTANT, AND FORMS A "CAP" PROTECTING THE UNDERLYING WEAKER SEDIMENTARY ROCK. SOME BURIED INTRUSIVE FEATURES ARE VERY DEEP, SO THAT EROSION OF THE SURROUNDING ROCK EXHUMES AND LEAVES THEM AS PROTRUDING LANDFORMS.
Insert Figure 18.11 [Ship Rock topographic map]
FIGURE
18.5 USGS Topographic map
Ship Rock,
1:24,000
contour interval = 20 feet