MAR110 LECTURE #11 Case Study: · PDF filelegends about sightings and/or ... Figure 11.2...

23 September 2007 mar110_L11_CS_Krakatoa_23sep07.doc

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MAR110 LECTURE #11 Case Study: Krakatoa

Figure 11.3 Krakatoa: 1680 An etching of eruptions of Krakatoa around 1680. In general, evidence of historic eruptions is derived from oral legends about sightings and/or weather observations at distant sites (e.g., a 535 A.D. eruption was inferred from reports of the lack of sunlight in Rome).(??)

Figure 11.1 Explosive Volcanoes Due to large amounts of gases contained in their magma, subduction zone volcanoes are much more likely to be explosive than other oceanic volcanoes. The explosions can become even larger when seawater enters the magma chamber through cracks and flashes into steam. The latter happened with the largest of the 1883 Krakatoan explosions – one that almost completely obliterated the island. (NG??, ItO)


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Figure 11.4 Mount Etna Display Mount Etna - an active subduction zone volcano that explodes on ocassion. (NG??)

Figure 11.2 Regional Geography and Plate Tectonics Indonesia is dominated by volcanic island arcs that are formed by the subduction of the Philippine and Indian-Australian plates under the Eurasian plate. These collisions and those associated with a number of other smaller plates make this a very seismically active region. In 1883, the island of Krakatoa was situated in the Sunda Strait - almost at the intersection of two significant faults. A Dutch volcanologist, who was standing on a nearby mountain, reported seeing the ocean split open far enough for him to see the glowing magma underneath - just prior to the massive explosion of Krakatoa. The 1883 explosion was the culmination of a long history of Krakatoan volcanism. (SW)


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Figure 11.6 Krakatoa - 1883- In the 1883 explosion of Krakatoa much of the original island (top) was destroyed. The only remaining portion of the original island was a section of the Rakata cone (bottom). Several smaller islands were created from the debris while Verlaten Island was significantly enlarged and Lang island lost some land around its perimeter. (ItO)

Figure 11.5 Krakatoa Before 1883 Two maps of Krakatoa prior to the 1883 eruption. The one on the left shows the locations of the earlier cones: Perboewatan, Danan, and Rakata, while the one on the right shows the topography of the surveyed portions of the islands. Verlaten and Lang islands are thought by some geologists to have been the remains of a giant caldera left over from an earlier and much larger eruption-possibly the suspected 535 A.D. eruption. (??)


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Figure 11.7 Krakatoa Explosion “The Sound Heard ‘Round the World” The sound of the largest explosion was heard all around the Indian Ocean and in other parts of the world including Cuba. The sound was described as ships firing their guns. Even where nothing was heard, barometers recorded air pressure changes; which traveled around the world seven times and “echoed” for fifteen days. (SW?)


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Figure 11.8 Krakatoa Explosion - Followed by Tsunami ! The series of 1883 Krakatoa explosions triggered a series of tsunami. The first few were minor (couple of feet), but the final one was so powerful that it carried an iron ship two miles inland. The tsunami triggered by the final explosion was unusual in that it “skipped” over islands and other landmasses to hit the coast on the opposite side of the landmass before the wave had a chance to refract/reflect around the island/landmass. There were reports of the wave, which traveled around the world, reaching England. (SW)


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Figure 11.9 Tsunami impact The worst damage from the tsunami occurred in the Sunda straight (right) where in some places wave run ups were as much as forty two meters above sea level (lower left). (NH)


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Figure 11.10 Tsunami Measurement


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Figure 11.11 Tsunami “Sleigh Ride” The 1886 eruption of Krakatoa triggered a series of tsunamis which were some of the largest volcanically induced tsunamis in history. The waves were so large that they carried heavy iron clad ships such as those shown-and their moorings-several miles inland. The tsunami carried several heavy iron ships a couple miles inland and deposited them in the jungle. Many other ships were carried back out and deposited on the beaches as debris or washed all the way out to sea. (SW?) The only known eruption to create tsunamis on a comparable scale was the eruption of Santorini around the year 1450BC that covered the entire island of Crete in over a meter of water and destroyed the Minoan civilization. (NH)


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Figure 11.12 The Volcanic Ash Hazard Large amounts of fine volcanic ash are ejected during an eruption can burn and/or smother life near the volcano as it settles. Farther away the fine dust can coat everything and cause roofs to collapse from the excessive weight of only a few centimeters. Even small amounts of the ash when breathed in can also damage lungs and other tissues. (??)


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Mt EtnaMt EtnaSmoke/AshSmoke/Ash

TrailTrail

Figure 11.13 Mt Etna Ash from volcanic eruptions can be carried a long distance away by the wind before it has a chance to settle. The large smoke and ash plume from Mount Etna is clearly visible from satellite. (NG??)


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Figure 11.14 Volcanic Ash Injection and Distribution A volcano ejects ash into the stratosphere where the winds distribute it before eventually falling out, sometimes after several years. (NG??)


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Figure 11.15 Stratospheric Volcanic Ash Volcanic ash and dust in the stratosphere reflects sunlight back into space (top). This reduction in the amount of sunlight reaching the earth creates a cooling effect. Krakatoa’s eruption released enough ash into the atmosphere to cause noticeable cooling (bottom). Also note Tambora in 1815, that eruption released even more ash than Krakatoa and combined with ash from the unusually high number of other volcanoes that erupted at the same year, the cooling effect was extreme compared to other events. (??, NH)


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Figure 11.16 Rare But Huge Volcanic Hazard Super volcanoes (sometimes referred to a mega volcanoes) are gigantic volcanoes that are many times more powerful than both Krakatoa and Tambora combined. The second most recent one to erupt was Toba 74,000 years ago in Sumatra and was thought to have triggered an ice age. The mass extinction that followed from the sudden drop in temperature as well as from the ash and other fallout could have wiped out as much as three fourths of all the life on the planet. (SA??)


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Long Valley Long Valley CalderaCaldera

Figure 11.17 Ancient Super Volcanoes There are at least three known super volcanoes in the United States. The directly affected regions for each is shown with Mount Saint Helens shown for comparison. Note the relative amounts of debris. <the crater lake eruption is not considered a super volcano but it definitely is one a scale much different that what humans are familiar with> (SA??)

MAR110 LECTURE #11 Case Study: · PDF filelegends about sightings and/or ... Figure 11.2...

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