lab 9

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Marshalltown Community College *

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101

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Geology

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Dec 6, 2023

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Lab: Volcanoes A) Hot Spots – The Hawaiian-Emperor Island Chain A dramatic aspect of plate tectonics is the estimated 50 to 100 hot spots across Earth's surface. These are individual sites of plumes of upwelling material from the mantle. Hot spots occur beneath both oceanic and continental crust and appear to be deeply anchored in the mantle, tending to remain fixed relative to migrating plates. An example of an isolated hot spot is the one that has formed the Hawaiian-Emperor Islands chain. The Pacific plate has moved across this hot, upward-erupting plume for almost 80 million years, with the resulting string of volcanic islands moving northwestward away from the hot spot. Thus, the age of each island or seamount in the chain increases northwestward from the island of Hawai'i. The big island of Hawai'i actually took less than 1 million years to build to its present stature. Hawai'i Volcanoes National Park is on the island of Hawai'i where volcanic activity is presently nearly continuous. The youngest island-to-be in the chain is still a seamount, a submarine mountain that does not reach the surface. It rises 3350 m (11,000 ft) from the ocean floor but is still 975 m (3200 ft) beneath the ocean surface. Even though this new island will not see the tropical Sun for about 10,000 years, it is already named Lo'ihi. The map on the next page shows the ages of each portion or island in the chain called the Emperor-Hawaiian Seamount Chain. Use the map in Figure 1 to complete the following questions: 1. Define “seamount”: 2. Define “island”: Look up the definition of “Hawaii an islands ”: 3. The Meiji Seamount (larger shape in top left corner of the larger map) traveled all the way from the hot spot where it was created to its current location. Its path is marked by the chain of seamounts and islands connecting it to the hot spot. Use a string and the map scale to measure how much distance the Meiji Seamount traveled (note: this was not a straight path). 4. a) Which is the youngest Hawaiian Island? b) Which is the oldest Hawaiian Island? 5. Read through the paragraph above about the seamount that will become the newest Hawaiian Island. If Lo’ihi was growing through summit eruptions at 2 m (6.5 ft) per year (hypothetical: in reality it grows a lot more slowly), in how many years approximately would it break through the surface of the Pacific Ocean? (Show your work.) _ _ 6. a) The age of Midway Island is . The distance from Midway to the active hot spot is miles (use map scale). b) Given these values what was the average speed of the Pacific Plate movement per year over this time frame? cm in
Figure 1: Hawai'i and the Emperor Seamounts. Ages are shown in millions of years (after D. A. Clague, "Petrology and K-Ar Ages of Dredged Volcanic Rocks from the Western Hawaiian Ridge and the Southern Emperor Seamount Chain," Geological Society of America Bulletin 86 (1975): 991).
B) Kilauea Caldera in Hawai'i On the island of Hawai'i the continuing Kilauea eruption represents the longest eruptive episode in Hawaiian history, active since 1983! During 1989-1990, lava flows from Kilauea actually consumed several visitor buildings in the Hawai'i Volcanoes National Park and homes in Kalapana, a nearby town. Kilauea has produced more lava than any other vent on Earth. Characteristic of a hot spot, eruptions emanating from the asthenosphere are low-viscosity magmas that are very fluid and yield a dark, basaltic rock (less than 50% silica and rich in iron and magnesium). Gases readily escape from this magma because of its texture, causing an effusive eruption that pours out on the surface, with relatively small explosions and little tephra. However, dramatic fountains of basaltic lava sometimes shoot upward, powered by jets of rapidly expanding gases. These are relatively gentle eruptions that produce enormous volumes of lava annually on the seafloor and in places like Hawai'i. An effusive eruption may come from a single vent or from a flank eruption through side vents in surrounding slopes. If such vents form a linear opening, they are called fissures and sometimes form a dramatic curtain of fire during eruptions. In Iceland, active fissures are spread throughout the plateau landscape. In Hawai'i, rift zones capable of erupting tend to converge on the central crater, or vent. The interior of such a crater is often a sunken caldera, which may fill with low- viscosity magma during an eruption, forming a molten lake, which may then overflow lava down- slope to the sea. Mauna Loa is an enormous mountain rising 5800m (19,029 ft) from the sea floor or 4169 m (13, 678 ft) above sea level-a total height of 9969 m (32,707 ft)! Its lava dome is 18 km by 9 km (11 mi by 5.5 mi). The topographic map in Figure 2 illustrates a portion of the Kilauea Volcano that has formed on the southeast slope of Mauna Loa. The topographic map was prepared in 1981. Use Topographic Map #1 to answer the following questions: 7. Define “relief” as it relates to the map: 8. a) What is the scale of this map? b) What is the contour interval? 9. What is the approximate width of the Kilauea Crater from North Rim horizontally across the map to Byron Ledge (be careful: the zero (0) is in the middle of the top scale bar, the entire bar’s length represents 2 miles) ? a) in miles: b) in km: 10. From the rim of the crater at Uwekahuna Bluff at the volcano observatory to the bottom (floor) of the main crater at the 3524-foot BM marker represents how much relief? . (Hint: check the note under the map.) 11. Within the crater is a "fire pit" named Halemaumau. This pit has varied in size during eruptions. (Use contour lines inside the pit to find lowest point in the pit.) a) Does the 3412 ft label in the pit represent the lowest point inside the pit? b) How many feet was this pit below the main floor of the Kilauea Crater (see previous question for the “floor” ) at the time of this map? 12. Record the dated lava flows noted in the Kilauea Crater, from oldest to youngest (find 9):
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