Hazards 4

Most volcanic eruptions occur near constructive and destructive plate margins:

Constructive margins:

• Basaltic lava is formed here - it is very hot and has a 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:

• Andesitic and rhyolitic lava are formed here - they are cooler and more viscous than basaltic lava, so they flow less easily. Andesitic and rhyolitic lavas usually erupt intermittently 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.

A few volancoes 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.

Volcanic eruptions can create primary hazards - hazards that come from the eruption itself, for example:

Pyroclastic flows:

• 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 (around 10-15km).
• Because they travel fast and can happen with relatively little warning, pyroclastic flows can cause widespread death and destruction. 

Lava flows:

• 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 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.

Volcanic gases:

• 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.

Pyroclastic and ash fallout:

• 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. Hevaier particles are deposited earlier than light ones…