Volcanic processes and hazards Flashcards
why are volcanoes both destructive and constructive?
the can destroy human infrastructure but also add material to the Earth’s surface
3 things effected by large volcanic eruptions
- atmosphere composition (addition of gases)
- bioshpere (kills plants, animals, humans)
- hydrosphere (tsunamis)
define VOLCANO
landform built by volcanic activity
define VOLCANISM
process in which magma rises through the Earth’s crust and erupts at the surface as lava flows and/or pyroclastic materials and gases
what is a VOLCANIC EDIFICE
part of the volcano above the ground
seamount
a volcano under the sea which doesn’t reach the surface
______ volcanoes form intra-plate, subduction volcanoes form at _______ margins, and rift volcanoes form at _______ margins.
hot-spot volcanoes form intra-plate, subduction volcanoes form at convergent margins, and rift volcanoes form at divergent margins.
Active, dormant and inactive/extinct volcanoes?
active - erupted in historic time.
dormant - haven’t erupted in historic time, but they could.
Inactive - not expected to erupt again
Flood basalts form due to gigantic volcanic eruptions of ______ viscosity lava. They erupt due to __________ _______. A single eruption can cover 200,000km^2 and form large ________ ________. They have been cited as the cause of mass-extinction events.
Flood basalts form due to gigantic volcanic eruptions of very_low viscosity lava. They erupt due to continental rifting. A single eruption can cover 200,000km^2 and form large igneous provinces. They have been cited as the cause of mass-extinction events.
shield volcanoes
- form from low viscosity lava
- gentle slopes
- build up layers over time
- eruption is effusive (not explosive) and can be continuous.
- e.g. rangitoto
cinder cones
- steep, scoria cone
- formed from effusive eruptions of low viscosity lava
- gases can vent long after the end of the eruption
- e.g. mt eden, one tree hill
what is scoria and why is it red?
scoria forms from low viscosity, basaltic lava cooling rapidly as it flies through the air. It is red because it gets oxidised by steam.
composite/ stratovolcanoes
- tall, conical shaped volcanoes
- explosive eruptions
- form from higher viscosity (than mafic) felsic magma
- no extensive lava flows
- e.g. Mt Taranaki
rock types associated with felsic magma
- rhyolite
- dacite
- andesite
Volcanic domes
- formed from lava too viscous to flow (high silica content)
- lava accumulates around the vent
- basaltic or rhyolitic
- large pieces of rock (BRECCIA) break off as the dome grows
- may erupt explosively, as gases get trapped and pressure builds up
what is the name given to a collapsed volcano? why do they collapse?
caldera. Volcanoes collapse when the magma chamber is emptied and the weight of the edifice is no longer supported.
2 mechanisms by which a caldera can forms
- sudden emptying of the magma chamber in an explosive eruption, causing rapid collapse of the edifice through formation of a ring fault.
- Gradual emptying of the magma chamber associated with shield volcanoes (low viscosity), causing subsidence of the edifice
4 types of volcanic eruption
- Hawaiian - mostly low viscosity lava, not much gas
- Plinian - large volume of gas, pumice and ash
- Strombolian - mixed gas/ pyroclastic material and lava
- hydrovolcanic - highly explosive, hot magma comes into contact with water in the ground and vaporises it.
what is used to measure the size of a volcanic eruption?
Volcanic explosivity index.
Similar to the Richter scale.
Based on ammount of material, duration, and how high ejection is
rhyolite, dacite, andesite and trachyte are all ____ viscosity lavas.
high
igneous rock found with bubbles (vesicles), would’ve formed from …. why?
formed from high viscosity lava, because since the lava doesn’t really flow, the gases get trapped. (the opposite is the case with low viscosity lava)
basalt and andesite are _____ viscosity lavas
low
pyroclastic materials
- form from explosive volcanism
- classified as airfall deposits
- range from ash to volcanic bombs (breccia)
deposits from a pyroclastic flow are called?
Ignimbrite
Ignimbrite composites comprise?
- volcanic ash
- pumice
- lithics
- they can be unconsilidated or welded by heat.
do volcanic gases (SO2, HCl, HF, CO2) cause global warming?
no they form global cooling by reflecting sunlight back into space.
lahar
fast flowing slurry of water, ash and rock. Occur if the eruption melts a lot of snow or ice or if there is heavy rain after an eruption.
whats the most destructive volcanic hazard?
pyroclastic flow - mixture of hot (500 degrees c) gases and ash which travel at several hundred kph, destroying everything in their path
5 main hazards of a volcano
- ash falls
- airborne ash
- lava flows
- pyroclastic flows
- lahar
dangers of ash
- abrasive (especially to aircraft)
- corrosive
- electrically conductive
- suspend in the atmosphere for weeks
- ash loading on buildings – increase the dead-load on the roof (even worse when wet) which may lead to collapse
- impact water supplies (change composition, shortages during clean-up)
does ash affect water quality?
yes, 55 soluble compounds (mineral acids). can change the pH of water and increase turbidity (addition of flocculants)
Mitigation of the volcanic hazards at Mt Ruapehu.
Eastern Ruapehu Lahar Alarm and Early Warning System (ERLAWS)
- installed after 1995 and successfully predicted 2007 lahar
- array of geophones detect vibrations
- buried tripwires detect dam collapse
- water level sensors in crater lake detect sudden emptying
- automatic barriers on SH1 if lahar detected
other lahar mitigation techniques
- bund preventing flow
- overflow channel
- sediment retention basin
- channelisation of flow
main things monitored on a volcano
- gas monitoring
- seismic activity (tiltmeters, extensometers and gps)
- deformation monitoring (large scale bulging)
what is the most widespread hazard caused by a volcano?
ash fall
