5.3- Volcanic Hazards Flashcards
Most volcanic activity is associated with plate tectonics and so is mainly located
Along plate margins
Volcanic activity is found at the following sites:
- along ocean ridges
- on or near subduction zones
- associated with rift valleys
- over hot spots
Explain how volcanic activity can occur along ocean ridges
- along ocean ridges where plates are moving apart (constructive) and magma is forcing its way to the surface, cooling and forming new crust
- as the plates move further apart, this new crust is carried away from the ridge (sea-floor spreading)
- best example is the mid-Atlantic ridge where Iceland represents a large area formed by volcanic activity
- volcanoes formed here have fairly gentle sides due to low viscosity of basaltic lava
- eruptions are frequent but relatively effusive
Explain how volcanic activity can occur on or near subduction zones
- the ‘ring of fire’ that surrounds the Pacific Ocean is associated with plate subduction
- the deeper the oceanic plate descends, the hotter the surroundings become
- this together with the heat generated from friction, begins to melt the oceanic plate into magma in a part of the subduction zones called the Benioff zone
- as it is less dense than the surrounding material, this molten magma begins to rise as plutons of magma
- eventually, these reach the surface and form volcanoes
- the andesitic lava, which has a viscous nature creates complex, composite and explosive volcanoes (compared with the basaltic emissions of the constructive margins)
- if the eruptions take place offshore, a line of volcanic islands known as island arcs can appear e.g. the West Indies
Explain how volcanic activity is associated with rift valleys
- at constructive margins in continental areas such as east Africa, the brittle crust fractures as as sections of it move apart
- areas of crust drop down between parallel faults to form rift valleys
- the crust here is much thinner than in neighbouring areas
- it is through this thinning crust that magma forces its way to the surface to form volcanoes
- there are a number of active volcanoes situated along the rift in east Africa
Explain how volcanic activity can occur over hot spots
- in certain places, a concentration of radioactive elements below the crust causes a hot spot to develop
- from this, a plume of magma rises into the plate above
- when this lava breaks through to the surface, active volcanoes form above the spot
- the basaltic lava flows slowly and forms huge but flattish volcanoes, sometimes referred to as shield volcanoes
- e.g. Hawaiian Islands in Pacific Ocean
- the hot spot is stationary, so as the Pacific plate moves over it, a line of volcanoes is created
- the one above the hotspot is active and the remainder form a chain of islands with extinct volcanoes
Volcanic eruptions show an enormous variation- on the one hand they can involve the tranquil effusion of sluggish lava; on the other
They can take the form of huge explosions that eject gas and dust and can blot out the sun for many years, bringing on global climate change
The main method of measurement of magnitude has been the
Volcanic explosivity index (VEI)
Explain the VEI
- a logarithmic scale running from 0-8
- quiet lava-producing eruptions score 0-1 on the index whereas the eruptions of Mt St Helens (USA) scored 5
- colossal eruptions on the scale of 7-8 occur very infrequently
- volcanologists estimate that the last eruption at 7 on the scale was in Indonesia in 1800s
Critics of the VEI point out
That it does not take into account gas emissions or the atmospheric/ climatic impact of eruptions
To determine the frequency of eruption of any volcano,
Its previous history of activity can be interpreted by volcanologists using the deposits associated with the volcano itself
A volcanic event can produce a variety of effects, the impact of which can range from the area immediately around the volcano to
The whole planet
Primary effects are brought about by material ejected from the volcano (and are described below):
- tephra= solid material of varying grain sizes ranging from volcanic bombs to ash, all ejected into the atmosphere
- pyroclastic flows (nuées ardentes)= very hot, gas charged, high-velocity flows made up of a mixture of gas and tephra- flow down the sides of volcano with speeds up to 700km per hour. Roman city of Pompeii was destroyed by such flows
- lava flows
- volcanic gases= these include carbon dioxide and monoxide, sulphur dioxide
What are some secondary effects of volcanic eruptions?
- lahars (volcanic mud flows)= melted snow and ice as a result of the eruption combined with volcanic ash forms mud flows that can move down the course of river valleys at high speeds- capability to destroy towns
- flooding (when an eruption melts glaciers and ice, serious flooding can result)
- acid rain= volcanoes emit gases which include sulphur, which when combined with atmospheric moisture acid rain results= weathering!
- climatic change= the ejection of huge amounts of volcanic debris into the atmosphere can reduce global temperatures and is believed to be an agent in past climatic change
When does volcanic activity become hazardous?
When they impact upon people and the built environment, killing and injuring people, burying and collapsing buildings, destroying infrastructure and bringing agricultural end other economic activities to a halt
