Hazards- Volcanoes Flashcards
Shield volcanoes
- gentle slopes and wide base
- frequent eruptions of basaltic lava
- lava flows at high speeds and for long distances before cooling
- usually non violent eruptions
- non viscous lava due to basaltic composition
- found at constructive boundaries
Composite volcanoes
- steep sided with a cone shape
- high with narrow base
- explosive eruptions of lava and ash
- viscous lava builds in layers and does not flow far from the crater
- found at destructive boundaries
- secondary cones formed when the main vent gets blocked
Icelandic volcanoes
- low viscosity and highly effusive
- may have water therefore phreatic
Hawaiian volcanoes
-effusive, minor explosivity, with fire fountain and low viscosity lava flow
Strombolian volcanoes
Eruption with gas bubbles so ejects tephra (ash and lapilli)
Vulcanian volcanoes
- higher gas build up than Strombolian, highly viscous lava
- a series of short lived explosions with tephra including bombs
Pelean volcanoes
-explosive eruptions with a range of tephra with nuee ardent (glowing cloud) pyroclastic flows
Plinian volcanoes
- highly explosive, large eruption columns (up to 45km)
- range of tephra and pyroclastic fallout (including flows)
The Volcano Explosivity Index (VEI)
- scale used to measure intensity of volcanic eruptions
- similar to the Richter scale and Mercalli scale
- VEI measures explosivity, volume of ash (tephra) and the height ash reaches into the atmosphere
- it also gives a description and name of the eruption and the total no. of those eruptions in human history
- allows die comparison between events
Lahars
- mixture of hot/cold water and rock fragments
- form when erupted volcanic material gets saturated during periods of heavy rainfall
- may be some time after an eruption
- can grow as they travel (picking up material)
- most common on strato-volcanoes (steep gradient required)
- capable of setting like concrete
- can destroy anything in their path
Mudflows
- consist of flows of ash, cinder, soil and rock
- changed into clay by acids in volcanic gases
- can be hot or cold depending on origin including:
- eruptions effecting water directly
- rapid snow/ice melt
- eruption induced by heavy rainfall
Tephra
-rocks ejected up into the atmosphere
-largest pieces called blocks
-often fall near to the site
-small particles called lapilli- remain in atmosphere for years
-moving away from volcano particles get smaller
-thickening occurs when ash+water join
Distance tephra travels affected by:
-temperature of air
-wind direction/speed
-height of eruption column
Nuées Andentes
- a huge cloud of ash
- relatively cool (compared to pyroclastic flows)
- linked with Plinian and Vulcanian activity
Lava flow
- effected by the type of lava
- as the thickness of lava increases- viscosity increases
- this makes the lava flow slower
Gases
- consists mainly of water vapour
- remaining gases include CO2, SO2, H, N, Ar
- hydrogen and water combine with other elements to produce toxic compounds eg. Hydrochloric acid
- volcanoes emit gas even when they’re not erupting
- increase of levels of gas indicates an eruption in near future
- thermal imaging camera and gas samples used
Acid rain
- SO2 emissions can result in acid rain and air pollution downwind of the volcano
- acid rain can cause health problems
- SO2 can be ejected more than 10km into the air
- SO2 becomes sulphate aerosols- reflect sunlight and have a cooling effect on Earths climate
- makes soil acidic- can reduce crop yield
Ash fallout
- clouds of ash moved by the wind
- known as ash plumes
- move away from the volcano vent (no longer supported by gases)
- unsupported particles fall out
- ash deposits thicken near the vent
- if there’s heavy rainfall ash can form volcanic mudflows
Pyroclastic flows
- fast moving currents of hot gas and rock
- can reach up to 1000 degrees
- one of the most dangerous-destroy everything in path
- high density mix of hot lava blocks, pumice, ash and volcanic gas
- move at high speed
- consist of lower flow and a turbulent cloud of ash
- formed by collapse of eruption columns/lava domes due to them becoming too steep
Monitoring volcanoes- geological observations
- topography measured to detect ground movements that occur when a volcano is active
- tiltmeter used to measure change in slope angle of a volcano
- installed when the volcano is relatively inactive
- when magma rises the volcano expands-tiltmeter measures this
- horizontal movements using GPS
Monitoring volcanoes- seismic activity
- occurs as volcanoes prepare to erupt
- increase in activity may indicate its likely to erupt
- type of earthquakes that occur are also key
- measured by seismographs
- infra sound also monitored to detect and locate eruptions
- difficult to distinguish between plate movements and seismic activity
Monitoring volcanoes- gas emissions
- increasing amounts of SO2 indicate increasing amounts of magma near the surface
- sometimes levels drop prior to eruptions (harden f magma seals gas passages)
- emissions measured at the volcano (in the crater) by helicopter
Protection/preparation on an individual level
- avoid river areas and low lying land
- close windows and stay indoors and block chimneys
- ash should be swept off roofs
- avoid driving as ash can damage engines
Protection/preparation on a government level
- lava cooled by water which turns into steam allowing the lavas heat to dissipate
- build a barrier to prevent the lava flows from reaching major towns/cities
- artificial trenches built in order to divert flow, then concrete blocks are added to fully divert and stop the flow (MICs only)
