Volcanoes Flashcards
Shield volcanoes
Gentle slopes with wide base, frequent eruptions of basaltic lava, flows at high speeds for long distances, usually non-violent eruptions, non-viscous lava due to basaltic composition, found at constructive boundaries, examples are Kilauea and Ejjafjallajökull
Composite volcanoes
Steep sided with distinctive cone shape, explosive eruptions of lava and ash, layers of alternating ash and lava, viscous lava builds in layers and doesn’t flow far from crater, found at destructive boundaries, secondary or parasitic cones when the main vent gets blocked
Icelandic
Low viscosity, highly effusive (molten then solidifies), may have water therefore phreatic (caused by heating & expansion of groundwater)
Hawaiian
Low viscosity, effusive, minor explosiveness with fire fountain
Strombolian
Eruption with gas bubbles so eject tephra (ash and rock)
Vulcanian
Highly viscous, series of short explosions with tephra, higher gas build up than Strombolian
Pelean
Explosive eruptions with range of tephra and nuee ardent (glowing cloud), pyroclastic flows
Plinian
Highly explosive, range of tephra fall including pyroclastic flows, large eruption column (up to 45 km)
Lava flow- usually 1-10m thick can be up to 100m, over 1100 degrees, hardens into igneous rock, rarely threaten human life due to slowness but more so for property, can construct barriers or diversion paths
Mount Nyiragongo in 2002 reached speeds up to 100km/h, killed around 245 people, destroyed everything in its path
Tephra- rock fragments (ash, lapilli or bombs/blocks) erupted out, larger fragments propelled less (usually 1 mile) while smaller fragments usually 50km, can cause respiratory problems, conduct lightning, problems for communication, acid rain, low visibility, reduce risk with mask and storing food, can cause soil to be very fertile
Tombora eruption in 1815 produces ash causing acid rain leading to famine and disease in nearby islands killing 80000
Nuee ardente- fast moving cloud of hot gas and ash, can travel 50km from source, form during explosive eruptions as column collapses or non-explosive eruptions when volcanic rock collapses, dangerous due to speed, temperature and being near to ground, incinerate nearly all in its path, can travel over 100mph, evacuate to higher places away from valley
Mount Panatubo had deposits up to 220m thick in some valleys
Lahar- a destructive mudflow on the slope of a volcano, tiny particles can cause respiratory and other health problems, usually flows in valleys, ash usually gets blown miles away from the source, prevented by evacuated valley and nearby area, sloped roofs reduce weight exposure
Mount St. Helens 1980 sruoted ash and gas more than 60000 feet in air, nearby houses and roads up to 4 inches in ash, over a day 500 million tonnes of ash fell on Washington and Montana, stranded over 10000 people, over $1 billion in property damage, disrupted flights
Monitoring- geological observations
- tiltmeter used to measure changes in slope of volcanoes as it expands when more magma rises into the magma chamber
- electronic distance measurements (edm) measures horizontal movement when a volcano expands before an eruption
- gps used today, retrievers place all over the volcano, distance from satellite can be detected and movement recorded
Monitoring- seismic activity
Seismic activity always occurs as volcanoes are about to erupt, similar to earthquakes seismographs used to detect infra-sound, as many volcanoes are on plate boundaries it has hard to decide whether seismic activity is linked to eruption or general plate movement, also all volcanoes react differently to seismic activity
Monitoring- gas emissions
As magma nears surface and pressure decreases gases escape, the sulphur dioxide levels rising show the arrival of increasing amounts of magma near the surface, sometime emissions drop to low levels prior to eruption due to sealing of gas passages by hardened magma, this leads to increased pressure in volcano’s plumbing system and more likely chance of eruption
