The geology of New Zealand

This aerial view into the Red Crater of Mount Tongariro with the green Emerald Lakes in New Zealand reveals the extraordinary and ever-changing beauty of explosive volcanism. Mount Tongariro is a 1978 m (6490 ft) high, steep stratovolcano on the North Island of New Zealand and belongs to the Taupo volcanic zone. The volcano consists of an explosive type of lava called andesite, interspersed with pyroclastic layers of tephra; tiny rock fragments formed during volcanic explosions. Mount Tongariro first erupted 275.000 years ago. The huge volcanic complex consists of 12 individual cones. Red Crater is one of them and last erupted ash in 1926 and still contains active fumaroles. One of the explosion craters at Mount Tongariro filled with water and created the beautiful Emerald Lakes.
About 85 million years ago, when dinosaurs roamed the earth, the ocean floor of the Pacific Ocean consisted of many individual tectonic plates. Due to the continuing spreading, drifting and subduction of the volcanic seafloor, most of these plates were subducted into the interior of the Earth and melted along the Ring of Fire surrounding the entire Pacific Ocean. At present, the Pacific Plate consists of a single gigantic plate. Its tectonically active boundaries form deep-sea trenches and the famous volcanoes of the Ring of Fire, including such notorious supervolcanoes like Krakatoa in Indonesia. One of these huge and explosive volcanoes is New Zealand’s Mount Tongariro.
The geological processes of the Ring of Fire in the region of New Zealand are complicated. Geologically New Zealand is a sunken continent, a fragment of the former southern supercontinent Gondwana and consists of continental granite. However, it is also interspersed with volcanic rock. The collision zone of the Pacific and Indo-Australian ocean plates stretches exactly through New Zealand, beneath the volcanoes of the Tongariro chain.
The alpine climate around Mount Tongariro allows abundant snowfall in winter, but is currently not sufficient to form glaciers. During the last ice age cycles, however, valley glaciers existed here about 18,000 years ago, leaving behind their moraines and cirques.
The green color of the Emerald Lakes is created by sunlight reflected from a white layer of marl on the bottom of the lakes. Marl is a white clay of calcium carbonate. It is formed when carbonate from dissolved limestone reacts with calcium in supersaturated water. The clay precipitates and deposits on the lake floor as sediment. The limestone itself was transported here by the glaciers of the last ice age and consists of tiny fossil shell fragments. They lived about 150 to 200 million years ago in a coral sea, very similar to today’s Great Barrier Reef in northeastern Australia. It is remarkable that geological processes over millions of years have resulted in these organisms landing in a crater lake of an active volcano.