Anna Meikle, of Ardgowan School asks :-

What makes a volcano explode?

Margaret Bradshaw, a geologist who regularly works in the Antarctic, responded.

Volcanoes are points on the Earth's surface where melted rock pours out from deep inside the earth.

A volcano explodes because gases trapped within the melted rock become bottled up be- neath an obstruction in the vent. It is rather like a bottle of fizzy drink that has been vigorously shaken. Everything looks alright until the top is removed, at which point the pressure is re- leased, and the gas expands to make the liquid froth up and out of the bottle neck.

Some volcanoes explode more violently than others do. Volcanoes which have thick, sticky lava tend to block the vent more easily. Below the "stopper", gas pressure steadily increases in the lava below ground level until it blows the "stopper" out, causing a tremendously violent explosion which may blow an enormous hole in the ground. Lake Taupo in the North Island is believed to have formed in this way from an eruption 25,000 years ago. Other volcanoes with more runny lava do not block up so easily and explosions are much smaller.