17: Igneous Landforms 1

17 IGNEOUS LANDFORMS

1. An explosive eruption empties a large magma chamber beneath a volcano. The mountain sides collapse into the empty magma chamber, creating a caldera. A caldera is distinguished from other types of craters by its size (it’s much larger, both in diameter and depth, than other craters) and by the way it forms (it’s a collapse structure).

2. Crater Lake diameter: X mi = 5.4375 in × 62,500 in × 1ft × 1 mi = 5.4 mi 1 1in 12 in 5280 ft

3. a. Approximate water depth 2000 ft

b. Highest benchmark elevation 8156 ft

Water surface elevation 6176 ft

Local relief 1980 ft

c. Total crater depth 2000ft + 1980ft = 3980 ft

4. a. Mt. St. Helens crater diameter: X mi = 1.6875 in × 48,000 in × 1 ft × 1 mi ≈ 1.3 mi 1 1 in 12 in 5280 ft

b. Mt. St. Helens crater depth: 8365 ft – 6300 ft = 2065 ft

5. a. Capulin Mountain crater diameter: X mi = 0.625 in × 24,000 in × 1 ft × 1 mi ≈ 0.2 mi 1 1 in 12 in 5280 ft

b. Capulin Mountain crater depth: 8182 ft – 7770 ft = 412 ft

6. a. Capulin Mtn base elevation: 7100 or 7200 ft

b. Capulin Mtn local relief: 1082 or 982 ft

c. Distance (ft): (measured from 7100 ft contour) X mi = 1.375 in × 24,000 in × 1 ft ≈ 2750 ft 1 1 in 12 in

Distance (ft): (measured from 7100 ft contour) X mi = 1.094 in × 24,000 in × 1 ft ≈ 2188 ft 1 1 in 12 in

d. Mountain side gradient (ft/ft): measured from 7100 ft contour: 1082/2750 ≈ 0.39 ft/ft measured from 7200 ft contour: 982/2188 ≈ 0.45 ft/ft

e. Capulin Mtn slope: tan-1(0.39) ≈ 23° or tan-1(0.45) ≈ 27°

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7. Mauna Loa highest elevation (ft) 13,333

Mauna Loa base elevation (ft) 0

Mauna Loa Local relief (ft) 13,333

Distance (ft) ≈109,375 X ft = 5.25 in × 250,000in/in × 1ft/12in

Gradient (feet/foot) ≈0.12 13,333ft/109,375ft

Slope (degrees) ≈7°

8. Crater Crater Local Slope Volcano Width (mi) Depth (ft) Relief (ft) (degrees) Type

Crater Lake 5.4 3980 2151 12° composite

Wizard Island 0.06 90 762 27° cinder cone

Mount St. Helens 1.28 2085 4765 21° composite

Mount Rainier 0.3 270 9410 19° composite

Capulin Mountain 0.24 412 1082 23° or 27° cinder cone

Mauna Loa 2.4 600 13,300 7° shield

9. a. Convergent boundary; subduction

b. Composite volcanoes and cinder cones

c. Mount. St. Helen’s crater is much larger than a typical crater (e.g. Mt. Rainier), but not as big as Crater Lake, a caldera. Mount St. Helens is still visibly a mountain, while at Crater Lake the former Mt. Mazama is not evident. Calderas are collapse structures, and with the information we have, it’s hard to know whether the Mt. St. Helen’s crater is actually a collapse structure or whether its size is due to the top of the mountain being blown away. Most likely the crater size is partially due to collapse, making the crater more of a caldera than just a large crater.

10. Type of Igneous Tectonic Volcanic Landform Rock(s) Setting

Wizard Island Cinder cone Andesite Subduction Basaltic andesite Red Cone Cinder cone & andesite Subduction

Crater Lake Caldera Andesite & dacite Subduction

Mount St. Helens Strato volcano Andesite & dacite Subduction

Mauna Loa Shield volcano Basalt Oceanic hot spot Ancient Capulin Mountain Cinder cone Basaltic to andesitic continental rift zone Ancient Ship Rock Volcanic neck Basaltic continental rift zone

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11. Tectonic Setting Igneous Rock(s)

Subduction zone andesite and dacite

Oceanic hot spot basalt

Continental rift zone basaltic to andesitic

Igneous Landform Igneous Rock(s)

Cinder cone basalt, basaltic andesite, & andesite

Caldera andesite and dacite

Composite volcano basalt, andesite, dacite

Shield volcano basalt

12. Subduction zones primarily explosive

Continental rift zones effusive and explosive

Oceanic hot spots effusive

13. Oceanic rift zones experience only effusive eruptions, while continental rift zones experience a mix of effusive and explosive eruptions.

14. a. Pre-Mazama lava and pre-Mazama rhyodacite (rhyodacite is a mix of rhyolite and dacite)

b. They were either buried by younger deposits or they were blown away during the final eruption.

c. Wizard Island. It’s composed of post-caldera andesite.

d. Weathering. Fluvial erosion. Hillslope processes pulling material downhill.

15. a. 20 m. The thickest deposits are located northeast of Crater Lake.

b. Southwest (wind was coming from the SW). The majority of the ash spread to the northeast, and there appears to be little/no ash to the southwest.

c. The areas to the north and northeast appear flatter.

d. Ash and pyroclastic flow deposits filled in the low spots helping to level the landscape.

16. a. West-northwest. Ash spread to the east and southeast. There appears to be no ash to the west.

b. Mount St. Helens ash fall is much thinner. The thickest deposits are only 2–5 in (5–13 cm).

c. There is a series of ridges and valleys.

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d. The valleys would have been filled in with more ash decreasing the relief, but not totally filling the valleys. The local relief would be less but the overall relief wouldn’t be obliterated – we’d still be able to see a series of ridges and valleys.

17. a. See Figure E.15.

b. Dikes

c. 4–5

d. See Figure E.15.

e. Irregular. There are numerous closed contour lines along the top of the ridge. Each closed contour line indicates a high spot.

f. ridge rocks: basaltic rocks (igneous)

surrounding rocks: shale with sandstone & siltstone (sedimentary)

g. The sedimentary rock. It’s at a lower elevation than the ridges. The ridges are dikes, a type of intrusive igneous landform, so the shale must have surrounded the dikes when they formed.

18. 7178ft – 5500ft = 1678 ft

19. a. Weathering and erosion by water and wind.

b. The basaltic igneous rock is fairly resistant to weathering and erosion, particularly in comparison to the surrounding shale. In addition, the solidified lava extends down to great enough depths below the surface so that as the surrounding shale erodes, the dikes continue to form prominent features on the landscape.

20. The volcanic activity at Ship Rock occurred long before the volcanic activity at Crater Lake. According to the introduction, it takes millions of years of weathering and erosion to expose volcanic necks and associated dikes. Crater Lake, on the other hand, erupted only thousands of years ago and we can still see remnants of the mountain that was once there.

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FIGURE E1.15 USGS TOPOGRAPHIC MAP SHIP ROCK, NEW MEXICO, 1979. SCALE 1:24,000 CONTOUR INTERVAL = 20 FEET