In explosive volcanoes the power of the eruption pulverises rock into a fine ash inside the vent and cone. The ash is thrown many kilometres into the sky and may be carried by winds across the world and can disrupt air travel. When lava is thrown from the volcano it is whipped up and becomes full of air
bubbles. If it cools quickly enough these bubbles are trapped in the rock and it is now called pumice. Some pieces contain so much air that they float in water.
5. Water vapour
Many volcanoes along subduction zones release huge amounts of water vapour (from the sea water carried into the mantle by the sinking oceanic plates). As this vapour rises into the air it cools rapidly creating torrential rain during the eruption. The intense rainfall can trigger lahars, e.g. Mount St Helens, 1980.
IN DEPTH - Lahars
Lahar is an Indonesian word that describes a mixture of water and rock fragments flowing down the slopes of a volcano. When moving, a lahar looks like a mass of wet concrete that can carry rock debris up to 10 m in diameter. Lahars vary in size and speed. Small lahars less than a few metres wide and several centimetres deep may flow at speeds of a few metres per second. Large lahars hundreds of metres wide and tens of metres deep can flow at over 80 km/h – much too fast for people to outrun, e.g. Nevado del Ruiz, Colombia. Volcanic eruptions may trigger one or more lahars by quickly melting snow and
ice on a volcano. Usually lahars are formed by intense rainfall during or after an eruption – rainwater can easily erode loose volcanic rock and soil on hillsides and in river valleys. Some of the largest lahars begin as landslides of saturated rock on the sides of a volcano.
OLCANOES
Fig. 7 A lahar flows from Mount St Helens’ crater. 41