When most of us think about a volcanoes, we picture a cone-shaped mountain, like Mt Egmont in the North Island or Mt Fuji in Japan. And it probably has clouds of white ash rearing up in a huge cloud from its top and raging rivers of red hot lava pouring down the slopes.
Some are like this. But only a few. Volcanoes are complex and different in both the way they look and how they behave.
It's common for volcanic debris to become mixed with water and form thick, rock-laden slurries that suddenly break out and pour rapidly down slopes. The water may come from melting of snow and ice. Or, from the up welling of a crater lake at the top of the mountain. But whatever they are composed of these are known as lahars, and they're destructive and dangerous.
There are excellent examples of lahars on the Summit Road on Banks Peninsular.
Most volcanoes have a crater, at or near the summit. These craters could be just a few hundred metres across from one side to the other. But some are tens of kilometres in diameter.
Craters are formed by explosion, subsidence, erosion, or a combination of these. Probably the best known crater is that formed in 1980 at Mt St Helens, USA, through collapse of a sector of the cone which triggered a violent eruption.
Erosion craters, or Calderas, are formed when a large volume of magma drains from the underlying chamber, and a cylindrical block of the superstructure collapses into the void beneath. Lyttelton and Akaroa volcanoes are two of the world's best examples of erosion craters.
Most volcanoes have soft centres made mostly of pyroclastic materials. This means they are more erodable at the center than the uniform, lava-coated flanks.
On conical volcanoes, such as Mt Egmont or Mt Fuji, streams cut radial drainage systems deep into the volcanic materials on the gently sloping outer flanks. Often one of these stream channels becomes dominant and when its head cuts back into the central area of the volcano, a sizeable crater may be excavated quite rapidly.