Pyroclastic flows and lava

Damage from lava

Heimaey 1973

In 1973 the Edfell volcano erupted behind the town of Heimay in Iceland.

Initially the town was covered in ash, but soon after this lava began to flow down into the town and the harbour.  Houses that had lava flow into them were set on fire and eventually pushed over.  Concern mounted over the harbour being sealed off because of its fishing industry, the major employer of the town.  All efforts were put into trying to divert the lava flow from entering the harbour entrance and thus keep it open.  By using every available pump and fire hose, the lava was cooled and eventually diverted.  However, 300 homes were lost to lava flows and another 80 collapsed under the weight of the ash.  No lives were lost due to an emergency plan well executed.

The eruption lasted 4 months and extended the island considerably.

One of the problems of lava is dealing with the immense heat and flows.  It is not always possible to divert lava flows but luckily for Heimay, the towns engineers succeeded.

Pyroclastic Flow

Mt Unzen Japan (1991)/Mt Pinatoubo (1991)

In November of 1989, an earthquake swarm broke nearby and continued throughout the succeeding year. In May of the following year (1991), fresh lava began to emerge. This forced authorities to order the evacuation of some 12,000 residents. Despite the precautionary measures made against it, the volcano managed to claim 43 lives when it eventually erupted.

A pyroclastic flow is a mixture of solid to semi-solid fragments and hot, expanding gases that flows down the flank of a volcano.  The gases contain sulphur oxides, carbon dioxide which are fatal if inhaled in significant quantities. Pyroclastic flows are heavier-than-air and move much like a snow avalanche, except that they are extremely hot, contain toxic gases, and move at often over 100 km/hour. They can travel up to 50 km away from the site of volcanoes that produce them.

They are the most deadly of all volcanic phenomena.  Pyroclastic flow killed the French volcanologists Katia and Maurice Kraftt when Mount Unzen erupted. A Japanese colleague described Maurice's body as being completely carbonised (turned into carbon) from the immense temperatures.