The crater is the funnel shaped depression at the top of the vent in a volcano. There may be more than one crater, just as there may be secondary vents and parasitic cones on the side of the main volcano.
Where the lava at the top of the vent cools off, there may be the potential to water to collect in the depression and form a lake.
They also form in maars: large circular craters which are the result of previous hydro-volcanic eruptions.
Gases from beneath the earth are likely to continue to be emitted, and the bottom layers of the lake are therefore full of dissolved gases, which can include hydrogen sulphide (the ‘rotten eggs’ smell, which is also produced by the action of a car’s catalytic converter), carbon dioxide and even cyanide.
The sulphur is also deposited as a solid at the entrance to small vents (fumaroles) at the edges of the main volcanic crater (these are known as flank eruptions)
(Sulphur mines are seen in the video extract)
These were produced by the incident on Mt. Ruapehu featured in the video. Lahar is an Indonesian term for a volcanic mudflow. These lethal mixtures of water and tephra have the consistency of wet concrete, yet they can flow down the slopes of volcanoes or down river valleys at rapid speeds, similar to fast-moving streams of water (up to 100kmh). These mud slurries carry debris ranging in size from ash to lapilli, to boulders more than 10 meters in diameter. Lahars can vary from hot to cold, depending on their mode of genesis. The maximum temperature of a lahar is 100 degrees Centigrade, the boiling temperature of water. They can travel over 300km. It is not a ‘primary product’, but can be more devastating than a lava flow.
Lahars will tend to follow existing river valleys. When the lahar comes to a halt, the mud will set like concrete. They can therefore block lines of communication. In the case of Mt. Ruapehu they cut across ski pistes and affect tourism.