Mount Ruapehu & Crater Lake

At 2797 m, Mount Ruapehu is not just the highest peak in the North Island, it is also the highest volcano on mainland New Zealand. It is a stratovolcano composed of several layers (strata) that form a steep volcanic cone which is surrounded by debris of past eruptions. The eighteen small glaciers on the volcano are left-overs of the Otira glaciation and are the only remaining ice caps in the North Island. Geological research has estimated Ruapehu to be more than 250,000 years old.

Mount Ruapehu seems to have major eruptions in a cycle of about every 50 years. The latest occurred between 1995 and 1996 after an increase of seismic activity recorded in the Taupo Volcanic Zone. Crater lakes are usually formed between eruptive periods.

However, the occurrence of potential crater lakes between these cycles are of great concern to the park's officials. The reason for this is the Tangiwai Train Disaster that occurred in 1953. After a major eruption in 1945, a crater began to grow causing debris barriers to break down and give rise to an enormous lahar (debris flow) moving south. The mudflow hit the Tangiwai Bridge with force, causing it to collapse. Six out of nine train carriages crashed into the river, killing 151 passengers.

The Crater Lake is situated to the west of Pyramid Peak (2645 m) and north of Tahurangi Peak (2797 m). It is estimated to be around 100 m deep and to measure between 500 and 600 m in width. Depending on the lake's colour, it indicates volcanic activity by changing its appearance with temperature. Green tones signify colder temperatures, darker tints may indicate the water to rise over 50°C.

Besides sudden eruptions and giant lahars, chemical pollutants formed through crater lakes or released by fumaroles, pose a third major threat within the park. While the majority of volcanoes liberate mostly water in form of steam, they may also release carbon dioxide (CO
2) and monoxide (CO) as well as sulphur (S), chlorine (Cl) and fluorine (F). In combination with the water melted from ice and snow, these substances might form dangerous chemical compounds. These may include sulfur dioxide (SO2), hydrogen chloride (HCl), and hydrogen sulphide (H2S). In fact, Ruapehu's crater is known for its increased amount of sulphur dioxide gas which is stored within the lake. Acid rain, therefore, is very possible to occur.
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