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What's the Difference? Aegean and Black Sea 2006 Expedition What’s the Difference? (adapted from the 2005 New Zealand American Submarine Ring of Fire Expedition) FOCUS SEATING ARRANGEMENT Volcanic processes at convergent and divergent Classroom style if students are working individu- tectonic plate boundaries ally, 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 Asthenosphere LEARNING OBJECTIVES Lithosphere Students will be able to compare and contrast Magma volcanoes at convergent and divergent plate Fault boundaries. Transform boundary Convergent boundary Students will be able to identify three geologic Divergent boundary features that are associated with most volcanoes Subduction on Earth. Tectonic plate Students will be able to explain why some volca- BACKGROUND INFORMATION noes erupt explosively while others do not. The geographic region surrounding the Aegean and Black Seas has been the stage for many MATERIALS spectacular performances in Earth’s geologic and Copies of “Submarine Volcanism Worksheet,” human history. Human activities on the region’s one copy for each student or student group stage began during Paleolithic times; artifacts dis- covered near Istanbul are believed to be at least AUDIO/VISUAL MATERIALS 100,000 years old. Well-known Aegean cultures (Optional) Computer projection equipment to include the Minoans (ca 2,600 – 1,450 BC), show downloaded video materials Mycenaeans (ca 1,600 – 1,100 BC), Ancient Greeks (776 – 323 BC), and Hellenistic Greeks TEACHING TIME (323 – 146 BC). Istanbul—“the only city that One 45-minute class period, plus time for student spans two continents”—has been a crossroads research of travel and trade for more than 26 centuries. Mariners have traveled the Aegean and Black 1 Aegean and Black Sea 2006 – Grades 9-12 (Earth Science) Focus: Volcanic processes at convergent and divergent tectonic plate boundaries oceanexplorer.noaa.gov Seas since Neolithic (“Stone Age” times; 6,500 to move several centimeters per year relative to – 3,200 BC), probably for a combination of each other. purposes, including trading, exploration, and warfare. The junction of two tectonic plates is known as a plate boundary. Where two plates slide horizon- Interactions between these cultures and many tally past each other, the junction is known as a others were often violent and destructive. So, transform plate boundary. Movement of the plates too, were interactions with geological processes. causes huge stresses, breaks portions of the rock, Some of these processes are directly related and produces earthquakes. Places where these to the same forces that are believed to have breaks occur are called faults. A well-known caused the breakup of Pangaea (see “Volcanoes, example of a transform plate boundary is the San below). One of the most dramatic and destruc- Andreas Fault in California. tive events was the eruption of a volcano near a small Aegean island called Thera (also known as Where tectonic plates are moving apart, they Santorini), sometime between 1,650 and 1,450 form a divergent plate boundary. At these bound- BC. Estimated to be four times more powerful aries, magma (molten rock) rises from deep than the Krakatoa volcano of 1883, the eruption within the Earth and erupts to form new crust on left a crater 18 miles in diameter, spewed volca- the lithosphere. Most divergent plate boundaries nic ash throughout the Eastern Mediterranean, are underwater (Iceland is an exception), and and may have resulted in global climactic form submarine mountain ranges called oceanic impacts. Accompanied by earthquakes and a tsu- spreading ridges. nami, the volcano destroyed human settlements, fleets of ships, and may have contributed to the If two tectonic plates collide more or less head- collapse of the Minoan civilization on the island on, they produce a convergent plate boundary. of Crete, 110 km to the south. On Thera, the larg- Usually, one of the converging plates moves est of the islands near the volcano, the ancient beneath the other in a process called subduc- city of Akrotiri was completely buried beneath the tion. Subduction produces deep trenches, and ash. Excavation of the city began in 1967, and is earthquakes are common. As the sinking plate ongoing. The Bronze Age eruption of the Theran moves deeper into the mantle, increasing pressure volcano was by no means its last. In fact, the and heat release fluids from the rock, causing volcano erupted at least 12 times between 197 the overlying mantle to partially melt. The new BC and 1950 and most geologists agree that a magma rises and may erupt violently to form violent eruption will happen again. volcanoes that often form arcs of islands along the convergent boundary. These island arcs are Many volcanoes, including the one near Thera, always landward of the neighboring trenches. are caused by movement of tectonic plates. These This process can be visualized as a huge con- plates are portions of Earth’s outer crust (litho- veyor belt on which new crust is formed at the sphere) about 5 km thick, as well as the upper 60 oceanic spreading ridges and older crust is recy- - 75 km of the underlying mantle. Tectonic plates cled to the lower mantle at the convergent plate move on a hot flowing mantle layer called the boundaries. asthenosphere, which is several hundred kilome- ters thick. Heat within the asthenosphere creates The tectonic setting of the Aegean/Black/ convection currents (similar to the currents that Mediterranean Sea area is complex, and includes can be seen if food coloring is added to a heated two major plates (the Eurasian and African container of water) that cause the tectonic plates Plates) as well as several minor ones (including 2 Aegean and Black Sea 2006 – Grades 9-12 (Earth Science) oceanexplorer.noaa.gov Focus: Volcanic processes at convergent and divergent tectonic plate boundaries the Hellenic, Turkish, Arabian, and Van Plates). and possibly mud that flowed down the sides Boundaries between these plates are not always of the volcano at speeds of 60 miles per hour clear, but motion at plate boundaries is undoubt- or more. In the final and most violent stage of edly responsible for earthquakes and volcanoes all, the pyroclastic flow reached the coast with throughout the region. This is not considered a temperatures approaching 400°C. Visit http://www. highly volcanic area, but even one Thera-type vol- immersionpresents.org/content/ for more information cano can do plenty of damage! about the Minoan eruption, including photo- graphs of other volcanoes in similar eruptive Thera and its neighboring islands are the most stages. active volcanic center in the Aegean Arc, and are part of a single volcano that has erupted Exploring the history of the Theran volcano is one many times. At one time, there were fewer than of the primary goals of the Aegean and Black the five islands visible today. In fact, one of it Sea 2006 Expedition. In this lesson, students will older names is “Strongyle,” which means circular investigate the characteristics of volcanoes at or rounded, suggesting that the overall appear- divergent and convergent plate boundaries, and ance was different in the past. During some of make inferences to account for observed differ- its eruptions, the floor of the volcano’s crater col- ences between volcanoes at these locations. lapsed into the magma chamber below, forming a caldera. Over time, the caldera was partially LEARNING PROCEDURE re-filled by other volcanic activity. Before the erup- 1. To prepare for this lesson: tion during Minoan times, the rim of the caldera • Review the background essays for the was an almost-continuous ring, with only a single Aegean and Black Sea 2006 Expedition at entrance. The floor of the caldera collapsed http://oceanexplorer.noaa.gov/explorations/06blacksea/wel- again during the Minoan eruption, destroying come.html and part of the above-water ring, thereby creating two • The Submarine Ring of Fire 2004 back- new channels. ground essay, “Submarine Volcanism 2004” (http://oceanexplorer.noaa.gov/explorations/04fire/back- Studies of volcanic ash deposits suggest that the ground/volcanism/volcanism.html). Minoan eruption happened in at least five stages. The first was a series of precursory volcanic If students do not have internet access, you will events that took place over several months and also need to download diagrams of the struc- may have provided the inhabitants with a warn- ture of mid-ocean ridges and subduction zones ing of things to follow, perhaps accounting for (http://oceanexplorer.noaa.gov/explorations/03fire/logs/ridge. the absence of human remains at Akrotiri. The html and http://oceanexplorer.noaa.gov/explorations/03fire/ next stage should have left no doubt about what logs/subduction.html), as well as reference materials was coming: An eruption column 36 km high cited on the “Submarine Volcanism Worksheet.” was ejected into the atmosphere, blanketing the Eastern Mediterranean with hot pumice and ash. 2. Introduce the Aegean and Black Sea 2006 Then came an even more violent stage. Cracks Expedition, emphasizing the Bronze Age erup- in the crater floor allowed seawater to contact tion of the Theran volcano. Briefly review the hot magma, producing explosions of steam that concepts of plate tectonics and continental hurled large blocks of rock and clouds of steam drift. Be sure students understand the distinc- and ash over the islands and adjacent ocean. tion between mid-ocean ridges and subduction These explosions were followed by a classic zones. “pyroclastic flow;” a river of ash, pumice, gases, 3 Aegean and Black Sea 2006 – Grades 9-12 (Earth Science) Focus: Volcanic processes at convergent and divergent tectonic plate boundaries oceanexplorer.noaa.gov 3. Provide each student or student group with a Sheet lavas resemble broad blankets and are copy of the “Submarine Volcanism Worksheet,” formed by lava that is very fluid and flows and have students answer the worksheet ques- quickly (high effusion rate), while pillow lavas tions.
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