Volcanic hazards

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Volcanic hazards

This ranges from being extremely thick and viscous, to highly fluid. The amount of pyroclastic material also varies. The two types are:

Aa flow: This is a few metres thick, a mix of uneven shaped, sharp edged ash and cinder blocks. It is unstable.

Pahoehoe flow: Is fluid rather than viscous, but does not move quickly. Often the surface layer is static whilst flow continues beneath.

Types of eruption

The diagram above shows the main types of eruption that occur. The characteristics of each are outlined in more detail below:

Hawaiian Type Lava slowly and easily escapes from vent. It is runny, basaltic. Gases escape easily & gentle sided volcanoes result.
Strombolian type Gas explosions occur more frequently, small but frequent eruptions. Cone shaped volcano.
Vulcanian type Gas emissions involved, more violent but less frequent than above. Plugs of cooled lava may be ejected in blast.
Vesuvian type Extremely strong explosions, often after volcano has been dormant for a while. Gas and ash clouds, which fall over a large area.
Krakatoan type Exceptionally violent.
Plinian type Usually most violent. Massive amounts of lava, gas and pyroclastic material emitted. Part of the volcano may be removed.

Lava flows: A greater threat to property than human life due to opportunity for evacuation. Lava flows are more dangerous when released very quickly. They usually cause large but localised economic losses.


Pyroclastic flows: These are hot rock fragments, lava particles ash and hot gases. They are linked with subduction zone volcanoes, and the flow moves very quickly from its source.

Ash and Tephra fall: Ash is material below two millimetres in diameter whilst tehpra is anything above this. It is usually formed when magma is fragmented by explosions, and can stay in the atmosphere causing global variations in weather patterns. Ash fall do not cause many deaths but can lead to breathing difficulties.

Volcanic gases: Gases emitted from volcanoes include, water vapour, carbon dioxide, sulphur dioxide, hydrogen sulphide, helium and carbon monoxide. They rarely cause death but can be problematic as many are heavier than air.

Lahars: Mud flows of volcanic material due to ash and debris mixing with water. On steep slopes speeds can reach 22 metres per second.

Volcanic landslides: These are slides of rock and loose volcanic material, which are driven by gravity.

Tsunamis: A rare event, but the 1883 eruption of Krakatoa, did create a 30m high tidal wave.