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Hazardous Earth Essay QuestionsWhat are the main hazards generated by volcanic activity?
To what extent is there a variety of volcanic activity and resultant landforms and
landscapes?
To what extent do volcanic eruptions generate distinctive hazards?
Overall, I would argue to some extent the hazards generated from volcanic eruptions can be
distinctive but they share many common features. Volcanoes can form in different tectonic settings
such as at convergent plate boundaries or hotspots, these volcanos can have distinct hazards as a
result of different factors influencing them.
EXTENT TO WHICH THEY ARE DISTINCT VARIES DEPENDING ON TYPE OF VOLCANIC ERUPTION
- VEI suggests different eruptions share similar hazards - characteristics distinct
- TheVEI scale is a scale that uses both quantitative measurements and subjective descriptions
of eruptive phenomena to provide a relative measure of the explosivity of volcanic
eruptions.
- lower VEI = effusive, scale increases with explosivity.
- The index discusses a range of hazards - showing hazards produced are interrelated to
explosivity
- index shows the tephra volume
- Tephra = ash that has settled
- Volcanic ash - formed during explosive volcanic eruptions when dissolved gases in magma
expand and escape violently into the atmosphere. The force of the gases shatters the
magma and propels it into the atmosphere where it solidifies into fragments of volcanic rock
and glass.
- as VEI increases - volume of tephra increases
- Yellowstone 600,000 years ago, VEI 8, erupted 1000km3 of tephra
- Poas eruption in costa rica 2009-present, 1 VEI scale, produces 0.001 km3 of tephra.
- difference that volcanos VEI has on the hazards it produces
- Both volcanos produce tephra but amount produced is different, higher VEI scale volcanoes
produce far more tephra.
- compare column height of a volcano.
- An eruption column is a cloud of super-heated ash and tephra suspended in gases emitted
during an explosive volcanic eruption. The volcanic materials form a vertical column that
may rise many kilometres into the air above the vent of the volcano.
- as VEI increases so does column height
- Toba, a VEI 8 super volcano, occurred 75 million years ago, the column height was over 25
km,
- poas, column height - ranges between 0.1-1km
- difference in the amount of ash and gas produced as more ash and gas results in a larger ash
cloud,
MJ -Therefore, I would argue the extent to which volcanos have distinct features depends on the
types of volcanos you assess and how you assess the features. Most volcanos are shown to produce
some volcanic ash and gases resulting in an ash cloud and tephra accumulating showing they do not
have distinct hazards, however, the amount of ash and gas produced varies hugely depending on the
,VEI of the volcano thus it could be said the features of these hazards are distinct. Volcanos of a very
similar VEI will likely produce similar amounts of ash, thus, if only assessing volcanos of one VEI you
will find their hazards have similar feature, however, it must be noted the scale is logarithmic, with
each interval on the scale representing a tenfold increase in observed ejecta criteria, thus any small
change in VEI will have a large increase in the hazards seen and the amount of ash produced.
VARIES BETWEEN TECTONIC SETTINGS
- Similar hazards – different features
- E.g. movement of magma occurs at both locations and can be a hazard itself but it results in
a range of different other distinct hazards at each location.
- oceanic continental convergent plate boundary
- volcanoes form as the subducted plate + sediment on earths floor are pulled into the
asthenosphere (650km deep)
- Higher temps - rock and sediment melt and under high pressure.
- Bouyancy force acts upon magma - upwelling - denser than the material around it.
- magna rises to earths surface – eruptions
- E.g. Mt St Helens - oceanic plate subducted under the North American plate forcing magma
up - formation of this volcano
- plinian style eruptions associated with explosive and dangerous hazards
- As the oceanic plate is subducted - sediment on the ocean floor melts.
- E.g. shells contain CaCO3 - put under high amounts of pressure - gasformation, e.g. CO2
- increases pressure in magma - eruption = explosive.
- Also, magma in Mt St Helens = dacite (63-68% silica) - highly viscous - gas cannot easily
escape - pressure builds up.
- eruption = explosive - hazards e.g. projectiles - occur as the magma is under high pressure -
rocks = launched(land 2km away).
- Also, high viscosity = cools quickly + solidifies - not able to move far from the vent - magma
plug - increase in pressure in magma chamber – results in lateral blast(eruption directed
laterally not upwards)
- Occurs when pressure increases - stress on rock face next to the magma chamber exceeds
the strength - erupts laterally to release the pressure.
- CONVERSELY, volcanism also occurs at hotspots - locations where theres a plume of magma
to the earths surface - asthenosphere is much closer.
- magma melts through lithosphere so lava comes out of the crust - volcanic activity
- crust moves by convection currents - hotspot remains in same place which results in a chain
of islands.
- crust moves + plume of magma no longer able to supply to the volcano and it becomes
dormant.
- Example of chains of islands can be seen in the pacific as the plate is moving rapidly, e.g.
Hawaii.
- Magma melts ocean pacific plate - becomes predominantly basalt
- Basalt = low silica – up to 52% - not viscous - gas can easily escape
- Only hazard = lava flows
- lava moves quickly - able to flow far before it cools.
- E.g. flows from Mauna Loa collectively cover 806 km 2, nearly 16% of the volcano's surface.
- contrasted to Mt St Helens - lava doesn’t flow further than vent.
, MJ - Therefore, it could be said when considering tectonic setting different eruptions share some
similar hazards but the characteristics of each hazard can have distinctive features. Both volcanoes
on convergent plate boundaries and at hotspots are associated with the movement of magma.
However, the different viscosities of the magma results in a range of different knock on hazards. The
highly viscous magma results in explosive hazards such as projectiles and lateral blasts. Whereas, at
oceanic hotspots the less viscous lava means lava flows themselves are a more prominent hazard
but there are less other hazards as pressure cannot build up to the same extent.
distinct hazards when comparing volcanoes of different structures
- profile of Mt St Helens = a strato volcano
- lava is viscous + cannot move far before it cools
- also, ash clouds are associated with explosive eruptions - ash settles - material stacking -
volcano is steep and tall
- E.g. Mt St Helens = 2.5 km tall but only 9.7 km in diameter
- can result in landslides - steep relief
- weathering weakens rock by processes such as freeze thaw - water freezes in cracks +
expands
- gravity can exert stress + when stress exerts strength mass movement of the rock and ash
can occur causing a landslide.
- VS landslides do not occur often on shield volcanoes e.g. Mauna Loa - relief is flatter - gravity
exerts less pressure.
- Shield volcanos = height is 1/20 of base.
- highly runny and not viscous lava can move far before it cools + is not thick enough to stack
- volcanos are low in profile + not steep
- E.g. Mauna Loa where the height is 4.1 km but the diameter is 100km, thus landslides are
not associated as a hazard with these volcanoes.
MJ - Therefore, it could be said when considering tectonic setting different eruptions there are
examples of distinct hazards. Strato volcanos are more subjected to pressures such as gravity which
can result in landslides as a hazard, whereas, shield volcanos aren’t exposed to this same stress so
landslide do not occur to the same extent, thus it could be said between these types of volcanoes
there are distinct hazards.
Overall, I would argue to some extent the hazards generated from volcanic eruptions can be
distinctive but they share many common features. Volcanoes can often be categorised, for example,
by their VEI or shape. When comparing volcanoes of different VEIs I would argue volcanos share
some hazards, thus suggesting they are not distinct, however, the features of these hazards are very
different and occur on different scales. Similarly, when comparing volcanoes in different tectonic
settings it could be said they share similar hazards, such as magma movement but this features of
this magma is highly different and can result in other distinct hazards occurring. Moreover, it could
be said that the shape of the volcano can result in distinct hazards and it determines the extent to
which external factors such as gravity influence it and cause hazards such as landslides.
Evaluate the extent to which the processes leading to explosive and effusive volcanic
eruptions are different?
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