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What's the Difference? 2005 Submarine Ring of Fire Expedition What’s the Difference? FOCUS SEATING ARRANGEMENT Volcanic processes at convergent and divergent tec- Classroom style if students are working individually, tonic plate boundaries or groups of two to four students GRADE LEVEL MAXIMUM NUMBER OF STUDENTS 9-12 (Earth Science) 30 FOCUS QUESTION KEY WORDS How do volcanic processes differ at convergent Volcano and divergent tectonic plate boundaries? Caldera Hot spot LEARNING OBJECTIVES Ring of Fire Students will be able to compare and contrast volca- Asthenosphere noes at convergent and divergent plate boundaries. Lithosphere Magma Students will be able to identify three geologic fea- Fault tures that are associated with most volcanoes on Transform boundary Earth. Convergent boundary Divergent boundary Students will be able to explain why some volca- Subduction noes erupt explosively while others do not. Tectonic plate MATERIALS BACKGROUND INFORMATION Copies of “Submarine Volcanism Worksheet,” one The Ring of Fire is an arc of active volcanoes and copy for each student or student group earthquake sites that partially encircles the Pacific Ocean Basin.The location of the Ring of Fire coin- AUDIO/VISUAL MATERIALS cides with the location of oceanic trenches and (Optional) computer projection equipment to show volcanic island arcs that result from the motion of downloaded video materials large pieces of the Earth’s crust (tectonic plates). Tectonic plates consist of portions of the Earth’s TEACHING TIME outer crust (the lithosphere) about 5 km thick, as One 45-minute class period, plus time for student well as the upper 60 - 75 km of the underlying research mantle. The plates move on a hot flowing mantle layer called the asthenosphere, which is several hundred kilometers thick. Heat within the astheno- 1 2005 Submarine Ring of Fire Expedition – Grades 9-12 (Earth Science) 2005 Submarine Ring of Fire Expedition – Grades 9-12 (Earth Science) Focus: Volcanic processes at convergent and divergent tectonic plate boundaries oceanexplorer.noaa.gov oceanexplorer.noaa.gov Focus: Volcanic processes at convergent and divergent tectonic plate boundaries sphere creates convection currents (similar to the The Mariana Arc is part of the Ring of Fire that currents that can be seen if food coloring is added lies to the north of Guam in the western Pacific. to a heated container of water). These convection Here, the fast-moving Pacific Plate is subducted currents cause the tectonic plates to move several beneath the slower-moving Philippine Plate, cre- centimeters per year relative to each other. ating the Marianas Trench (which includes the Challenger Deep, the deepest known area of the The junction of two tectonic plates is known as Earth’s oceans). The Marianas Islands are the result a plate boundary. Where two plates slide hori- of volcanoes caused by this subduction, which zontally past each other, the junction is known frequently causes earthquakes as well. In 2003, as a transform plate boundary. Movement of the the Ocean Exploration Ring of Fire Expedition sur- plates causes huge stresses that break portions of veyed more than 50 volcanoes along the Mariana the rock and produce earthquakes. Places where Arc, and discovered that ten of these had active these breaks occur are called faults. A well-known hydrothermal systems (visit http://oceanexplorer.noaa.gov/ example of a transform plate boundary is the San explorations/03fire/welcome.html for more information Andreas fault in California. on these discoveries). The 2004 Submarine Ring of Fire Expedition focussed specifically on hydro- Where tectonic plates are moving apart, they form thermal systems of the Mariana Arc volcanoes, a divergent plate boundary. At these boundaries, and found that these systems are very different magma (molten rock) rises from deep within the from those found along mid-ocean ridges (visit http: Earth and erupts to form new crust on the litho- //oceanexplorer.noaa.gov/explorations/04fire/welcome.html for sphere. Most divergent plate boundaries are under- more information). The 2005 Submarine Ring of water (Iceland is an exception), and form subma- Fire Expedition will explore hydrothermally active rine mountain ranges called “oceanic spreading volcanoes in the Kermadec Arc, an area where tec- centers” or “mid-ocean ridges.” tonic plates are converging more rapidly than any other subduction zone in the world. If two tectonic plates collide more or less head-on, they produce a convergent plate boundary. Usually, In this lesson, students will investigate the char- one of the converging plates moves beneath the acteristics of volcanoes at mid-ocean ridges and other in a process called subduction. Subduction convergent plate boundaries, and make inferences produces deep trenches, and earthquakes are com- to account for observed differences between volca- mon. As the sinking plate moves deeper into the noes at these locations. mantle, increasing pressure and heat release fluids from the rock causing the overlying mantle to par- LEARNING PROCEDURE tially melt. The new magma rises and may erupt 1. To prepare for this lesson, read the Submarine violently to form volcanoes that often form arcs Ring of Fire 2004 background essay, “Submarine of islands along the convergent boundary. These Volcanism 2004” (http://oceanexplorer.noaa.gov/ island arcs are always landward of the neighbor- explorations/04fire/background/volcanism/volcanism.html). ing trenches. This process can be visualized as a If students do not have internet access, you will huge conveyor belt on which new crust is formed also need to download diagrams of the structure at the oceanic spreading ridges and older crust is of mid-ocean ridges and subduction zones (http: recycled to the lower mantle at the convergent plate //oceanexplorer.noaa.gov/explorations/03fire/logs/ridge.html boundaries. The Ring of Fire marks the location of and http://oceanexplorer.noaa.gov/explorations/03fire/logs/ a series of convergent plate boundaries that sur- subduction.html), as well as reference materials cited rounds the Pacific Ocean basin. on the “Submarine Volcanism Worksheet.” You may also want to visit the Magic Mountain Web 2 3 2005 Submarine Ring of Fire Expedition – Grades 9-12 (Earth Science) 2005 Submarine Ring of Fire Expedition – Grades 9-12 (Earth Science) Focus: Volcanic processes at convergent and divergent tectonic plate boundaries oceanexplorer.noaa.gov oceanexplorer.noaa.gov Focus: Volcanic processes at convergent and divergent tectonic plate boundaries pages (http://oceanexplorer.noaa.gov/explorations/02fire/ (4) What are two primary factors that affect the logs/magicmountain/welcome.html) to see the “land- viscosity of magma? scape” around an active hydrothermal site. Silica (SiO2) content and temperature of the magma. 2. Briefly review the concepts of plate tectonics and continental drift. Be sure students understand the (5) What is the difference between sheet lavas distinction between mid-ocean ridges and sub- and pillow lavas? duction zones. Sheet lavas resemble broad blankets and are formed by lava that is very fluid and flows 3. Provide each student or student group with a quickly (high effusion rate), while pillow lavas copy of the “Submarine Volcanism Worksheet,” are bulbous mounds formed by slow-flowing and have students answer the worksheet ques- lava (low effusion rate). tions. (6) Do volcanoes at mid-ocean ridges and sub- 4. Lead a discussion of students’ responses to ques- duction zones erupt explosively? tions on the worksheet. The correct responses are: Volcanoes at subduction zones often erupt (1) What are three geologic features that account explosively; volcanoes at mid-ocean ridges for most volcanoes on Earth? usually do not. Oceanic spreading centers at divergent plate boundaries (e.g, the Atlantic mid-ocean (7) What is a caldera? ridge), subduction zones at convergent plate A large depression on the summit of a vol- boundaries (e.g., Pacific Ring of Fire), and cano caused by the downward collapse of the “hot spots” which are believed to be rela- summit when large amounts of magma are tively small regions in the Earth’s mantle that suddenly removed from the magma chamber are especially hot (e.g., volcanoes of the beneath the summit; this removal is often the Hawaiian Islands and Yellowstone Park); you result of a large explosive eruption. may want to point out that more than half of the world’s volcanoes are located on the (8) Are calderas more likely to occur at mid- Pacific Ring of Fire, but that more than three- ocean ridges or subduction zones? Why? fourths of all lava produced on Earth comes Calderas are moe likely to occur at subduction from mid-ocean ridges. zones, because subduction zone volcanoes are more likely to erupt explosively. (2) How is the shape of volcanoes at mid-ocean ridges different from the shape of volcanoes (9) Would you expect to find more primitive lava at subduction zones? composition at mid-ocean ridges or subduc- Mid-ocean ridge volcanoes tend to be linear tion zones? Why? and look like long, low ridges, while volca- More primitive lava would be expected at noes at subduction zones tend to be cone- mid-ocean ridges, because this lava is pro- shaped and isolated. duced by magma directly from the Earth’s interior, in contrast to the magma at subduc- (3) What causes a volcano to erupt explosively? tion zone volcanoes which includes material Stiff, viscous magma that traps gases and from the surface of the subducted plate. allows pressure to build up until an explosive eruption occurs. 2 3 2005 Submarine Ring of Fire Expedition – Grades 9-12 (Earth Science) 2005 Submarine Ring of Fire Expedition – Grades 9-12 (Earth Science) Focus: Volcanic processes at convergent and divergent tectonic plate boundaries oceanexplorer.noaa.gov oceanexplorer.noaa.gov Focus: Volcanic processes at convergent and divergent tectonic plate boundaries (10) How quickly do biological organisms colo- thorough publication of the U.S. Geological nize newly-erupted lava? Survey on plate tectonics written for a non- Biological colonization occurs rapidly, often technical audience within a few months of the lava’s eruption.
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