FEATURES OF VOLCANIC ERUPTIONS AT DESTRUCTIVE PLATE MARGINS
Andesitic and rhyolitic magmas are viscous, so gas cannot easily escape from them. As the
magma travels up towards the surface, pressure is reduced and the gases expand. When the
gas finally escapes, it does so explosively, blasting volcanic material out across the surrounding
area. Fragments ejected during an explosive volcanic eruption are called pyroclasts from the
Greek words pyro, ‘fire’ and klastos, ‘broken’, or tephra, from the Greek word for ‘ ash’.
NUEES ARDENTES: Mixture of ash and gas which can travel at very high speeds – 50 m per
sec. This is a dense ground hugging cloud containing poisonous gases – sulphur dioxide,
carbon dioxide, hydrogen sulphide. Very dangerous , e.g. 30 000 people died when Mt Pelee in
Martinique exploded in 1902. May change direction if the wind changes.
PYROCLASTIC FLOWS: solid materials – pumice, rock etc as well as ash and gas, Travelling
at between 20 and 50 m per sec. These move downhill in response to gravity and again they are
very destructive , e.g. Montserrrat, 1997.
LAVA FLOWS: at destructive margins tend to be viscous,as the lava is high in silicates, so thay
do not flow far. Gas is trapped in the lava – as it cools it forms pumice.
IGNIMBRITES are rocks formed when particles of pumice and ash from poorly sorted
pyroclastic flows are welded together.
PYROCLASTIC FALL DEPOSITS
These are deposited from the air.
NAME
SIZE
VOLCANIC BOMBS
BLOCKS
LAPILLI
ASH
SCORIA OR PUMICE
Over 10mm
Over 65 mm
2 – 65 mm
Under 2mm
Variable
STATE DURING
EJECTION
Molten or plastic
Solid
Solid or molten
Solid or molten
Solid or molten
SHAPE
Spindle
Angular
Round or angular
Very fine
Vesicular
ADDITIONAL HAZARDS
LAHARS: Volcanic mudflows. Hot ash mixed with water from heavy rain, melting glaciers, or
draining crater lakes. Lahars can move downslope at up to 80 km/hr.
Deposits are finegrained and clasts are orientated in the direction of flow.
Example: Nevada del Ruiz, 1985. Lahars affected a very densely populated area of the Andes
in Colombia, killing 23 000 people.
TSUNAMIS: More frequently associated with earthquakes than volcanoes, but a large volcanic
eruption may displace the seafloor – e.g. the tsunami following the eruption of Krakatoa in 1883
caused the death of 30 000 people.