Volcanoes Key Question: How are volcanoes hazardous to human populations and what controls risk? Key Words Risk and Hazard Primary and Secondary Effects

What is the difference between a hazard and risk? Anything that poses a threat to humans or property Hazard = Risk = Probability or likelihood of being affected by the hazard

Why do volcanoes erupt in different styles Red (Shield) One control on risk of eruptions in volcano type. Task: Decide what type of plate boundary each forms at and think of some of the hazards associated with each Grey (Cone)

Why do volcanoes erupt in different styles Grey (Cone) Red (Shield) Volcano Type Hazards Task: Decide what type of plate boundary each forms at and think of some of the hazards associated with each

Grey (Cone) Hazards: Ash / tephra (roof collapse and breathing difficulties) Lava flows (High Viscosity Andesite and Rhyolite ) Pyroclastic Flows Famine Tsunami Volcanic Gases Lateral Blasts Climate Change Mudflows (lahar)

For these highlighted explain how they occur Grey (Cone) Hazards: Ash / tephra (roof collapse and breathing difficulties) Lava flows (High Viscosity Andesite and Rhyolite ) Pyroclastic Flows Famine For these highlighted explain how they occur Volcanic Gases Lateral Blasts Climate Change Mudflows (lahar)

Red (Shield) Hazards: Lava (low viscosity basalt –effusive) Ash Gas Climate change

Why do volcanoes erupt in different styles Grey (Cone) Red (Shield) Volcano Type Mudflows (lahar) Lava (low viscosity basalt –effusive) Climate Change Hazards Lateral Blasts Ash Task: Now decide which are primary and secondary Volcanic Gases Pyroclastic Flows Gas Famine Lava flows Climate change Ash / tephra tsunami

Primary Hazards

How can their explosivity be measured? VEI 0 The Volcanic Explosivity Index (VEI) is used for classifying explosive eruptions. VEI 1 & 2 open-ended VEI 5 10 times larger than the previous value (logarithmic) VEI 6 VEI 7 VEI 8

Measuring Volcanic Eruptions Non-explosive eruptions. gentle effusion of low-viscosity basaltic magma e.g. Hawaiian and Icelandic volcanoes. EFFUSIVE ERUPTION. VEI 0 Small explosive eruptions that eject enough debris (100,000m³) to cover London with a thin dusting of ash! VEI 1 & 2 More violent explosive eruption of the more viscous andesitic magmas such as the 1980 Mount St Helens eruption in the USA VEI 5 Mt. Pinatubo in the Philippines in 1991. This was the largest eruption in the 20th Century and blasted out sufficient material to bury the City of London to a depth twice that of the height of the former World Trade Centre towers in New York! VEI 6 1815 eruption of Tambora in Indonesia, the largest eruption since the end of the last Ice Age - 10,000 years ago. The Tambora eruption blasted out over 100km³ of debris - enough to bury the City of London to a depth of 1km! VEI 7 Toba, in Sumatra in Indonesia over 70,000 years ago. This blast ejected sufficient gas and debris into the atmosphere to bury the whole of Greater London to a depth of 1km, as well cause a rapid and dramatic change to the global climate. SUPERVOLCANIC ERUPTION. VEI 8

Explosivity of Volcanic Eruptions Red volcano Explosiveness Effusive Grey volcano Low gas pressure Gas pressure Viscosity Fluid Silica content in magma 52-45% SiO2 Magma type Basaltic Plate tectonic setting Constructive Plate Margin

Volcanic risk? What controls your overall risk? Eruption Style Level of Development Volcanic risk? Population Density Technology Finally Is living in Indonesia or Iceland is more dangerous and why (for volcanoes). Task: Explain how each changes the risk