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Main hazards generated by volcanic activity p1 - Coggle Diagram
Main hazards generated by volcanic activity p1
Different types of eruptions
Effusive -> Explosive:
MAFIC (Fe-rich) -> Basalt -> Andesite -> Rhyolite -> SILICIC (Si-rich)
increasing in SiO2 (silica) content
increasing gas content
decreasing ability for gas escape
decreasing temp at eruption
Eruption style classification:
volcanic terminology (Mercalli) based on characteristics of well-known volcanoes
1) Hawaiian
2) Strombolian
3) Vulcanian
4) Sub-plinian or Vesuvian
5) Plinian
6) Ultra-plinian
from 1 (effusive) to 6 (explosive)
Mercalli types of eruptions:
volcanoes can be classified according to the type of eruption
Icelandic lava eruptions, persistent fissure eruption, large quantities of basaltic lava build up vast horizontal plains, e.g. Deccan Plateau, Columbia Plateau
Hawaiian eruptions, more noticeable central activity than Icelandic, runny, basaltic lava travels down sides of volcano in lava flows, gases escape easily, occasional pyroclastic activity occurs but less important than lava
Strombolian eruptions, frequent gas explosions that blast fragments of runny lava into air to form cones, very explosive, large quantities of pyroclastic rock thrown out, white cloud of steam emitted from crater
Vulcanian eruptions, violent gas explosions blast out plugs of sticky or cooled lava, fragments build up into cones of ash & pumice, occur when very viscous lava solidifies rapidly after explosion, often eruption clears a blocked vent and spews large quantities of volcanic ash into atmosphere
Vesuvian eruptions, very powerful blasts of gas pushing ash clouds high into the sky, more violent than vulcanian, lava flows occur, ash falls to cover surrounding area
Plinian eruption, gas rushes up through sticky lava and blasts ash and fragments into the sky in a huge explosion, violent eruptions create immense clouds of gas and volcanic debris several km thick, gas clouds and lava can rush down slopes, part of volcano may be blasted away during eruption
The modern approach:
the modern classification of eruptions is based on the physical characteristics of the eruption:
eruption rate
volume of products
eruption column & altitude
duration
these form the Volcano Explosive Index (VEI):
0 (least explosive) to 8 (mega-colossal)
mega colossal would be global or at least regional scale
0 = Hawaiian, 8 = Plinian / Ultra-plinian
Phreatic eruptions:
driven by steam
phreatic = superheated water reaches flashpoint and rapidly expands
phreatic-magmatic = water heated by contact with lava / magma
Features of a typical volcano:
magma chamber
dyke
flank eruptions
parasitic cone
crater & main vent
new cone
pyroclastic flows
ash falls & deposits
cloud of ash & rock
lava flows
Phreatic eruption - Mt. Ontake, Japan:
Japan has 110 active volcanoes, several are submarine volcanoes, 70% of Japan’s land mass is mountainous
Japan is located in one of the most tectonically active zones in the world; 4 tectonic plates meet, and widespread subduction gives rise to intense volcanic activity
evidence records nearly 1200 volcanic eruptions in Japan in past 2000 years
Mt. Ontake:
classic strato-volcano 200km west of Tokyo on Japan’s largest island, Honshu
just over 3000m, summit is often snow-covered
dormant for many centuries until a sequence of eruptions from Oct 1979 – April 1980
further eruptions (some of them small phreatic eruptions) followed in 1991 & 2007
27th Sept 2014, just before midday, it erupted violently - killed 63 people and large areas surrounded affected by ash fall, pyroclastic flows, volcanic bombs and lahars
for a while, air space in the vicinity of the eruption was closed as a precaution against the possible damaging effects of fine volcanic ash on aircraft jet engines
How:
magma heat source can flash vaporise water into steam and expand violently
increased inputs leads to pressurisation & fracturing
moisture from snow & rain seeps into geothermal system
tephra (ash & bombs) generated from pre-existing materials
eruption has no glass (chilled magma) or ash, indicating it was steam driven
Global distribution of volcanoes:
island arcs (explosive) above subduction zones
within plate hot spots (effusive)
andean margins above subduction zones (explosive)
isolated oceanic islands (effusive)
linear chains, rift zones (effusive)
linear chains MOR (effusive)
Shape of volcanoes:
Fissure:
effusive, fissure eruption, Fe-rich (basalt)
mid-ocean ridges (diverge boundaries). No crater. Lava spreads over large areas creating plateaus.
e.g. Laki Fissure, Iceland
Shield:
Effusive, Hawaiian eruption.
Fe-rich (basalt)
build up over time after many eruptions from a single vent. Slope around 15°. Can be many km wide.
e.g. Mauna Loa, Hawaii
Dome:
mildly explosive, Vulcanian. Si-rich Andesite.
highly viscous lava doesn't spread out far. Vent can plug, building pressure to large eruptions.
e.g. Puy de Dome, France
Ash-cinder:
explosive, Vesuvian. Si-rich Andesite.
layers of ejecta (solid fragments) build up over time. Very steep slopes
e.g. Hekkla, Iceland
Composite:
very explosive, Plinian. Si-rich Rhyolite.
subduction zones. Alternating layers of lava and ejecta. Can be very high 2,500m altitude. Viscous lava clogs vents building pressure to explosive eruptions.
e.g. Mt. Etna, Sicily, Italy
Caldera:
very explosive, Plinian. Si-rich Rhyolite.
over 1km diameter. Has an inverse crater. Explosive eruptions have caused the summit to collapse in on itself
e.g. Santorini, Greece
Lake Nyros, Cameroon – Gaz hazards:
One of several deep lakes that occupy volcanic craters in Cameroon in West Africa
2km wide and 200m deep
Aug 1986, 1700 people died and all animal life in area around volcano asphyxiated
cause was a leak of CO2 from a volcanic crater lake
gas had built up at bottom of the lake after being emitted from underlying magma chamber
CO2 is dense, flowed down volcanic slopes as 50m thick ground-hugging layer travelling at about 70km/h
not known for certain what caused CO2 to escape the lake, could be a deep movement of magma, earthquake, change in water temp of lake, strong winds stirring up lake waters
White Island, New Zealand:
phreatic eruption
9th Dec, 5 deaths, 8 missing, 47 on island at time of eruption, 31 in hospital with injuries, 27 burns on more than 71% of their bodies
Impact of intrusion on landscapes
Batholith:
a large, usually discordant igneous intrusion >100km across, that may be an aggregate of plutons
effect on landscape & typical landforms:
Up-doming of ground surface. After erosion form resistant peninsulas, headlands and steep cliffs. Formation of tors and islands.
impact on landscape system:
Erosive products form depositional landforms
example:
Lands End St. Michaels Mount
Pluton:
10km
discordant
effect on landscape & typical landforms:
same as Batholith
impact on landscape system:
same as Batholith
example:
same as Batholith
Laccolith:
a 30-500m across concordant dome shaped intrusion
effect on landscape & typical landforms:
Upward doming of surface after erosion of overlying rocks.
impact on landscape system:
Dome shaped hills.
example:
Biddlestone, Northumberland
Dyke:
a discordant, 1-20m across sheet-like intrusion
effect on landscape & typical landforms:
Cut through layers in sedimentary rock, magma is forced along cracks in the rock.
impact on landscape system:
Forms rock ‘walls’ – causes contact metamorphism in surrounding country rocks.
example:
Kildonan Dyke Swarm, Isle of Arran
Sill:
a concordant, 10-30m across sheet-like intrusion
effect on landscape & typical landforms:
Formed when magma is forced along between layers of sedimentary rock.
impact on landscape system:
Flat-topped cliffs, waterfalls.
example:
Drumadoon, Isle of Arran
Plug/neck:
a small <1km diameter body, discordant
effect on landscape & typical landforms:
Exposed after erosion of overlying volcanic rock.
impact on landscape system:
Cone-shaped hills that are steep sided, cylindrical in shape.
example:
Ship Rock, New Mexico USA
Volcanic activity
Volcanic landscapes and landforms are controlled by:
1) Composition of the magma
2) Processes in magma generation (magma generation is controlled by plate tectonic setting)
3) Intrusive and eruptive behaviour
4) Geological structures: faults and joints
5) Exposure level, weathering and erosion
Intrusion:
is when magma cools & solidifies before it reaches the surface
sills, form when magma intrudes between the rock layers, forming a horizontal or gently-dipping sheet of igneous rock
dyke, form as magma pushes towards the surface through cracks in the rocks, dykes are vertical or steeply-dipping sheets of igneous rock
Extrusion:
is the result of magma coming from deep within the earth to surface, where it cools as lava
How magmas form and get to the surface:
partial melting starts in the mantle or crust
melts, coalesces & rises along fractures
isolated melt droplets -> veins -> dykes -> pluton (magma chamber) -> dykes & sills -> volcano -> lava flow
segregation -> ascent -> emplacement -> extrusion
When will a volcano erupt?
active, erupted since the last glacial period (10,000 years ago)
dormant, not erupted during the last 10,000 years but may erupt in the future
extinct, not expected to erupt again
Types of volcanic landforms & eruptions are determined by:
1) the type of lava
2) the materials ejected
3) the eruption style (how it takes place, size and frequency)
Eruptions are either:
effusive, fluid non-explosive eruptions
explosive, viscous highly explosive eruption
Primary hazards produced by volcanic activity:
Lava flows:
basic (basaltic) lava is free-flowing
acidic lavas such as rhyolite are think & palsy so do not flow easily
impacts dependent on the type of lava
basaltic flow large distances e.g. Hawaii July 2015 20km flow
everything in path of lava either burned, bulldozed or burried
destroy infrastructure, property & crops
rarely causes injuries or fatalities
Pyroclastic flows:
a combination of very hot gases (500 degrees) a shard rock fragments travelling at high speed (100km/hr)
follow ground contours and destroy everything in path
inhalation of hot & poisonous gas causes almost instant death
e.g. Pomeii pyroclastic flow from mount Vesuvius in AD79
Tephra:
any material ejected from volcano into air
size vary from very fine as he to large volcanic bombs (>6cm)
lighter debris such as pumice
potential hazardous cover farmland in layers of ash & destroying crops
transport ground & air disrupted
buildings can collapse due to weight of accumulated ash
people with respiratory diseases may have difficulty breathing
e.g. Iceland volcano, April 2010, 100,000 flights cancelled
Toxic gases:
wide range of toxic gases including CO, CO2 & SO2
deadly threat to humans
SO2 combines with atmosphere water, acid rain produces, enhances weathering and can damage crops & pollute surface water and soils
Secondary hazards produced by volcanic activity:
Lahars:
type of mud flow with consistency of wet cement
speeds up to 50km/h
different rock fragments & ash & soil mix together
snow & ice on volcano summit melt & flow down
everything in path either destroyed or buried under thick layers of debris
e.g. 1984, Nevado Del Ruiz in Armero was overwhelmed by lahars causing deaths of 23,000 people
places like Southeast Asia, ash covered slopes of volcanoes continue to generate lahar hazards after periods of heavy rain
Floods:
volcanic eruptions beneath an ice field or glacier cause rapid melting
e.g. Iceland, several active volcanoes lie under the vanrajokull ice field
vast quantities of water accumulate during eruption as they find exit from ice, resulting in torrent of water causing devastating floods - locally called jokulhaulps
Tsunami:
violent eruption some island volcano cause mass displacement of ocean & tsunami waves can travel speeds up to 600km/h
deep water, usually less than 1m & long wavelength of up to 200km
near shore, rapidly increase in height and break releasing vast energy & water along shore & inland
e.g. Krakatoa, in 1883, tsunamis believed to have drowned 36,000 people