What is volcano? • A volcano is a vent or 'chimney' that connects molten rock (magma) from within the Earth’s crust to the Earth's surface. • The volcano includes the surrounding cone of erupted material. vent cone conduit
why do volcanoes erupt? • Hot, molten rock (magma) is buoyant (has a lower density than the surrounding rocks) and will rise up through the crust to erupt on the surface. • 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.
Types of Eruptions • Explosive eruptions ejectlarge amounts of gas and a high viscosity (sticky) magma (Andesitic or Rhyolitic magma).
Explosive Eruptions • 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 Above: Large eruption column and ash cloud from an explosive eruption at Mt Redoubt, Alaska
Explosive Eruptions • Three products from an explosive eruption • Ash fall • Pyroclastic flow • Pyroclastic surge Pyroclastic flows on Montserrat, buried the capital city.
Explosive Eruptions Direct measurements of pyroclastic flows are extremely dangerous!!!
Types of Eruptions • Effusive eruptions releasesmall amounts of gas and (or) low viscosity (runny) magma (Basaltic magma).
Effusive Eruptions • These form at hot spots or spreading centers creating either a shield volcano or basalt flood plane.
Volcanic Dangers • Pyroclastic flow • Pyroclastic surge • Pyroclastic fall (Tephra) • Lava flow • Earthquakes • Noxious Gases
1. Pyroclastic Flows • They are hot, turbulent, fast-moving, high particle concentration clouds of rock, ash and gas. • They can reach > 100 km from a volcano. • They can travel 100’s km/h and are commonly >400°C. • They will destroy everything in their path including buildings, agriculture and forests. Although because they contain a high concentration of particles and a low concentration of gas, • they are dense and usually are confined to, and flow along, topographic lows (valleys). • It is extremely important to understand them as they are often the most hazardous component of an explosive eruption.
2. Pyroclastic surge • These are low particle/high gas concentration (low density) flows of volcanic material. • Pyroclastic surges can travel up to 360 km/h and be greater than 600oC • They can go overtop of high topographic features, and therefore are not confined to valleys. • Pyroclastic surges can travel up to at least 10 kilometers from the source.
3. PYROCLATIC fall (Tephra) • rock fragments thrown into the air by an explosive eruption. • They can range in size from dust to the size of cars (lava bomb) • They can collapses roofs, brings down power lines, kill plants, contaminates water supplies, and be a respiratory hazard for humans and animals.
4. Lava flow • It is not just explosive volcanic activity that can be hazardous. Effusive (lava) activity is also dangerous.
4. Lava flows • Lava flows have temperatures in excess of 200oC. Therefore will burn any flammable material it contacts with.
4. Lava Flows • Thick lava flows will bury all in it’s path including infrastructure (buildings, roads, waterways etc.) and agricultural land…
5. Earthquakes • Earthquake activity commonly precedes an eruption • Earthquakes are a result of magma pushing up towards the surface. The increase volume of material in the volcano shatters the rock.
6. Noxious Gases • Volcanoes produce large quantities of gases, mostly water (H2O), but also significant amounts of carbon dioxide (CO2), hydrogen sulphide (H2S), sulphur dioxide (SO2), and hydrofluoric acid (HF) as well as some chlorine (Cl) and nitrogen (N) compounds. • Each of these gases has caused loss of life and damage. • Sulphur dioxide vented from the Laki volcano in Iceland in 1783 damaged crops and killed livestock and people. • When SO2 reacts with water in the atmosphere, it forms sulphuric acid and acid rain. Acid rain formed from volcanic clouds can damage crops. • When CO2 gas, which is heavier than air, is released by a volcano, it collects in low areas; concentrations can become high enough to be deadly.
6. Noxious gases • Lake Nyos (Cameroon) is a crater lake inside a dormant volcano. • The lake had become laden with carbon dioxide gas. • This gas had suddenly bubbled out of the lake and asphyxiated nearly every living being in thesurrounding valley.
Types of Volcanoes • Shield Volcano • A large, circular volcano with gentle sloping sides. • Created from layers of non-explosive, basaltic lava flows. • Has a large caldera in the middle.
Types of Volcanoes • Cinder Cone Volcanoes • Form from the fallout of the ejected material of an explosive eruption. • Have steep sides and are generally less than 500 m high.
Types of volcanoes • Mt Edziza (B.C.)
Paricutinvolcano began to erupt in a corn field in Mexico in 1943 and continued until 1952. The farmer had noticed a fissure (vent) had opened in the field one morning and from it was pouring black ash. In the first year the volcano grew to 336 m (almost 1 metre per day). Rate of growth decreased steadily; by 1952 the volcano was 424 m in height.
Types of Volcanoes • Cinder Cone volcanoes sometimes appear on old Shield volcanoes
Types of Volcanoes • Composite or Stratovolcanoes • Volcanoes that alternate between periods of lava flows (constructive phase) and periods of explosive eruptions (destructive phase). • Commonly called “composite volcanoes” because they are made up of both lava and pyroclastic deposits. • Can be much larger than a Cinder Cone volcano
Mt. St. Helens Before Mt. St. Helens After
Extensive ash falls and ash flows are commonly produced during explosive phases.
http://www.youtube.com/watch?v=xExdEXOaA9A&feature=channel • http://www.youtube.com/watch?v=NBPwwt0HuVo&feature=related • http://www.youtube.com/watch?v=Bz7WCttwXQk&feature=related • http://www.cbc.ca/documentaries/passionateeyeshowcase/2010/volcano/