Lesson 13: Oceanic Hotspots
During the next couple of weeks we'll be looking at parklands shaped by the processes that occur above mantle plumes or "hotspots". Most geologists now accept that such plumes rise from the deep mantle and have little direct connection to the processes that occur along plate boundaries. This view is supported by the fact that many hotspot volcanoes occur far from plate margins although a few, like Iceland, do lie along active boundaries. This week we'll visit the parklands of Hawaii to explore how volcanoes are built by oceanic hotspots and how the resulting islands change after volcanism ceases. Even though we'll be focusing on Hawai'i you should bear in mind that not all oceanic hotspots produce island chains that fit the "classic" Hawaiian model. Some, like the hotspot that built the Galápagos Islands on young, thin oceanic lithosphere, do not build linear island chains at all!
Because many of the islands produced by oceanic hotspots lie in the out in the open ocean they are subject to tsunamis triggered by distant earthquakes or massive submarine landslides. Our exercise this week will look at risks posed by these "seismic sea waves" as well as the "storm surges" that accompany powerful cyclonic storms.
As you read through this chapter and browse through the supporting websites please take careful notes so that you can keep track of major points and recall them more easily when we refer to them later in the semester. Be sure that you are prepared to meet the learning objectives outlined below before you move on to the quiz at the bottom of the page.
Weekly Learning Objectives
Upon successful completion of this week's lesson, a student is expected to be able to:
- Determine the general direction and the average speed an oceanic plate is moving from a map of the locations and ages of islands produced by a hotspot like Hawaii.
- Rank the following features developed above an oceanic hotspot in order of their relative ages, from youngest to oldest: volcanically inactive island, seamount, atoll, and volcanically active island.
- Explain why volcanic islands on an oceanic plate grow both smaller and shorter after plate motion carries them off the hotspot.
- Recognize the following features associated with Hawaiian volcanism, and briefly describe how each is formed: shield volcanoes, spatter cones, cinder cones, lava tubes, calderas, and pit craters.
- Cite four changes that are commonly observed on Hawaiian shield volcanoes prior to an eruption, and describe how each is related to the accumulation of magma in a shallow reservoir beneath the summit.
Reading and Browsing Assignment
- Read Chapter 8, focusing on the topics outlined in the learning objectives above.
- For a review of volcanic features, including those formed at oceanic hotspots, take a few minutes to browse through the How Volcanoes Work site authored by Vic Camp at San Diego State University. The site has sections on topics ranging from eruption dynamics to volcanic landforms that will supplement our text, and each section includes a brief "self-test" that you can try.
- To learn more about what's going on right now in Hawaii Volcanoes National Park check out the Hawaiian Volcano Observatory site. In addition to background information and live webcam shots of Kilauea and Mauna Loa, you can also find a brief write-up on Haleakala, volcanic hazards, seismicity, and more.
- Sulfur dioxide gas emissions by Kilauea and other vents in Hawaii Volcanoes National Park are a health concern and led to a recent park closure. To learn more about how such emissions are monitored check out the NPS Kilauea SO2 Advisory site.
- Finally, to learn a little more about how tsunamis are forecast through a combination of monitoring and modeling check out the NOAA Center for Tsunami Research. There's lots to see here, but the DART program of tsunami-detecting buoys is a particularly good example of how technology is being deployed in an effort to save lives.
Exercise 13 (Due by 9:00 AM on Monday, 19-Apr-2010)
Please load up your Hazard City CD, click on the Tsunami/Storm Surge exercise, and select version 3. Read through each of the introductory panels and then print the report and fill it out. Just a couple of pointers. First, in order to calculate the speed of the tsunamis in the "Tsunami Travel Time" question I simply laid a ruler against my computer monitor, estimated how may centimeters was equivalent to 2000 miles, and then measured to see how many hours each tsunami required to travel this distance. This is not a very sophisticated method but it works; also, remember, speed = distance/time. Second, in the questions about the town note that Ocean Village covers 20 blocks and the people are distributed evenly. This will make figuring how many people or buildings will be affected by a wave of a given run-up pretty easy. Be sure to have your final results in hand before you go to the Etudes "Assignments, Tasks and Tests" tool to complete Exercise 13.
Quiz 13 (Due by 9:00 AM on Monday, 19-Apr-2010)
After you feel you have mastered the learning outcomes outlined above, please complete Quiz 13 in the Etudes "Assignments, Tasks and Tests" tool. There are ten questions, each worth one point. If you can answer all of them correctly it means that you know your way around oceanic hotspots pretty well and are ready to move on to learn about parklands formed at continental hotspots next week.