In the middle of New Zealand's North Island is a large lake called Taupo. Lake Taupo is actually a huge caldera, which is a depression left after a massive volcanic eruption. 

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The Lake Taupo eruption of around 230AD was the biggest on earth in the last 5,000 years. The eruption itself was not witnessed by humans, as far as we know, but the peculiar sunsets and ash clouds were possibly recorded half a world away, by the Chinese. There is more than one crater in the caldera (or floor of Lake Taupo) and one of them seems to blow at several thousand-year intervals.  Taupo had an even bigger eruption 26,500 years ago. (The Oranui eruption). This may be the eruption that caused the caldera to form by collapse into the magma chamber. Most of the ignimbrite flows around Taupo came from this eruption. Since then, the magma from eruptions has become hotter. When eruption clouds combine with water, sticky rain produce a layer of tephra mud which is draped over the older ignimbrite rocks.

There have been much smaller eruptions from Lake Taupo and it is suspected one may be coming up in the near future. A reef in the south west corner of the lake is slowly rising at the moment - so watch this space! The vulcanologists say it will be a small eruption, if any, to make a rhyolite dome, of which New Zealand has many. However predictions about volcanoes are as good as earthquake predictions - rather poor. If Lake water meets hot magma in a confined space, the result is not always gentle!

The biggest volcanoes do not have classical cone shapes. Taupo is probably the most powerful caldera in the world, yet looks like an innocent lake. Smaller calderas are near by, in New Zealand's volcanic zone.

Although this is an active part of the Pacific Ocean's "Ring of Fire" you are very unlikely to see an eruption during a visit to New Zealand. It is no more dangerous than California and Japan, which are on the same volcanic ring.

Hot springs and geysers are common near the edge of a caldera. This is the Lady Knox Geyser, south of the Rotorua caldera. The caldera is currently filled by Lake Rotorua (Press on the picture for a bigger and better version) Photo by Paul Maggs

Lake Rotorua at sunrise

That is surely not a picture of a volcano!
But it truly is.

The peaceful lake is actually sitting in the Rotorua caldera, which erupted long before Lake Taupo and is responsible for the extensive flat topped hills to the west of Rotorua. Those hills are made of ignimbrite, "fire rock", named originally by the New Zealand geologist, Prof Marshall. (The late Roger Marshall, a famous New Zealand Radiologist, often told me of trips he took with his father, checking the abundant evidence of volcanism in central North Island.) 

Many innocent-looking landscape features tourists pass without a thought are actually the result of violent eruptions. The flat topped ignimbrite is not in the picture - it is behind us. The black swan is a native of Australia, but common on New Zealand waterways.

Photo by Paul Maggs

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The pyroclastic flow from the Taupo eruption did not manage to surmount Mt. Ruapehu and ancient forest is preserved on the protected southern slopes of the big volcano, below the Turoa ski field.

(The Turoa field is to the right of the crater, this view from the west.)

 

Craters of the Moon

On the edge of the Taupo caldera, hot springs are found where ground water has sunk down to the hot magma close below and then returned to the surface as steam. The Craters of the Moon is a park near Taupo township and adjacent to the hydrothermal power station at Wairakei. Bores drilled into the hot ground-water layer supply steam to drive generators in the power station. The Waikato River provides cooling water and a tropical shrimp farm uses the hot discharge. The shrimps cannot survive in the usual cold temperature of the river: in fact they do not normally live in New Zealand at all. 

Since the hydrothermal power station started using up ground water, the fumaroles and mud-pools have become even more active. There is less water for cooling the magma and the ground cavity, previously packed with hot water, is now filled by high pressure steam, which breaks out at the Craters of the Moon. Much of the water from the power station is pumped back into the ground, but this is not enough to replace the losses.

Vegetation growing beside the steam vents includes algae and plants demanding a warmer climate than Taupo provides, but they can survive in the hot, moist craters. Plants away from the vents include poor quality scrub (Manuka shrubs). Some shrubs are dead, killed by heat. Steam rising through the dead, distorted trunks gives a look of devastation to the park. The craters are close to the main road and there is no charge to stroll amongst them, on good board-walks. (They look nothing like moon craters, incidentally!)

2002: There has been an eruption of one of the craters of the moon. Nobody was injured - it was just a big steam blast.

     
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Volcanic sinter: Bridal veil falls, Waiotapu