1 MAR110 LECTURE #6 Volcanoes I Ocean Basin Volcanism Mid-ocean ridge volcanism produces ocean crust and occasional volcanic islands which when underwater are called seamounts. Isolated hotspots produce mid-ocean island chains. In a subduction zone, volcanoes are formed by the rising of melted lithosphere magma. (ItO) Mid Ocean Ridge Magma As new crust is formed along the ridge axis the older crust is pushed to either side forming bands of rock parallel to the ridge. The upward moving magma in the rift zone forces its way through the oceanic crust, forming underwater volcanoes where the magma breaks through as pillow lava. (?) 1
2 Underwater Volcanoes - Seamounts Underwater volcanoes can be formed in the region of mid-ocean ridges when mantle magma chambers remain connected. Once the volcano is carried away and disconnected it becomes extinct" and is called a seamount. (NG) Subduction Zone Volcanism A map of the major tectonic plates showing the types of boundaries and where they may be found. Red lines indicate areas with significant subduction zone volcanic activity near or on land. (LEiO) 2
3 Juan de Fuca Plate Subduction The plate subduction in the northeast Pacific (offshore of Washington and Oregon) generates a chain of volcanic mountains known as the Cascades. ashore, (NG) 3
4 Subduction Zone Volcanism Northeast Pacific Deep magma chambers - formed by the friction between the subducting Juan de Fuca plate and the North American plate and feed the active volcanoes and spawn earthquakes in the Cascade Range. An active erupting volcano produces volcanic bombs (small to large pieces of solidified magma) and at times huge amounts of smoke (fine particles) and somewhat larger ash which tends to settle back to earth in the region. Besides slower lava flows, this kind of volcanic action can also produce superheated, high-speed pyroclastic flows that in turn can trigger landslides and mudflows. On occasion when the magma conduits are blocked major explosions can occur like mt St Helens in the 80s.(NG) Subduction Zone Volcanism - Mt St Helens 4
5 Mt St. Helens Explosion Impact Mt Vesuvius (1663) in coastal Italy. (ItO?) 5
6 An active Mt Vesuvius Buries Pompeii -79 AD. (ItO?) Mummified Remains of Pompeiian Victims. (NG) 6
7 A Slumbering Mt Vesuvius hovers high over Pompeii Ruins. (ItO?) Krakatoa 1883. (NH) 7
8 Explosive Volcano Energy 8
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) 9
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?) 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) 10
11 Krakatoa Tsunami Travel Times 11
12 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) Tsunami Measurement 12
13 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 1450 BC that covered the entire island of Crete in over a meter of water and destroyed the Minoan civilization. (NH) Krakatoa Lives On 13