Volcanic hazards

Where do most volcanic eruptions occur?

Near constructive and destructive plate margins.

What is formed at constructive margins?

Basaltic lava is formed here - it is very hot and has a low viscosity (runny), so it flows easily and quickly.

What are eruptions of basaltic lava like?

They are frequent and go on for a long time, but they’re not very violent.

What happens if a constructive margin is underwater?

Magma rises to fill the space left by plates moving apart, forming ocean ridges.

What happens if a constructive margin is on land?

As plates pull apart, forming rift valleys, they come thinner, and magma is able to break through at the surface.

What is formed at destructive margins?

Andesitic and rhyolitic lavas are formed here - they are cooler and more viscous (thicker) than basaltic lava, so they flow less easily.

What are eruptions of andesitic and rhyolitic lava like?

Usually erupt intermittently (every once in a while) and the eruptions are short-lived.

What happens at subduction zones?

One plate is pulled beneath another, and melting of the plate forms magma, which rises to the surface as volcanoes.

What happens when magma rises to the surface at destructive margins?

Because the lava is viscous, it forms blockages in volcanic vents, causing pressure to build.

This is cleared by a violent eruption.

How do some volcanoes occur away from plate margins?

At hotspots above magma plumes.

Most hotspots have basaltic lava that flows quickly, forming volcanoes with gentle slopes (shield volcanoes).

What can volcanic eruptions create?

Primary hazards - from the eruption itself.

What are the primary hazards that can be caused by volcanic eruptions?

• Pyroclastic flows.

• Lava flows.

• Volcanic gases.

• Pyroclastic and ash fallout.

• Mudflows (lahars)

• Acid rain.