3.2.1 Volcanic Hazards Flashcards
1
Q
Which plate margins do volcanic hazards occur at?
A
- Constructive plate margins
- Destructive plate margins
2
Q
Volcanic hazards at constructive plate margins
A
- Basaltic lava is formed here – it is very hot and has a low viscosity (it’s runny), so it flows easily and quickly. Eruptions of basaltic lava are frequent and go on for a long time, but they’re not very violent.
- If the margin is underwater, magma rises to fill the space left by plates moving apart, forming ocean ridges.
- If the margin is on land, as plates pull apart, forming rift valleys, they become thinner, and magma is able to break through at the surface.
3
Q
Volcanic hazards at destructive plate margins
A
- Andesitic and rhyolitic lavas are formed here – they are cooler and more viscous (less runny) than basaltic lava, so they flow less easily. Andesitic and rhyolitic lavas usually erupt intermittently (every once in a while) and the eruptions are short-lived.
- At subduction zones, where one plate is pulled beneath another, melting of the plate forms magma, which rises to the surface as volcanoes. Because the lava is viscous, it forms blockages in volcanic vents, causing pressure to build. The blockage is cleared by a violent eruption.
4
Q
Location of volcanoes
A
A few volcanoes occur away from plate margins at hot spots above magma plumes. Most hot spots have basaltic lava that flows quickly, forming volcanoes with gentle slopes (shield volcanoes).
5
Q
Primary hazards from volcanic activity
A
- Pyroclastic Flows
- Lava flows
- Volcanic Gases
- Pyroclastic and Ash Fallout
6
Q
Secondary hazards from volcanic activity
A
- Mudflows (Lahars)
- Acid Rain
7
Q
[Primary hazards]
Pyroclastic Flows
A
- A pyroclastic flow is a mixture of super-heated gas, ash and volcanic rock that flows down the sides of a volcano. It travels at high speed (often more than 80km/h) and flows a long way (generally around 10-15km).
- Because they travel fast and can happen with relatively little warning, pyroclastic flows can cause widespread death and destruction, through e.g. burning and burial under debris.
8
Q
[Primary hazards]
Lava flows
A
- Lava can flow from a volcanic vent down the side of the volcano.
- The speed of the flow and distance travelled depend on the temperature and viscosity of the lava, as well as the steepness of the slope – low viscosity (runny) lava can flow at up to 10km/h on a steep slope, and may travel tens of kilometres.
- Most flows are relatively slow, so people have time to evacuate areas that will be affected. However, lava flows destroy anything in their path, including buildings and vegetation, by burning, burying or knocking it down.
9
Q
[Primary hazards]
Volcanic Gases
A
- Lava contains gases such as carbon dioxide and sulfur dioxide, which are released into the atmosphere when a volcano erupts.
- Some of these gases can be harmful to humans and animals if they’re breathed in, e.g. sulfur dioxide can cause breathing difficulties.
10
Q
[Primary hazards]
Lava Flow: Case Study
A
- Case study: Hawaii – Recent lava flows have buried roads, housing developments and cars. Kilauea constantly erupting since 1983 has destroyed 180 houses (no deaths) and added 120km2 of new land and became a major tourist attraction.
11
Q
[Primary hazards]
Pyroclastic and Ash Fallout
A
- Pyroclastic fallout is material that has been ejected from a volcano during an eruption and falls back to the ground. When fallout consists mostly of ash, it’s called ash fallout.
- Fallout consists of material of a range of sizes – from large pieces of rock weighing several tonnes to microscopic ash particles. Material can travel thousands of kilometres from the volcano. Heavier particles are deposited earlier than light ones, so material ends up being well sorted, with larger, heavier particles deposited near the volcano and smaller, lighter particles (e.g. ash) further away.
- Large pieces of falling tephra can damage buildings and kill or injure people. Finer material can form a layer up to several metres in thickness, which can kill vegetation, hinder road and rail transport and cause buildings to collapse. Ash can also be harmful to people if it is breathed in.
12
Q
[Secondary hazards]
Mudflows (Lahars)
A
- Mudflows occur when volcanic material mixes with large amounts of water (e.g. from rainfall or from ice melted by the eruption). Flows move very quickly (over 80 km/h) and can travel for tens of kilometres.
- Mudflows can bury or destroy natural habitats, settlements and infrastructure (e.g. roads and bridges).
13
Q
[Secondary hazards]
Acid Rain
A
- Volcanic gases can react with water vapour in the atmosphere, which then falls as acid rain – e.g. sulfur dioxide reacts with water to form weak sulfuric acid.
- This can damage ecosystems, and can also cause stone and metal to deteriorate, damaging buildings, bridges, statues, etc.
14
Q
Magnitude of volcanic activity
A
Volcanic events range from small, slow lava flows to huge eruptions of lava, ash and gas.
15
Q
How is the magnitude of volcanic activity measured by?
A
Magnitude of eruptions can be measured using the Volcanic Explosivity Index, which grades volcanoes on a scale from 0 to 8 based on the amount of material ejected and how high the material is blasted.
