This page last updated on 03-Jul-2012 EENS 3050 Natural DisastersTulane University Prof. Stephen A. NelsonVolcanic Hazards & Prediction of Volcanic EruptionsVolcanic HazardsThis lecture will begin with a video entitled "Understanding Volcanic Hazards". This video was prepared by the International Association of Volcanology and Chemistry of the Earth's Interior in several languages, and was designed show people living in the vicinity of volcanoes the possible effects of an eruption. Although the terminology may vary slightly from what we have learned in this course, the effects are the same and the footage of volcanic eruptions in progress is spectacular. The video is dedicated to the 23,000 people who died as a result of mudflows (lahars) from the 1985 eruption of Nevado del Ruiz volcano in Colombia, and is intended to help prevent similar disasters in the future. The video covers:z Ash Falls (tephra falls)z Hot Ash Flows (pyroclastic flows)z Mudflows (lahars)z Volcanic Landslides (debris flows and debris avalanches)z Lava Flowsz Volcanic GasesPrimary Effects of Volcanismz Lava Flows{ Lava flows are common in Hawaiian and Strombolian type of eruptions, the least explosive.{ Although lava flows have been known to travel as fast as 64 km/hr, most are slower and give people time to move out of the way.{ Thus, in general, lava flows are most damaging to property, as they can destroy anything in their path.{ Control of lava flows has been attempted with limited success by bombing flow fronts to attempt to divert the flow, and by spraying with water to cool the flow. The latter is credited with saving the fishing harbor during a 1973 eruption of Heimaey in Iceland.Volcanic Hazards and Prediction7/3/2012Page 1 of 11z Violent Eruptions and Pyroclastic Activity{ Pyroclastic activity is one of the most dangerous aspects of volcanism.{ Hot pyroclastic flows cause death by suffocation and burning. They can travel so rapidly that few humans can escape.{ Lateral blasts knock down anything in their path, can drive flying debris through trees.{ Ash falls can cause the collapse of roofs and can affect areas far from the eruption. Althoughash falls blanket an area like snow, they are far more destructive because tephra deposits have a density more than twice that of snow and tephra deposits do not melt like snow.{ Ash falls destroy vegetation, including crops, and can kill livestock that eat the ash covered vegetation.{ Ash falls can cause loss of agricultural activity for years after an eruption, a secondary or tertiary effect.z Poisonous Gas Emissions{ Volcanoes emit gases that are often poisonous to living organisms. Among these poisonous gases are: Hydrogen Chloride (HCl), Hydrogen Sulfide (H2S), Hydrogen Fluoride (HF), and Carbon Dioxide (CO2). { The Chlorine, Sulfur. and Fluorine gases can kill organisms by direct ingestion, or by absorption onto plants followed by ingestion by organisms. { In 1984, CO2 gas escaping from the bottom of Lake Monoun, a crater lake in the African country of Cameroon, killed 37 people.{ In 1986 an even larger CO2 gas emission from Lake Nyos in Cameroon killed more than 1700 people and 3000 cattle.Volcanic Hazards and Prediction7/3/2012Page 2 of 11Secondary and Tertiary Effects of Volcanismz Mudflows (Lahars){ Volcanoes can emit voluminous quantities of loose, unconsolidated tephra which become deposited on the landscape. Such loose deposits are subject to rapid removal if they are exposed to a source of water. { The source of water can be derived by melting of snow or ice during the eruption, emptying of crater lakes during an eruption, or rainfall that takes place any time with no eruption. { Thus, mudflows can both accompany an eruption and occur many years after an eruption. { Mudflows are a mixture of water and sediment, they move rapidly down slope along existing stream valleys, although they may easily top banks and flood out into surrounding areas.{ They have properties that vary between thick water and wet concrete, and can remove anything in their paths like bridges, highways, houses, etc.{ During the Mt. St. Helens eruption of May 18, 1980, mudflows were generated as a result of snow melt on the volcano itself, and deposition of tephra in streams surrounding the mountain. { On November 13, 1985 a mudflow generated by a small eruption on Nevado del Ruiz volcano in Columbia flowed down slope and devastated the town of Armero, 50 km east of the volcano and built on prior mudflow deposits. The town had several hours of warning from villages higher up slope, but these warnings were Volcanic Hazards and Prediction7/3/2012Page 3 of 11ignored, and 23,000 people died in the mudflow that engulfed the town. z Debris Avalanches and Debris Flows{ Volcanic mountains tend to become oversteepened as a result of the addition of new material over time as well due to inflation of the mountain as magma intrudes. { Oversteepened slopes may become gravitationally unstable, leading to a sudden slope failure that results in landslides, debris slides or debris avalanches. We will cover these types of hazards in more detail later in the course and in the next lecture.{ During the May 18, 1980 eruption of Mt. St. Helens, Washington, a debris avalanche was triggered by a magnitude 5.0 earthquake. The avalanche removed the upper 500 m of the mountain, and flowed into the Spirit Lake, raising its level about 40 m. It then moved to the west filling the upper reaches of the North Fork of the Toutle River valley (see map above).{ Debris avalanches, landslides, and debris flows do not necessarily occur accompanied by a volcanic eruption. There are documented cases of such occurrences where no new magma has been erupted.z Flooding{ Drainage systems can become blocked by deposition of pyroclastic flows and lava flows. Such blockage may create a temporary dam that could eventually fill with water and fail resulting in floods downstream from the natural dam.{ Volcanoes in cold climates can melt snow and glacial ice, rapidly releasing water into the drainage system and possibly causing floods. Jokaulhlaups occur when heating of a glacier results in rapid outburst of water from the melting glacier.Volcanic Hazards and Prediction7/3/2012Page 4 of 11z Tsunami{ Debris avalanche events, landslides, caldera collapse events, and pyroclastic flows entering a body of water may generate tsunami.{ During the 1883 eruption of Krakatau volcano, in the straits of Sunda between Java and