Geology 101 Name(s): Lab 3: Igneous rocks, volcanoes and volcanic hazards Igneous rocks — Samples R4 – R17 (Tubs 5 – 18) 1. An igneous rock may be identified by either its texture or its characteristic mineralogy and grain size. The rocks in the table below can be identified by their grain size and mineralogy. Use Table B-1a to aid your identification. For predominant grain size, simply write either “coarse” or “fine”. Rock # Felsic, mafic or intermediate? Predominant grain size Rock name R4 R5 R6 R7 R8 R9 2. Look at samples R-10 and R-11, two very common rocks in the Cascades. a. R-11 is called granodiorite. Is it mafic, felsic, intermediate, or in-between two of the categories? If in-between, which two classifications is it in between? (Hint: look at the name) b. R-10 is called dacite. Same question as part a. (Hint: it is the extrusive equivalent of granodiorite) c. Why is it not weird that if you find a lot of one of these rocks in the Cascades, that you find a lot of the other as well? Hint: your answer should use the phrases “magma chamber” and “eruption”.3. The rocks in the table below can be identified by their texture. Use Table B-1b to aid your identification. Under the texture column, write the descriptive word or words from Table B-1b that helps you categorize the rock. Rock # Texture Rock name R12 R13 R14 R15 R16 Table B-1a. Igneous rocks which are identified by their composition/grain size Table B-1b. Igneous rocks which are identified by their texture 1. Is the rock glassy on any fresh surface? Yes — Obsidian No — Go to #2. 2. Is the rock vesicular (containing gas bubbles)? Yes — Go to #3 No — Go to #4 3. Is the rock dark-colored on a fresh surface? Yes — Scoria No — Pumice 4. Is the rock composed of large pieces of broken rocks? Yes — Breccia No — Go to #5 5. Is the rock well-cemented (doesn't crumble easily)? Yes — Welded tuff No — Tuff Tuff and pumice are sometimes difficult to tell apart; however, a simple test distinguishes them: pumice floats on water and tuff sinks. Coarse-grained (mostly visible grains) GRANITE Fine-grained (mostly invisible grains) RHYOLITE DIORITE ANDESITE GABBRO BASALT COMPOSITION (Minerals present) Felsic quartz orthoclase biotite Na-plagioclase Intermediate Na-plagioclase amphibole pyroxene biotite Mafic Ca-plagioclase olivine pyroxene TEXTURE4. Get a sample of rock R-17, which is cut and polished thunderegg. Thundereggs are a type of geode, which make beautiful bookends and household decorations. The exterior rock of these thundereggs, which are from the Priday Agate Beds near Redmond, Oregon, is an andesitic basalt, and the interior material is milky quartz (I tell you this because I don’t want you to perform the usual mineral tests on these!). Write a short history of how this rock came to be; in other words, start from magma and tell me what happened to make this rock. Hint: it’s not a one-event history. Volcanoes 5. The viscosity of a fluid is its ability to resist flow; higher viscosity means that the fluid doesn't flow very fast. Mafic magma is very fluid compared to more felsic magmas. As a result, mafic lava travels further from the volcano than does felsic lava, which tends to pile up near the eruption point. Felsic lava, because of its viscosity, also traps volatiles (gases). Circle the correct choice below: a. Which temperature magma would tend to have less viscosity (more fluidity)? 1200°C 900°C 600°C b. Which temperature magma would tend to have more iron in its molten state? 1200°C 900°C 600°C c. Which temperature magma would contain the least amount of dissolved gases, and therefore be more fluid? 1200°C 900°C 600°C d. So which word best describes the composition of the magma that satisfies the temperature requirements above? mafic intermediate felsic6. Using the results of question 5, answer the following questions with mafic, felsic, intermediate or any: a. Which magma will likely erupt explosively? b. Which magma will likely erupt effusively (quiescently)? c. Which magma will produce volcanoes with steep slopes? d. Which magma will produce volcanoes with gentle slopes? The "2. Volcanism" raised relief model 7. a. Give the name of the rock most likely to be found on: Volcano 21 Volcano 28 Volcano 25 b. Along the northern edge, find features 37 and 38 and identify them. Feature 37 (the specific kind is called a laccolith) Feature 38 8. How does feature 38 make it to the surface? In other words, how do features like 38 ever get exposed and allow geologists to study them? Volcanic hazards 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 volumeof ejecta, and other factors. This could be used then to assess the danger a given volcano might pose to its surrounding communities. VEI Plume height Volume of ejecta (km3) Classification Example 0 <100 m (< 0.1 km) 0.000001 Hawaiian Kilauea, now 1 100 – 1000 m (0.1 – 1 km) 0.00001 Hawaiian/ Strombolian Stromboli, now; Nyiragongo, 1982 2 1 – 5 km 0.001 Strombolian/ Vulcanian Colima, 1991; Galeras, 1992 3 3 – 15 km 0.01 Vulcanian Galeras, 1924; Ruiz, 1985 4 10 – 25 km 0.1 Vulcanian/ Plinian Sakura-jima, 1914; Galunggung, 1982 5 >25 km 1 Plinian St. Helens, 1980 6 >25 km 10 Plinian/ Ultra-Plinian Krakatau, 1883; Vesuvius, 79AD 7 >25 km 100 Ultra-Plinian Tambora, 1815 8 >25 km >1000 Ultra-Plinian Toba. 74000 yrs BP 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. 9. Look at the Mt. Rainier, Glacier Peak and Mt. Baker maps. Use the scale to measure 10 km away from the crater in any