Volcanic Hazards

Define Lahars

Mudflows composed of pyroclastic material and water which flows from a volcano, often through river valleys

Pyroclastic Flow (Nuées Ardentes)

High density mass of gases, hot ash and tephra that flows rapidly down the sides of volcano

Describe the Nature of Volcanoes on Constructive Plate Margins

• Where plates move apart and magma is forced to the surface. Also along rift valleys (e.g. Kilimanjaro in The Great African Rift Valley)

• Volcanoes here have fairly gentle sides due to low visc. and basaltic lava

• Eruptions are effusive (frequent and relatively gentle)

Describe the Nature of Volcanoes on Subduction Zones

• e.g. the Pacific ‘ring of fire’ is associated with this process

• The deeper the plate descends, the hotter the surroundings become. This melts the oceanic plate in the Benioff zone.

• The molten magma exploits weaknesses in the crust and rises as columns of magma. The magma collects in sub-surface reservoirs call PLUTONS.

• Viscous andesitic lava creates complex , composite explosive volcanoes.

• Creates island arcs if happening offshore.

Describe the Nature of Volcanoes over Hotspots

• The Hawaiian Islands in the northern Pacific are SHIELD VOLCANOES

• They have low angle slopes and low viscosity lava that floes great distances from the volcanic vent itself.

What defines the Magnitude of Volcanic Events?

• They only become hazardous when they impact upon people and the built environment e.g. fatalities and injuries, damage or destruction to buildings, disrupting economic activities or agriculture.

• The Kamchatka peninsular is the most geologically active part of the world (29 active volcanoes), but due to such a small human population, they are not considered a hazard.