27 August 2008 mar110_L11_Mega-Volcanoes_Krakatoa_27aug08.doc 1MAR110 LECTURE #11 Mega-Volcanoes: Krakatoa Total Energy Total Energy --Volcanic EventVolcanic Event…energy released by …energy released by volcanic earthquakesvolcanic earthquakes….energy necessary to ….energy necessary to fracture the overburdenfracture the overburden….energy for ….energy for ejecting materialejecting material….energy to ….energy to produce atmospheric shock wavesproduce atmospheric shock waves….energy to ….energy to producing tsunamiproducing tsunami.. Kinetic, Potential, and Thermal EnergyKinetic, Potential, and Thermal Energyas large nuclear devices as large nuclear devices --typically 1 x 10typically 1 x 101515joules.joules.KrakatoaKrakatoa--1x101x101818joulesjoules27 August 2008 mar110_L11_Mega-Volcanoes_Krakatoa_27aug08.doc 2 Figure 11.3 ''BandaianBandaian' Eruptions' Eruptions…...most destructive! …...most destructive! ….. lateral basal surge ….. lateral basal surge --150 km/hr, 150 km/hr, ….. Precedes pyroclastic flows ….. Precedes pyroclastic flows …..likened to ”cyclones of ash”. …..likened to ”cyclones of ash”. ….. Mt St Helens 1980….. Mt St Helens 1980Figure 11.2 ''StrombolianStrombolian' Volcanoes' Volcanoes....after Mt after Mt StromboliStromboliin Italyin Italy..toss out bombs upward 100s of meters..toss out bombs upward 100s of meters..rhythmically for years..rhythmically for years..very symmetrical cinder cones..very symmetrical cinder cones27 August 2008 mar110_L11_Mega-Volcanoes_Krakatoa_27aug08.doc 3 Figure 11.4 ''PlinianPlinian' Events' Events..truly cataclysmic explosive events..truly cataclysmic explosive events..km cubes of magma straight up..km cubes of magma straight up..at 600..at 600--700 700 m/sm/s, to heights of 25 km, to heights of 25 km..volcano collapses to form a caldera..volcano collapses to form a caldera..Mt Vesuvius eruption of 79 AD..Mt Vesuvius eruption of 79 AD Figure 11.5 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)27 August 2008 mar110_L11_Mega-Volcanoes_Krakatoa_27aug08.doc 4 Figure 11.7 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 11.6 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)27 August 2008 mar110_L11_Mega-Volcanoes_Krakatoa_27aug08.doc 5 Figure 11.8 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. (??) Figure 11.9 Krakatoa Explodes - 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)27 August 2008 mar110_L11_Mega-Volcanoes_Krakatoa_27aug08.doc 6 Figure 11.11 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) Figure 11.10 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?)27 August 2008 mar110_L11_Mega-Volcanoes_Krakatoa_27aug08.doc 7 Figure 11.12 Tsunami Measurement Figure 11.13 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)27 August 2008 mar110_L11_Mega-Volcanoes_Krakatoa_27aug08.doc 8 AnakAnakKrakatoa Future?Krakatoa Future?BeforeBefore18831883AfterAfter1883188319791979 Figure 11.15 “Krakatoa Lives On” Figure 11.14 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