Topic 2 - Volcanoes, Processes, Hazards, and their impacts Flashcards
what is an explosive eruption
- andesitic and rhyolitic lavas formed from silica rich, acidic magma that is very viscous
- often solidifies before reaching surface = build up of pressure and violent eruption
what is effusive eruption
- basaltic lava formed from magma that is low in silica
- more fluid magma that allows gas bubbles to expand on way to surface, prevents sudden explosive activity
what is a LAVA PLATEAUX and how is it formed + example
- flat and featureless
- from fissure eruptions with extensive runny basaltic lava travelling large distances
- eg Antrim lava plateaux, N. Ireland (Giants Causeway)
what is a SHIELD volcano and how is it formed + examples
- gentle sides covering large area
- free-flowing basaltic lava has low silicon content and travels long distance before cooling, built up slowly by accretion of thousands of highly fluid lava flows
- eg Mauna Loa, Hawaii
what is an ACID/DOME volcano and how is it formed + examples
- steep sided convex cones
- formed from viscous rhyolitic lava (high silicon content) cools quickly = steep sided convex sides
- volcanic/lava domes formed by relatively small, bulbous masses of lava too viscous to travel any great distance, dome grows largely by expansion from within
- eg. Puv region, central France and Mt Vesuvius
what is a CINDER CONE volcano and how is it formed + examples
- steep and relatively symmetrical sides
- formed from ask, cinder and volcanic bomb layers ejected from violent eruptions. as gas-charged lava blown violently into air, breaks into small fragments that solidify and fall as cinders around vent form circular/oval cone
- eg. Paricutin, Mexico
what is a COMPOSITE CONE/STRATOVOLCANO and how is it formed + examples
- classic pyramid shape, consisting of layers of ash and lava
- more explosive and viscous andesitic lava cools quicker than basic lava, violent eruptions expel large amounts of ash. lavas flow through breaks in crater wall or issue from fissures on the flanks of the cone. lava, solidified within fissures forms dikes that act as ‘ribs’ which greatly strengthen cone
- eg. Mt Etna, Sicily and Mt Fuji, Japan
what is a CALDERAS and how is it formed + examples
- huge craters from violent eruptions may become flooded by sea/lake forms
- gases build up and explosion removes summit of cone leaving opening several km in diameter
eg. Krakatoa, Indonesia and Santorini, Greece
what are the types of volcanoes
- fissure
- shield
- dome
- ash-cinder
- composite
- caldera
how is a cinder cone volcano formed
- built from particles and blobs of congealed lava ejected from single vent
- gas-charged lava ejected, breaking into small fragments that solidify and fall as cinders around vent to form circular/oval cone
- most have bowl-shaped crater at summit and rarely rise more than a thousand feet, numerous in western North America
what is a composite volcano comprised of
- most have crater at summit containing central vent or clustered group of vents
-lavas flow through breaks in crater wall or issue from fissures on the flanks of the cone - conduit system through which magma from reservoir rises to surface
- once volcano becomes dormant, erosion destroys cone
what is a fissure
- on volcanoes a fissure is an elongate fracture/crack at surface from which lava erupts
- fissure eruption typically dwindle to central vent after hours/days
- sometimes lava flows back into ground by pouring into crack/open eruptive fissure - drainback
- sometimes lava flows back into same fissure erupted from
what is an Icelandic eruption
lava flows gently from fissure, usually on flat slopes
what is a Hawaiian eruption
there is a small vent through which small amounts of lava flow
what is a Strombolian eruption
usually small but frequent eruptions occur
what is a Vulcanian eruption
less frequent than Strombolian but more violent
what is a Vesuvian eruption
long period of inactivity but erupts with enormous power
what is a Krakatoan eruption
has got exceptionally violent explosion which may remove peak of its cone
what is a pelean eruption
very violent eruptions accompanies by pyroclastic flows containing glowing cloud
what is a plinian eruption
large amounts of lava/pyroclastic are ejected
what is a pyroclastic flow
- contain high density mix of lava blocks, pumice, ash and volcanic gas moving at high speed down slopes
- contain 2 parts - lower BASAL flow (coarse material along ground) and turbulent cloud of ash above basal flow
what is a tephra (ash falls)
solid material of varying grain size ranging from volcanic bombs to ash ejected into atmosphere
how is a pyroclastic flow formed
- collapse of eruption column: column ejected upward into atmosphere cooling and becoming too dense to maintain upward momentum
- ‘boiling over’ from eruptive vent, material erupted without forming high plume, rapidly moves down slope
- collapse of lava domes/flows - fronts of lava become too steep they collapse due to gravitational force
impacts of a pyroclastic flow (PF)
- rock fragments can be ash/boulders, PF shatter and carry away objects and structures in their path
- varying in size, they can destroy buildings, forests, farmland and cause death/injury
- can deposit loose rock fragments depending on volume of rock debris
- cause secondary hazards (flooding, lahars)
- blocking streams creates lakes which erode lockage = rush of water and volcanic material downstream
toxic gases characteristics and impact
- 180-440 million tonnes of CO2 released in a year causes health risks (high heart rate, breathing difficulties)
- sulfur dioxide irritates skin and membranes of nose, eyes, throat and causes acid rain
- lake nyos eruption 1986 released 300,000 tonnes of CO2
formation of toxic gases
magma contains dissolved gases causing eruptions, magma rises towards surface and pressure decreases
gases released from liquid magma (melt) and travel upward, released into atmosphere and eruptions release enormous amounts of gas in short time
what are primary impacts of an eruption
- pyroclastic flow
- tephra/ash falls
- toxic gases
what are secondary impacts of an eruption
- lahar
- famine
- jokulaup
- landslides
- homelessness
- unemployment
characteristics of volcanic landslide
- 1km3 - 100km3
- high velocity and great momentum allows them to cross valley divides and run up slopes several hundred m high
formation of volcanic landslide
- common on volcanic cones because of tall, steep shape weakened by rise/eruption of molten rock
- magma releases gases that partially dissolve in groundwater = hot hydrothermal system weakening rock
- mass of thousands of layers, lava, loose fragmented rock = fault zones that move frequently
impact of volcanic landslide
- often bury valleys with tens to hundreds of meters of rock debris = small hills formed (hummocks) and closed depressions if deposit is thick enough tributaries may then form lakes which drain forming lahars and floods
- generate deadly lahars
- most deadly volcano occurred in 1792 when sliding debris from Mt Mayuyama in Japan slammed into Ariaka Sea and caused tsunami killing nearly 15k people
what is a lahar
- volcanic mud flow
- melted snow and ice because of eruption combined with volcanic ash forms mud flows that can move down course of river valleys at high speed
- size, speed, amount of material constantly changes
lahar exemplification
- 1985 lahar destroyed Colombian town of Armero creating 7000 deaths from 28000 population as lahars can grow in volume incorporating anything in its path
when do jokulhlaups occur
- when eruption melts glaciers and icecaps causing serious flooding
- happened in Iceland when Grimsvotn erupted in 1996
how are lahars formed
- pyroclastic flows can generate them when hot debris erodes and melts snow and ice
- high-volume/low-duration rainfall occurring on steep slopes
- lake breakout floods without eruption lead to lahars, commonly occuring after stream becomes blocked by landslide/PF
lahar hazards
- erosion/transportation of loose volcanic deposits = severe flooding
- can crush/bury anything in path, buildings and land destroyed as well as bridges/roads trapping people in vulnerable areas
what are environmental impacts of volcanoes
- volcanic winters (depends on size)
- acid rain (CO2 adds to greenhouse effect, sulfur dioxides converted to sulfuric acid = acid rain, sulfate aerosols formed = reflect solar radaiation, absorb heat, cool earth)
- volcanic soils (basaltic lavas in tropical countries beneficial to farming)
- plants (destroyed, but volcanic soils restore them)
- animals (killed by lava flows, PF, tephra, gases, famine, forest fires)
- weather patterns (rain, lightning, thunder due to ash particles collecting water droplets, warming of pacific ocean = el nino effect, torrential rain, impair air quality with volcanic fog)
how does an eruption affect health
- 600m people live close to active volcanoes to be affected when erupted
- modest casualties (around 300,000 in last 200 years)
- long term exposure to volcanic fumes aggravate existing respiratory problems
- cause headaches, fatigue in healthy people
what are economic costs of an eruption
- Eyjafalljokull caused 0 deaths but disrupted millions of peoples travel
- ash is bad for jet aircraft engines, some engines operate at temperatures high enough to melt ash ingested so lava may get in engine
effects of an eruption on settlements
- vary from none to catastrophic
- atmospheric conditions may carry ash away from city or topography may direct lahars/pyroclastic flows to unpopulated areas
- however this could also happen and cause death/destruction
effects on farming and tourism of volcanoes
- initial eruption = famine but rich fertile soils created
- great rift valley of kenya hub for agriculture
- volcanism attracts tourism, Mt St Helens in 1980 was costliest disaster but more money now generated from tourism than initial cost
