DEMO #5

HOTSPOTS, YELLOWSTONE, AND EXPLOSIVE VOLCANOES

General Description:

This "demonstration" is actually a lecture beefed up with two demo's, part of a video, and some good images (overheads). The accompanying handout was given to students just to drive home some key points and to keep them tuned in. The first demo uses a lava lamp to illustrate mantle plumes and hot spot traces. The second demo involves heating beakers of motor oil and tomato sauce to illustrate explosive volcanism.
 
  Materials: Demo #1 Demo #2 Also: 4 small bags of Yellowstone ash (Huckleberry Ridge eruption) collected by Mike Perkins from a 1 foot deep bed in the Mojave Desert. Mantle Plumes: After giving a general explanation of hotspots, I used the lava lamp to demonstrate how mantle plumes originate and rise. I also showed Figure 1 so students could see the similarities between the lava lamp diapirs and modeled mantle plumes. Tracing Plate Motions: I then did Demo #1 to illustrate a hotspot trace. Two students held a long strip of paper and slowly walked it over the lava lamp. A third student standing stationary at the lamp made marks on the paper (over the lamp) every few seconds. The result was a hotspot trace on the paper. I then showed Figure 2 and Figure 3 of Hawaii as an example of how hotspot traces show plate motions. Next, I introduced the Yellowstone hotspot (showed Figure 4), and we worked through the three questions on this portion of the handout with the help of Figure 5. Yellowstone Eruptions: Students then watched a portion of the video "Volcano!" (near the end of tape 2). This video is in Kim's office. The video clip showed the aftermath of Mt. Pinatubo, including major lahars, effect on climate, etc. The point was to show them what a "big" eruption can do. I then introduced the eruptive history of Yellowstone and showed them Figure 6 and Figure 7 to drive home how REALLY BIG the recent Yellowstone eruptions were. I then passed out the ash samples and discussed Figure 8 and Figure 9. Fig 8 shows bones of a group of rhinos being excavated from a Huckleberry Ridge ash bed in northeastern Nebraska. The rhinos died from ash inhalation (or infections related thereto) during or days after the eruption. Causes of Explosive Volcanism: I performed Demo #2. I put oil in one beaker and tomato sauce in the other, covered each with foil, then heated them up on a hot plate. While they were heating, I explained that these are both thick liquids, sort of like magma, and obviously a magma has to be thick enough (contain enough silica) to be explosive. I then posed the question of what ELSE is needed. Eventually, the tomato sauce became highly explosive, and the oil did nothing, even at higher T’s (verified with thermometer). The class pondered why, and eventually realized that it’s the WATER in the sauce (like water/gas in magma), in addition to its being thick, that made it explosive. I wrapped up with a discussion of where (which plate boundary type) one would most likely find such explosive magmas.