9.1 Volcanoes Flashcards
what are volcanoes and there resultant hazards?
- volcanoes form where there is an opening in the earths crust (vent) and magma deep below crust may come out in:
- molten lava
- explode as volcanic bombs
- appear as ash
two types of lava
-
Basaltic:
- more effusive
- quieter eruptions and most lava flows, with high temperature of the lava - 1000 degrees
- associated with spreading centres and hot spots -
Acidic:
- more explosive with ash clouds, pyroclastic flows, volcanic bombs etc
- lava is cooler: 700-800 degrees and mostly associated with subduction zones
types of eruption
-
Plinian:
- magma is rhyolitic and high viscosity prevents escape of volcanic gases = highly explosive eruptions -
Vulcanian:
- short lived, only few hours and found where highly-viscous lava, build up of pressure within volcano - relatively intense eruption -
Hawaiian:
- fluid basaltic lava thrown into air in jets from vent or line of vents (fissure) at summit/flank of volcano
- jet can last for hours/days - known as fire fountaining
- these flows very fluid = travel for miles before they harden
pyroclastic flows
- mixture of pulverised rock, ash and hot gases that can move at speeds of 100 km/h
- gravity driven - flow down side of volcano
- gases within flow might occasionally ignite to form nuee ardente
- up to 400 degrees - hot
lahars (mudflows)
- made up of volcanic debris
- form when water mixes with ash on slopes of volcano
- can be detected in advance by acoustic (sound) monitors and can also sometimes be channelled away from buildings and people by concrete barriers - but impossible to stop completely
Example:
- Nevado del Ruiz eruption produced devastating Lahar - killed almost 25,000 in town of Armero
Volcanic landslides
- commonly originate as massive rockslides/avalanches that disintegrate during movement
Example: Mt St Helens landslide reached speeds of 180-288 km/hr and surged over 400 metre tall ridge 5km away from volcano
factors triggering it:
- intrusion of magma into a volcano
- explosive eruption
- large earthquake directly beneath volcano/nearby
- intense rainfall that saturates volcano or adjacent ash covered hillslopes with water, especially before/during large earthquake
what is volcanic ash and its impacts?
- powder size to sand size particles of igneous rock material blow into air by eruption and composed of irregularly-shaped particles, with sharp. jagged edges and can be abrasive
- insoluble so form mud when wet = lahars
Impacts:
1. People: respiratory problems
2. Livestock: same eye/respiratory problems as humans and may be unable to eat if ash covers grass
3. Buildings: collapse/damaged under weight of ash, when mixed with water, ash corrosive to metal roofing, AC clogged
4. Vehicles: damage engine parts/filters, frosted surface on glass - cant see through
5. Aircraft: ash pulled into het engine melt engine and stick to the inside restricting airflow and cans top engine
6. Water supply systems: have to filter out ash
life span of a volcano
- can be either active, dormant or extinct
- active: erupted recently/likely to erupt again
- dormant: erupted in last 2000 years and may again: dangerous as difficult to predict
- extinct: long since finished erupting, e.g UK’s
how can volcanoes be predicted?
- satellite imagery and data and air photographs can measure extremely small changes in topography/ground deformation of landscape/identify any bulging of land surrounding volcano as magma approaches surface under a volcano, also monitor electrical/magnetic fields which indicate ascent of magma towards surface
- heat sensitive probes indicate rising magma and to gauge size/growth of underground magma chambers: but only limited amount of time before eruption, temp of ground and ground water/groundwater level indicate imminent volcanic activity
- gravity meters: record changes in gravity in rocks as become stressed/detect rising dense plumes of magma
- strain meters measuring stretching/compression of crust
- tilt meters: detect bulging of land surface caused by raising magma
- radon gas emissions/analysis of gases released from fumeroles: indicate disturbance to underlying rock minerals
- study of volcanic periodicity/historic records: estimate
predicting earthquakes
- cannot be with any sense of certainty/accuracy
- study locations/frequencies of past earthquakes to calculate probability of occuring in future
ways of monitoring:
- Seismic monitoring indicate series of minor earthquakes preceding major one
- monitoring seismic gaps: identify stretches of faults that have not had an earthquake for a long time (seismic gap), in theory with stresses building up in this zone, earthquake is likely
role of silica in eruptions
- rhyolitic magma has high silica content and so is far more viscous and leads to explosive eruptions, such as Mt St Helens
- this is because silica has more molecular bonds and so is thicker
types of basic lava
Pahoehoe:
Smooth, rolling
Less viscous
Heated magma flows beneath the cooling crust
Hotter than Aa
Less silica
Aa:
Rough and rubble like
More viscous, does not flow as freely
Higher silica
acidic lava
- rhyolite
- more viscous and occurs at subductions zones - so high silica
shield volcanoes
formed from basic lava
less than 50% silica
fluid, flows long distances, solidifies slowly
constructive PMs
e.g Mauna Loa in Hawaii islands: largest active in world, formed at hotspot
strato volcanoes
- destructive
- magma gains added silica as it rises through continental rocks
lava is acidic, so is viscous - resulting in steeper sides, that steepen towards the summit
- long dormancy periods, feeding into risk perception
- during dormancy a thick plug of solidified magma builds up, and as the lava is such high viscosity, a lot of pressure has to build up for it to explode
cinder cones
- small, steep-sided volcanoes that erupt basaltic lava
- form when blobs of gas charge lava are thrown into air and break into fragments
lava dome
- mound of viscous lava extruded from volcanic vent
- form slopes on/in craters of strato volcanoes
- silica lava
- very explosive
VEI
- 0-8
- factors based on:
Amount of material ejected
Height of cloud it creates
Amount of damage caused
Aa as a hazard
Lava:
fastest basalt
steep sloping front moves forward in a unit
sudden dangerous surges in speed
destroying everything
Pahoehoe as a hazard
Lava: basaltic:
flows in individual lobes
moves around obstacles
setting flammable objects on fire
less viscous so moves faster and further
source of water for lahars
intense rainfall
glacier melt
snow melt
dam failure
jokulhaups (glacial floods)
-Sudden release of water from a glacier or an ice dammed lake
- where there is a sub glacial lake, geothermal heat from the volcano heats the underside until the glacier melts
- and a jokulhaup is formed, picking up sediment along the way
ways in which volcanic hazards are reduced
- concrete shelters to protect against volcanic bombs/ash:
EV: concrete absorbs heat, protects against bombs but not lava flows - Evacuation routes clearly sign posted/regular drills:
EV: don’t know size of eruption, risk perception = people may not leave if last ones small
- poor people = less able to leave - transport/communication limited, not understand reason to leave (education) - wealth - concrete lahar channels divert**: relatively inexpensive, help flooding, known hazard = people awar
EV: might not always work: don’t know what kind of eruption will be - not lahars? based one evidence - no eruptions the same
- loss of fertile soil - economic cost (LICs)
- increased surface runoff?
earthquakes or volcanoes easier to prepare for? why?
Earthquakes:
- earthquake proof building built - effective in HICs
- number of ways to predict/monitor
- knowledge of location of plate boundaries to indentify areas at risk
However: can’t predict exact date/time making evacuation/mitigation difficult
- earthquake buildings expensive: wealth/inequality
Volcanoes:
- see them so easier for warning signs = more time to evacuate
- strategies, e.g lava diversion channel cheaper than earthquake strategies like tsunami wall
However:
- do not know type of eruption. so some preparation less effective, e.g concrete buildings against lava
volcanoes on destructive plate margins
- usually explosive due to high pressure magma is under
- composite volcanoes
volcanoes on continental/oceanic
- denser oceanic plate subducts below continental
- plate subducting leaves an ocean trench
- oceanic crust is melted as it subducts into the asthenosphere
- extra magma created causes pressure to build up
- pressurised magma forces through weak areas in the continental plate
- explosive, high pressure volcanoes erupt through the continental plate
Oceanic/oceanic destructive(convergent) volcanoes
leaving an ocean trench
- built up pressure causes underwater volcanoes bursting through oceanic plate
Volcanoes on constructive/divergent plate boundaries
- magma is under less pressure, so lava flows more freely
- shield volcanoes are mainly formed from these eruptions
Oceanic/oceanic divergent volcanoes
- magma rises in between the gap left by the two plates separating, forming new land when it cools
- less explosive underwater volcanoes formed as magma rises
- new land on ocean floor by lava filling gaps = due to sea floor spreading
Continental/continental divergent volcanoes
- any land in the middle of the separation is forced apart = rift valley
- volcanoes form where magma rises
Type of magma at constructive PM
Basic (pahoehoe/aa):
Non viscous
Low silica
Flows further
Type of magma at destructive PM
Acidic (rhyolite):
Viscous
Acidic
High silica
Doesn’t flow very far
secondary hazards of volcanoes
Lahars
Landslides
Jokulhaups
Tsunamis
Acid rain
Global temperature change
