Taupo Volcanic Zone: Okataina, Rotorua, and Taupo

GNSscience

GNSscience

The Taupo Volcanic Zone on New Zealand’s North Island hosts a wide variety of volcanoes. White Island is there, as are Mounts Ruapehu and Ngauruhoe.

These impressive stratovolcanoes have arisen on the outer edges of the Taupo Volcano Zone, where the degree of spreading is relatively small compared to the central section.

So, what’s in that very stretched out TVZ center…supervolcanoes?

Yes. New Zealand’s North Island hosts two: Okataina and Taupo.

Let’s also include a look at Rotorua caldera today, which has had a VEI 7 eruption. (Some experts have informally set the minimum volume for supersizing an eruption at roughly 72 cubic miles, which falls within the VEI 7 range. For comparison, the 1980 Mount St. Helens had a volume of around 0.1 cubic mile!)

Southern Zealandia, per Alexander Karnstedt.

Southern Zealandia, per Alexander Karnstedt.

New Zealand and Volcanism

Geologists consider New Zealand’s North and South Islands to be part of a Western Europe-sized continent called Zealandia that’s mostly submerged.

The geologic history is quite complicated here, but plainly put, volcanism is the only reason why the North and South Islands sit above sea level today.

Modern New Zealand began forming around the same time that the Pacific tectonic plate started subducting underneath the Australia plate, some 20-25 million years ago. Geoscientists are still trying (PDF) to figure out why that subduction began.

I have no idea which came first, the volcanism or the subduction. Let’s just watch a cool video from the Auckland Museum about how the North and South Islands developed.

Of course the museum focuses on the  Auckland hot spot, but other big blasts were happening on the North Island at the same time. 

Researchers have identified as many as eight calderas in the TVZ. Today we’ll just look at Okataina, Rotorua, and Taupo.

Okataina and Rotorua are neighbors.  Image source

Okataina and Rotorua are neighbors. Image source

Okataina

Somewhere between 400,000 and 50,000 years ago – scientists can’t be more precise – a series of eruptions at Okataina blew out some 120 cubic miles of volcanic tephra that buried the central North Island under as much as 300 feet of ignimbrite (the remains of pyroclastic flows).

The roof of the magma chamber then collapsed, forming 11 x 16-mile-wide Okataina Caldera.

From 50,000 to 25,000 years ago, the volcano erupted eight more times. Two of these were ignimbrite eruptions, the other six “merely” a little bigger than Krakatoa’s infamous 1883 eruption. Each of the eight covered the central part of North Island with ash and rearranged the original caldera’s shape.

Starting some 21,000 years ago, a series of 11 eruptive episodes built up Tarawera, Haroharo, and other vents within the Okataina complex. Most of these episodes were explosive, with multiple vents showering the Bay of Plenty area with pyroclastic flows and then wrapping up their activity by extruding thick rhyolite flows and building lava domes. (If you like geochemistry, White Island, Ruapehu, and Ngauruhoe volcanoes, in contrast, tend to have andesitic eruptions.)

Source

Tarawera’s White Terraces. Source

By 1886, the Tarawera vent region was a beautiful place, with hot springs and sinter terraces that rivaled those of Yellowstone both in beauty and fame.

A little after midnight on June 10, 1886, earthquakes rocked the area. Around 1:30 to 2:30 a.m., a huge and very explosive fissure eruption began – they heard it from Auckland to Christchurch.

The terraces disappeared. Local villages were buried, killing about 120 people. It was New Zealand’s most deadly eruption since the island was settled.

This was no supereruption – it was “only” about as strong as the 1980 Mount St. Helens eruption. It’s a reminder that supervolcanoes do also have common-sized eruptions.

Today you can check out the last 24 hours of seismicity at the Okataina Volcanic Center here.

It isn’t all online, but scientists do monitor Okataina with 8 seismographs, 7 continuous GPS stations, and lake measurements. This caldera is currently showing no unrest and its aviation code is green.

Rotorua

Some 140,000 to 225,000 years ago, this volcano erupted some 35 cubic miles of material. It blanketed parts of the North Island in as much as 475 feet of ash.

NASA

The lake takes up roughly the northeastern half of Rotorua Caldera. NASA

As at Okataina, the roof of Rotorua’s magma chamber collapsed once the chamber had emptied, forming a 14-mile-wide caldera.

Unlike Okataina, Rotorua did not then develop a series of active vents. Its most recent eruptive activity since that one and only “big one,” as far as I can tell, has been the construction of three rhyolite lava domes, possibly from a set of small magma bodies. This happened about 25,000 years ago, around the time when the Pacific plate subduction under Zealandia was starting up.

I did find mention (PDF) of “at least one lesser eruption (of the order of 1 km3 [0.2 cubic mile]) between 42,000 and 13,800 years ago.” This may refer to the domes, or perhaps it describes another event.

There is a lot of geothermal activity at Rotorua now, including geysers as well as a number of steam explosions, a/k/a/ hydrothermal eruptions. The most recent explosions there happened in 2005.

Today, Rotorua is showing no sign of unrest, and its aviation code is green. You can monitor its webicorder here.

Taupo

Around 1,800 years ago, Earth’s most violent eruption over the past 5,000 years happened at Taupo…and yet that wasn’t this volcano’s “big one.”

Taupo began operations about 300,000 years ago, or some time after an unidentified “large caldera,” somewhere in the central TVZ, had erupted more than 240 cubic miles of ignimbrite.

Taupo’s early history is complex and may have involved at least one caldera-forming eruption. Scientists have definitely identified at least 28 eruptions there that are recent on the geologic time scale.

Five of them happened between 65,000 and 27,000 years ago from vents that are now submerged in Lake Taupo. Then came the Oruanui supereruption, 26,500 years ago, when the volcano blasted out 127 cubic miles of magma, devastating much of the North Island.

This colossal event was the world’s most recent supereruption.

The Oruanui eruption laid the basis for Lake Taupo.  Yes, you need satellites to see New Zealand's calderas.  NASA

Yes, you need satellites to see New Zealand’s calderas. The Oruanui eruption laid the basis for Lake Taupo. NASA

The next 21 eruptions varied in intensity from VEI 3 (on the scale of the 1913-1944 eruption of Vesuvius) to VEI 6 (about 1883 Krakatoa size). Taupo has a chaotic eruptive pattern, erupting randomly. The intervals between these eruptions were anywhere from 50 to 5,000 years.

Then, in 181 AD, came what scientists call the Hatepe eruption. Some just call it the Taupo eruption. It wasn’t a supereruption this time, but it was incredibly violent. Experts say:

The first phases of the eruption produced a series of five pumice and ash fall deposits over a wide area of the central North Island, especially east of Taupo and beyond Napier into Hawke Bay. The eruption culminated with a large and very energetic pyroclastic flow that devastated an area of about 20,000 km2 [over 7,700 square miles] and filled all the major river valleys of the central North Island with pumice and ash. These pumice deposits can still be seen today and many of the major rivers in the North Island carry large amounts of this pumice when in flood. Rounded pumice found on the beaches of the North Island have come from this eruption.

At its worst, this eruption blew some 5.5 cubic miles of material 31 miles into the sky – the most powerful plinian blast ever documented.

Taupo hasn’t erupted since then.

It’s difficult to predict what Taupo’s next eruption will be like or when it will happen. Per GNSscience:

The most likely event will be a small – to medium-sized explosive rhyolite eruption, and the growth of a rhyolite dome. Basalt and dacite eruptions are also possible but less likely. A future vent will most probably be within Lake Taupo, between Horomatangi Reefs and Motutaiko Island. Tephra fall is most likely to be to the east or northeast, and may extend to the east coast of the North Island.

The volcano has had several minor episodes of unrest recently:
 

GNSscience constantly monitors Taupo Volcano by using Lake Taupo as a “spirit level,” as well as with 7 seismographs and 6 GPS stations.

Taupo currently is showing no signs of unrest, and its aviation code is green. You can check out its webicorder here. (Note: Webicorders pick up signals from traffic, wind and other natural events, and earthquakes around the world. This is why you may see what looks like activity on a webicorder when a volcano hasn’t been put on alert.)

Thus ends our visit to the Taupo Volcanic Zone. I don’t know any Kiwis personally, but consistently throughout all the videos I have watched and articles I have read, New Zealanders express great pride in the country’s volcanoes, as well as a sober acceptance of the risks they present. Such people perfectly complement the country’s natural energy and beauty.

 


 

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Front page image: GNSscience

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