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What is a volcano, and why do they occur??

What is a volcano, and why do they occur??

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  • This photo was taken in 2008, in Chile, when the Chaitén volcano erupted for the first time in 9000 years. The eruption also caused a spectacular lightning storm. This can happen when the hot gas and ash rises through the cooler atmosphere, causing a transfer of charge. The excess of electrons within the cloud makes it act like a capacitor, and if all the conditions are correct, huge electrical discharges may be observed as bolts of lightning during volcanic eruptions.


  • 1. What is a volcano? vent cone A volcano is a vent or 'chimney' that connects molten rock (magma) from within the Earth’s crust to the Earth's surface. conduit The volcano includes the surrounding cone of erupted material. magma chamber
  • 2. How and why do volcanoes erupt? Magma has a lower density than the surrounding rocks and will rise up through the crust to erupt on the surface. (Same principle as hot air rising, e.g. how a hot air balloon works) When magma reaches the surface it depends on how easily it flows (viscosity) and the amount of gas (H2O, CO2, S) it has in it as to how it erupts. A Lava Spigot
  • 3. How and why do volcanoes erupt? Sakurajima Volcano, Kyushu, Japan Large amounts of gas and a high viscosity (sticky) magma will form an explosive eruption! • Think about shaking a bottle of Coke and then releasing the cap. Small amounts of gas and (or) low viscosity (runny) magma will form an effusive eruption. Where the magma just trickles out of the volcano (lava flow). Ol Doinyo Legai Volcano, Tanzania
  • 4. Explosive Eruptions Below: Large eruption column and ash cloud from an explosive eruption at Mt Redoubt, Alaska • Explosive volcanic eruptions can be catastrophic • Erupt 10’s-1000’s km3 of magma • Send ash clouds >25 km into the stratosphere • Have severe environmental and climatic effects • Hazardous!!! Mt. Redoubt
  • 5. Explosive Eruptions Pyroclastic flows on Montserrat, buried the capital city. • Three products from an explosive eruption • Ash fall • Pyroclastic flow • Pyroclastic surge Pyroclast = fast moving hot gas and rock.
  • 6. Film crew attempting to outrun a pyroclastic flow in Indonesia. It was travelling at 100km/h and they made it…just.
  • 7. This is a pyroclastic cloud causing a lightening storm. The hot air and ash rise up through the colder air causing a charge. The cloud acts like a big capacitor and when enough energy is built up, we get lightning 2008, in Chile, when the Chaitén volcano
  • 8. Effusive Eruptions In effusive eruptions, lava pours out onto the ground. The lava is too viscous (sticky) for an explosive eruption. Hawaii
  • 9. Effusive Eruptions • Not generally as hazardous as explosive eruptions. • Lava flows can burn and bury buildings and forests. • Is a danger to people living on or near an active volcano. Hawaii