Geology 101 Name(s): Lab 5: Volcanic hazards and sediment The Volcano Explosivity Index (VEI) was developed by volcanogists Chris Newhall and Steve Self in 1982 to gauge the explosiveness of a particular eruption from characteristics like the height of the ejecta plume, the total volume of ejecta, and other factors. This could be used then to assess the danger a given volcano might pose to its surrounding communities. VEI Volume of ejecta (km3) 0.000001 Classification Example Hawaiian Kilauea, now 0.00001 Hawaiian/ Strombolian Stromboli, now; Nyiragongo, 1982 2 Plume height <100 m (< 0.1 km) 100 1000 m (0.1 1 km) 1 5 km 0.001 Colima, 1991; Galeras, 1992 3 4 3 15 km 10 25 km 0.01 0.1 5 6 >25 km >25 km 1 10 7 8 >25 km >25 km 100 >1000 Strombolian/ Vulcanian Vulcanian Vulcanian/ Plinian Plinian Plinian/ Ultra-Plinian Ultra-Plinian Ultra-Plinian 0 1 Galeras, 1924; Ruiz, 1985 Sakura-jima, 1914; Galunggung, 1982 St. Helens, 1980 Krakatau, 1883; Vesuvius, 79AD Tambora, 1815 Toba. 74000 yrs BP The effect of the 1980 VEI 5 eruption of Mt. St. Helens are given at http://vulcan.wr.usgs.gov/volcanoes/msh/may18/summary_may18_eruption.ht ml; one can only wonder at what effects a VEI 8 eruption might have!
Clearly, Mt. St. Helens is capable of a VEI 5 eruption. To translate this into human effects, two researchers, John Ewert and Christopher Harpel, came up with a Volcano Population Index or VPI. Through measurements of the eruptions of Central American volcanoes, which are similar to the Cascades volcanoes, they discovered that VEI 5 eruptions warrant evacuations of all people within 10 km (6 miles) of the volcano s crater. The VPI is the number of people who therefore are in need of evacuation for a potential VEI 5 event. 1. Let s assess the hazards that nearby Cascades volcanoes may have locally. Look at the Mt. Rainier, Glacier Peak and Mt. Baker maps. Use the scale to measure 10 km away from the crater in any direction (so a circle of radius 10 km around the crater). In the table, list any towns or cities within your boundary for these volcanoes. Volcano Mt. Rainier Towns and cities within 10 km VPI10 Glacier Peak Mt. Baker 2. Use a census reference like http://www.ofm.wa.gov/pop/ to calculate the VPI for each volcano. If there are no towns or cities within 10 km of a particular volcano, or else they are not listed in the census, write 0. 3. Compare and contrast this to the Central American country of El Salvador. Go to http://www.agiweb.org/geotimes/apr04/feature_vpi.html which is an article authored by Ewert and Harpel. Using figures from the article, complete the table: Volcano Volcan San Salvador Towns and cities within 10 km VPI10 Ilopango Caldera 4. The authors do caution about the limitations of using the VPI to predict volcanic hazards. What two factors do they cite might cause the VPI to be a serious underestimate of the true hazard?
What about those VEI 8 eruptions? VEI 8 eruptions are called super-eruptions, and they earn their name through the ejection of hundreds and possibly thousands of cubic kilometers of pyroclastic material, usually called ignimbrite, a rhyolitic or dacitic tephra. About 74,000 years ago (74000 yr BP), Toba volcano on the island of Sumatra in Indonesia erupted over a two-week period, and causing a six-year decline in worldwide temperatures plus significant acidification of rainfall. There is an ongoing debate over whether that eruption nearly wiped out Homo sapiens: http://www.sciencedaily.com/releases/2010/02/100227170841.htm Go to http://www.andaman.org/book/originals/weber-toba/textr.htm and look over the first three chapters of the report. 5. According to George Weber, the author of this report, what is the evidence that Toba was, indeed, a VEI 8 event? 6. What sort of tectonic setting generated this eruption? 7. Why isn t there really a volcanic peak here any more? The hint is that these types of volcanoes are called resurgent calderas. What is the evidence of the resurgent part here? Local volcanic hazards In addition to ejecta and explosions, volcanoes with glaciers or snow or just a lot of groundwater can produce mudflows when glacial meltwater combines with the rock and sediment on the mountain s flanks. These mudflows are known by the Indonesian word lahar. The map on the next page shows the paths of two lahars that descended from Mt. Rainier. What makes this a little scary is that neither of these events seems to be related to a major eruption of the volcano a lahar can happen simply because of increased heat flow from some deep magmatic source melting the glaciers.
8. Notice that the lahar paths are curved. Name the specific geographical feature these lahars follow. Hint: Find a different version of the same map online. Osceola Mudflow Electron Mudflow Citation of source where information was found (no Wikipedia or other encyclopedia this is college) Why do you trust this source? 9. Assume the Electron Mudflow (lahar) began at the summit of Mt. Rainier. The speed of the front of a lahar has been measured for several recent events. The average speed seems to be about 40 km/hr. How many hours after the initial event that triggered the mud to start moving at the summit did it take for the lahar to get to Orting? Show the detail of your calculations below. Hint: A piece of string may be handy here.
The final hazard discussed is tephra, or less formally, ash. This finely-pulverized rock from a volcano can drift a long way in the wind. Tephra is a respiration hazard, which can result in silicosis because the particles stick to the inner lining of the lung. Tephra can also destroy engines, since the fine particles can cause pistons and valves to seize (don t drive through a tephra fall if you can help it!). Finally, if an eruption can send tephra to the top of the troposphere, or even into the stratosphere, the fine particles can block light and therefore reduce the amount of sunlight that reaches the surface of the earth. In effect, a big eruption can cool the earth. Mt. Pinatubo in the Philippines ejected enough material in its 1991 eruption to cool the northern hemisphere by nearly 0.5 C for three years, which doesn t sound like much. This difference, though, shortened the growing season by nine days, which may have caused some crops in northern countries to fail. The US Geological Survey mapped the 1980 Mt. St. Helens ash plume. Go to http://vulcan.wr.usgs.gov/volcanoes/msh/maps/map_may18_ash_path.html 10. Which way was the wind blowing on May 18, 1980? The wind may have changed direction during the day (note the time intervals given). Now go to http://vulcan.wr.usgs.gov/volcanoes/msh/maps/may18_ashmap.html 11. Note that, as expected, ashfall was heaviest around the volcano. However, there seems to be another significant peak some distance from the volcano. Suggest a reason why ashfall amounts did not uniformly decrease from the volcano. Hint: what force keeps fine ash suspended in the air? Igneous rocks and volcanoes summary The distribution of the different types of volcanoes around the world seemed inexplicable to earth scientists even in the last century. Plate tectonics, though, provided the framework to allow explanations of why a particular type of volcano would occur in a given area. You ve seen (in Lab 2) some mechanisms that allow volcanologists to predict what type of rock with be produced by volcanoes in certain
plate tectonic settings, and, in this lab, how destructive an eruption may be. Here s a summary of the volcanoes and associated rock types: a divergent plate boundary volcano (such as one on a mid-ocean ridge) produces basalt a convergent plate boundary volcano (such as one located near a subduction zone) produces andesite or dacite, with minor basalt an oceanic hotspot volcano, located on the interior of an oceanic plate, produces basalt a continental hotspot volcano, located in the interior of a continental plate, produces rhyolite, andesite, dacite or basalt 12. Given those rules, fill in the table below. You may wish to search the Web or other sources to see what each of the volcanoes looks like. Volcano Long Valley Caldera (California) Vesuvius (Italy) Tectonic setting Subduction zone Subduction zone Name of rock the volcano is primarily made of VEI estimate Estimate of effect on climate (global, regional or local effect?) Cerro Azul (Galapagos Islands) Interior of oceanic plate What type of volcano (shield, composite or resurgent caldera) is each? Long Valley Caldera Vesuvius Cerro Azul
Sedimentary rocks Needed: Weathering samples W1 through W6 (Tub 18), sedimentary rock samples M15, 16, 17 (Tub 19), R18 R27 (Tubs 20 29), R33 (Tub 35) and S1 (Tub 36) Sedimentary rocks are those that are formed either by direct deposition of material by crystallization of minerals in aqueous solution or by organisms, or through sediment settling under gravity. The settled sediment or biological material will lithify, in which it will be compacted and then chemically cemented into a rock. To make sediment, a rock of any type must be weathered (either physically or chemically) on the Earth s surface, then erosion will transport the sediment to where the sedimentary rock forms. Weathering 13. Samples W1 and W2 are igneous rocks; you should verify that W1 is granite and W2 is gabbro. Sample W3 is a partially weathered igneous rock. Sample W4 is beach sediment. a. Identify the minerals in the clasts breaking off of W3. b. Did sample W3 start off as W1 or W2? c. Look at sample W4 with a hand lens. Did this sediment come from W1 or W2? d. When sample W4 compacts and lithifies, what will its basic rock name be? Hint: Would it still be an igneous rock? e. What would a beach composed of sediment from the weathering of a pluton made of W2 look like? (Hint: think of Hawaiian beaches) 14. Lithification involves both compaction and cementation. W5 and W6 are glacially deposited sedimentary rocks of the same composition. Which has undergone more compaction? How can you tell?
15. a. Sedimentary rocks are held together by cement, a non-mineral chemical compound, which forms bonds (though not as strong as chemical bonds) between mineral grains. The three common cements are silica (SiO 2 ), calcite or iron oxide (rust). How would you identify each cement (think of a test for each)? Iron oxide Calcite Silica b. Look at rock samples R18 and R19. What is the cement that holds each together? R18 R19 16. How does the cement get into these rocks? 17. Of course, like all rocks, sedimentary rocks are made of minerals. The most common minerals were given on the second page of Lab 3. Using the Mineral ID sheets from the lab manual, identify the following minerals: Mineral # M-15 M-16 M-17 Distinguishing features (color, cleavage, hardness, magnetism, density, etc.) Mineral name 18. Now consider the actual mineral grains in rock samples R18, R19, R24 and R25. Because sand-sized dark minerals are very hard to identify, sedimentary petrologists (much to the horror of igneous petrologists) use the term dark lithic fragments to categorize lots of little dark minerals. Sample Hint to ID the mineral Most common mineral in the rock R18 Use glass plate R19 R24 R25 Well, look at it Acid bottle? Carefully, get it moist and see what happens to its size