Volcanic Hazards Flashcards
Most volcanic eruptions occur near constructive and destructive plate margins.
Constructive margins:
>Basaltic lava is formed here- it is very hot and has low viscosity, 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.
Destructive Margins:
>Andesetic and rhyolitic lavas are formed here- they are cooler and more viscous (less runny) than basaltic, so they flow less easily. Andesitic and rhyolitic lavas usually erupt 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.
What are pyroclastic flows?
(Primary hazard)
> A pyroclastic flow is a mixture of superheated gas, ash, and volcanic rock that flows down the sides of a volcano. It travels at high speed (80km/h) and flows a long way (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.
What are Lava Flows
(primary hazards)
> Lava can flow from a volcanic vent on the side of the volcano.
> The speed of the flow and distance traveled 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 t 10km/h on a steep slope.
> 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, y burning, burying, or knocking it down.
What are volcanic gases?
> Volcanic gases are gases given off by active volcanoes.
> 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.
What is pyroclastic and ash fallout?
(Primary hazards)
> 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 tones to tiny ash particles. Material can travel thousands of kilometers from the volcano. Heavier particles are deposited earlier than the smaller ones, so material ends up being well sorted, with larger rocks being deposited near the volcano and smaller, lighter particles further away.
> Large pieces of falling tephra can damage buildings and kill or injure people. Finer material can form a layer up to several meters in thickness, which can kill vegetation, hinder rod and rail transport and cause buildings to collapse. Ash can also be harmful if breathed in.
What are mudflows?
(secondary hazards)
> Mudflows occur when volcanic material mixes with a large amount of water (e.g. from rainfall or from ice melted by the eruption). Flows move very quickly (80km/h) and can travel for tens of kilometers.
> Mudflows can bury or destroy natural habitats, settlements, and infrastructure (e.g. roads and bridges)
What is acid rain?
(secondary hazards)
> Volcanic gases can react with water vapor 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, damage buildings, brides, statues, etc.
