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Styles of Eruptions and Volcanic Hazards Styles of volcanic eruptions Some volcanoes may erupt only once - monogenetic (Diamond Head) Other volcanoes erupt many times polygenetic (Kilauea) Some erupt very gently (Kilauea) Others are very violent and explosive (Mt. St. Helens, Mt. Pinatubo) How come? Explosive volcanoes found associated with subduction zones Explosiveness is a function of magma viscosity (resistance to flow) Magmas that generate the explosive volcanoes are much more viscous than the non-explosive magmas More silica, cooler magma, more gas Hawaiian magmas Not very explosive Not very viscous Low silica, hot magma, moderate gas content Eruptive styles are Classified by level of eruption explosiveness I. Flood eruptions Least explosive eruptions Very fluid basalt erupted in very large quantities VERY FAST Lavas erupted over large areas Thought to develop as hot spots burn thru crust Columbia Plateau More than 420,000 cubic km of lava Flood basalts II. Hawaiian-type eruptions Hawaiian-type eruptions The next least explosive kind of eruption Fluid magmas with small amounts of gas Eruptions relatively gentle Periodically have a violent eruption but very rare - Drive in volcanoes Hawaiian-type eruptions Build shield volcanoes Fragmentation (grain size) Surtseyan phreatoplinian Plinian Peléan Vulcanian Strombolian Intensity (dispersal) Modified from Julia Sable Styles of explosive volcanism III. Strombolian eruptions Named for volcanic island off coast of Italy Characterized by less fluid lavas Moderate explosive activity Tephra common Lots of ash blankets country side Basalt and andesite common Large steep-sided composite volcanoes built by repeated Strombolian eruptions More explosive eruptions Build composite cones Mt. Shasta, California ‘A’a flows IV. Vulcanian eruptions Characterized by more viscous lavas “Stubbly” flows are common Lots of ash Andesite most common Arenal Volcano, Costa Rica V. Peléan eruptions Named for 1902 eruptions of Mount Pelée on Martinique in the Caribbean Mt. Pelé eruption Almost 30,000 people were killed instantly Peléan eruptions Lavas highly viscous, very explosive Explosive eruption of highly gas-charged lava leads to nuée ardente (glowing avalanche) or pyroclastic flow which move at 60+ kph and are 300deg C inside Don’t outrun these guys!!! Pyroclastic flows formed during 1968 eruption of Mt. Mayon Ash cloud is 30,000 ft high Flows caused by ash cloud collapse or lava dome collapse VI. Plinian Eruption most powerful eruption Named for Roman naturalist killed in explosion of Mt. Vesuvius in 79 A.D Very destructive Accompanied by major collapse Plinian Eruption Mt. St. Helens is good example before during Plinian = most powerful eruption Crater Lake in Oregon formed by similar explosive eruption Ash from this volcano (Mt. Mazama) found all over the Pacific Northwest & as far east as the Mississippi River! Plinian = most powerful eruption Greatest Plinian eruptions of modern times: 1813 explosion of Tambora volcano (eastern Indonesia) 1883 explosion of Krakatau (Sunda Strait between Java and Sumatra) Explosion km away heard in northern Australian, 2400 Other Recent Eruptions Pinatubo = Vulcanian-type eruption Mt. Unzen (Japan,1991) -- generated a number of lava domes that collapsed, sending pyroclastic flows down the side of the volcano, so it can be classified as a Peléan eruption Volcanoes & Plate Tectonics OK. So, where do you find all these volcanic eruptive styles? Flood eruptions - atop hot spots Hawaiian - at hot spots & along MOR All the rest are associated with subduction! Fragmentation (grain size) Surtseyan phreatoplinian Plinian Peléan Vulcanian Strombolian Intensity (dispersal) Styles of explosive volcanism What are the hazards? Volcanic Hazards 1. Lava Flows Volcanic Hazards 2. Falling Tephra Like one sees at the beginning of the movie Dante’s Peak. Falling tephra in Yakima, Washington during Mt. St. Helens eruption More Volcanic Hazards Pyroclastic flows Mudflows - Lahars Toxic volcanic gases Hazards in Hawaii Lava flows Note: The next few slides are courtesy of G&G graduate student Chris Gregg Lava Flow Hazard Zone Map of Hawaii Historic Eruptions Mauna Loa: 33 flows since 1843 5 reached ocean in Kona Hualalai: 3 flows since ~1800 2 reached ocean in Kona High effusion rates: 3-12 x 106 m3 day -1 Steep slopes > 6 %: Hualalai: >50 % Mauna Loa: 35 % Kilauea: <5 % Fast transit times: < 24 hours 1877 Lava Flows From Hualalai and Mauna Loa That Have Affected Kailua-Kona Resorts Airport Subdivision Hualalai’s last eruptions (c. 1800-1801) MAUNA LOA’s Radial Vents (33) and Rift Zones Historic radial vent eruptions: 1843, 1859, 1877, 1935 Typical Mauna Loa Eruptions: Summit followed by fissure eruptions What Controls the Flow Paths of Lava Flows? Topography What Controls the Speed of Lava Flows? Slope angle, surface roughness, eruption rate, lava type (viscosity) 1950 Eruption: Flow velocities 16-48 km/hr Note that all of these hazards are associated with the big island! How come? Worst volcanic hazards often occur after major eruptions Ash covers sides of volcano Ash becomes unstable during heavy rain Ash mud flow - LAHARS 10 years+ after Pinatubo eruption, mudflows were still killing people LAHAR - ASH MUD FLOW Volcanic gases Very dangerous CO2, CO, SO2, H2S, HCl and HF Hawaiian volcanoes Do not usually emit enough gas to harm people Except for Vog which forms as volcano erupts and as lava enters the ocean (Even reaches O`ahu with Kona wind) Attempts to control flows In the past, people have tried to control, direct lava flows by: Diverting the flow with barriers built with bulldozers Diverting by bombing one or all of the following locations - the vent, edge of a flow, tube entrance Water the flow front - not very successful! Avoiding volcanic hazards Predict them Volcanologists getting good at predicting eruptions of dangerous volcanoes Mt. St. Helens, Unzen and Pinatubo But, missed 1993 Mayon eruption Predictions eruptions Mostly made based on monitoring: Seismic activity increases Ground tilting Increased heat flow Increase in %sulfur in volcanic gas Because magma is moving into shallow levels under volcano! Cyclic nature of Kilauea eruptions Wai`anae Volcano Ko`olau Volcano Thurston Lava Tube Meteorites have struck the Earth in the past. Many meteorites are made of iron and nickel. We think this represents material of planets similar to Earth. Earth’s interior (core) is probably also composed of iron and nickel. The end!