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Volcanoes - Coggle Diagram
Volcanoes
Nature and Distribution
Oceanic Ridges:
Plates are moving apart and magma is forcing its way to the surface, cooling and forming new crust - continues spreading and crust is carried away (sea-floor spreading) e.g. mid Atlantic ridge
Eruptions are frequent but relatively gentle e.g. Eyjafjallajökull Iceland
Subduction Zones:
e.g. Pacific Ring of Fire
Deeper the oceanic plate descends, hotter the surroundings become - melts oceanic plate into magma in Benioff Zone. Magma rises as plutons, reach surface and form volcano.
Andesitic/viscous lava creates explosive volcanoes
Rift Valleys:
Constructive margins e.g. east Africa
Brittle crust fractures as sections move apart - crust drops between parallel faults - create rift valley.
Crust is thin, magma comes through and forms volcano e.g. Mt Nyiragongo
Hot Spots:
Radioactive elements below crust forms hot spot - plume of magma rises consumes plate above - volcano forms above spot (shield volcano) e.g. Hawaiian Island.
Spot is stationary, when plate moves, forms a chain of volcanoes.
Magnitude and Frequency
main method of measuring has been the volcanic explosivity index (VEI), a logarithmic scale running from 0 to 8.
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Impacts
Primary
Tephra: Solid material of varying grain size ranging from volcanic bombs to ash, all ejected into the atmosphere
Pyroclastic Flow: very hot (over 800 degrees(, gas charged, high-velocity flows made up of a mixture of tephra and gas. Flow around 700km per hour - Pompeii was destroyed by it.
Volcanic Gas: include CO2, CO, hydrogen sulphide, sulphur dioxide and chlorine. In 1986, CO2 from lake Nyos killed 1700 people.
Secondary
Lahars: Melted snow and ice combined with ash forms mud flows. In 1985, lahar destroyed Colombian town of Armero after Nevado del Ruiz erupted - only a quarter of the 28,700 population survived.
Flooding: glaciers and ice caps can melt from the eruption - e.g. Iceland 1996 when Grimsvotn erupted.
Tsunamis: Large sea waves reaching 10 foot tall e.g. Krakatoa 1883 killed 36,000 people
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Climatic Change: huge amounts of volcanic debris can reduce global temperatures and is an agent in climate change.
Management
Prediction: Very difficult to predict - could not predict Nevado del Ruiz - resulted in the death of 20,000 people.
Can only look at history to predict eruption - Yellowstone due now. Monitor land swelling, groundwater levels and gas emissions.
Protection: Monitoring a volcano can guide a rough guideline to when it will erupt and allow evacuation times. e.g. New Zealand have made risk assessments and alert levels to warn public.