Geology 103, Lab 07 Volcanoes

Transcription

1 Geology 103, Lab 07 In lecture we have looked at different types of volcanoes and how tectonic processes such as seafloor spreading and subduction give rise to volcanic eruptions. In this lab you will explore some fundamental observations regarding volcanoes including their size, the tectonic setting in which volcanoes occur, the relationship between the setting and the composition of the volcano s magma, and the depth to magma chambers beneath arc volcanoes (subduction zones). At the beginning of next lab you will turn in the following: Answers to the questions in the lab, the map with volcano locations plotted, and electronic and hard copies of the spreadsheet calculations. A. Size of some famous volcanoes: Volcanism has occurred on all the terrestrial planets and the Moon at some point in their history. Earth is the only one of these that has active volcanism today. Below are images comparing the size of some famous volcanoes on Earth and one on Mars. Use the image, our textbook and lecture notes to answer the following questions. 1) Based on the profile (shape), what type of volcano is Olympus Mons? What type is Fuji? 2) Which most likely had explosive eruptions, Fuji or Olympus Mons? Compare the size of these volcanoes: Create a simple spreadsheet to approximate the volume of these volcanoes; use the scales provided with the images and assume the volcanoes have a conical shape (V cone = 1/3pR 2 H). Note the separate scale for Olympus Mons 3) Approximately, how many times larger are Olympus Mons, Mauna Loa, Kilimanjaro, and Shasta relative to Fuji? Michael Stewart,

2 A. Tectonic Setting and Volcano Composition: We have learned in lecture that there are different types of volcanoes occurring at specific tectonic settings. Here you will plot the locations of some volcanoes along with some composition data on a map. Comparing this map of volcano locations to a map of tectonic plate boundaries you will see that magma composition also correlates with tectonic setting. The table below lists 30 volcanoes, their locations and average silica (SiO 2 ) content. Plot the locations of the volcanoes on the map (located at the end of this lab) using a colored dot. Hint: use the latitude and longitude information to plot the volcanoes. The color of the dot marking the volcano location will be Michael Stewart,

3 determined by the average silica content: Red if SiO 2 >60%, blue if 50<SiO 2 <60, and green if SiO 2 <50%. 4) Compare your plotted locations to the tectonic plate boundary map above. Where on the plates are most of these volcanoes located? How can you explain this? 5) Where (tectonic setting) are most of the high-silica volcanoes (red dots)? Table 1 : Location and Composition Western United States. Latitude Longitude SiO 2 Pacific Border (approximate) (wt%) Lassen, California 40 N 121 W 57.3 Crater Lake. Oregon 43 N 122 W 55.1 Mt. Rainier or Washington 47 N 122 W 62.2 Mt. Baker. Washington 49 N 122 W 57.4 Western United States Western Interior Yellowstone Park, Wyoming 45 N 111 W 75.5 Craters of the Moon. Idaho 43 N 114 W 51.5 San Francisco Peaks, Arizona 35 N 112 W 61.2 General America and West Indies Paricutin. Mexico 19 N 102 W 55.1 Popocatepetl, Mexico 19 N 98 W 62.5 Mt. Pelee, Martinique 15 N 61 W 65 Santa Marisa Guatemala 15 N 92 W 59.4 Mt. Misery. St. Kitts 17 N 63 W 59.8 South America Cotopaxi, Ecuador 1 S 78 W 56.2 Misti, Peru 16 S 71 W 60.1 Alaska and Aleutian Islands Area Katmai Alaska 58 N 155 W 76.9 Adak, Aleutians 52 N 177 W 60 Umnak island. Aleutians 53 N 169 W 52.5 Kamchatka. USSR 57 N 160 E 60.6 Japan Fuji. Honshu 35 N 139 E 49.8 Izu-Hakone, Honshu 35 N 139 E 53.8 East Indies Mayon. Philippines 13 N 124 E 53.1 Krakatoa (between Java & Sumatra6 S 105 E 67.3 Karkar, New Guinea 5 S 146 E 60.1 Central Pacific Mauna Loa or Kilauea, Hawaii 19 N 156 W 49.6 Galapagos Islands 1 S 91 W 48.4 Mariana islands 16 N 145 E 51.2 South Pacific White island, New Zealand 37 S 177 E 62.2 Auckland. New Zealand 38 S 176 E 49.3 Tahiti 18 S 149 W 44.3 Samoa 13 S 172 W 48.4 ways in which the mantle melts at different tectonic plate boundaries. 6) Where (tectonic setting) are most of the medium-silica volcanoes (blue dots)? 7) Where (tectonic setting) are most of the low-silica volcanoes (green dots)? 8) Provide an explanation for 5 through 7. Your lecture notes will help you here as will the textbook. You may want to consider (a) the type of crust that the magma must pass through before erupting, and (b) they Michael Stewart,

4 Spacing of Island Arc (Subduction Zone) and the Depth to Magma: We have learned in lecture that some volcanoes occur at subduction zones due to melting in the wedge of mantle asthenosphere overlying the subducting oceanic lithosphere (figure below). Melts from this mantel wedge then rise into the overlying crust to feed magma chambers beneath the volcanoes on the surface. If you consider this geometry in three dimensions, you will envision a three dimensional magma chamber extending horizontally through the crust paralleling the trench. This magma sheet then feeds the overlying volcanoes on the surface. The individual volcanoes of a subduction zone, such as those in the continental arc of the Cascade Range of the western US, are separated by a regular spacing interval. However, this interval of spacing is different for different arc systems. Interestingly, Geologists have learned that the spacing interval between volcanoes in a subduction zone is a function of the depth to the magma sheet in the crust: the shallower the magma sheet, the closer the spacing interval between volcanoes. Theoretically, one can show this relationship with the following equation: S = 2.568D, Where S is the spacing interval and D is the depth to the magma sheet. Notice this is independent of magma type, surface features, or gravity, and thus it should apply to other planets as well. Given the map (left) of the Cascade Range (continental volcanic arc), measure the spacing between the volcano pairs listed below. Input the data into a spreadsheet; calculate an average spacing interval and a depth to the magma sheet (in miles and kilometers). Note: the scale is 17mm=100miles. Michael Stewart,

5 Now Consider the Mariana Islands of the western Pacific Ocean (an island arc volcanic system). Again, use the map below to measure the distance between the listed volcano pairs and then calculate an average spacing interval for the Mariana volcanoes and a depth to the magma sheet (in miles and kilometers). Cascade Continental Arc Volcano Pair: Mt Baker-Glacier Peak Glacier Peak-Mt Rainier Mt Rainier-Mt St Helens Mt St Helens-Mt Adams Mt Adams-Mt Hood Mt Hood-Mt Jefferson Mt Jefferson-Three Sisters Three Sisters-Newberry Volcano Newberry Volcano- Mt Mazama Mt Mazama-Mt McLoughlin Mt McLoughlin-Medicine Lake Medicine Lake-Mt Shasta Mt Shasta-Lassen Peak 9) Compare the results you obtained for both the Mariana and Cascades systems and draw a general conclusion regarding the magma source of arc volcanoes. Hint: consider the difference in the type of crust between Island Arcs and Continental Arcs. Mariana Island Arc Volcano Pair: Farallon-Maug Islands Maug Ilsands-Asuncion Asuncion-Agrihan Agrihan-Pagan Pagan-Alamagan Alamagan-Guguan Guguan-Sarigan Sarigan-Anatahan Anatahan-Ruby Ruby-Esmeralda Bank Michael Stewart,

6 Michael Stewart,