Taupo Volcano
, in the centre of New Zealand’s North Island, is the caldera of a large rhyolitic supervolcano called the Taupō Volcano. This huge volcano has produced two of the world’s most violent eruptions in geologically recent times.
The Taupō Volcano forms part of the Taupō Volcanic Zone, a region of volcanic activity that extends from Ruapehu in the South, through the Taupō and Rotorua districts, to White Island, in the Bay of Plenty region.
Taupō began erupting about 300,000 years ago, but the main eruptions that still affect the surrounding landscape are the Oruanui eruption, about 26,500 years ago, which is responsible for the shape of the modern caldera, and the Hatepe eruption, dated 232 ± 5 CE. However, there have been many more eruptions, with major ones every thousand years or so.
Considering recent history alone, the volcano has been inactive for an unusually long period of time, but considering its long-term activity, it was inactive for much longer between 8100 and 5100 BC. Some volcanoes within the Taupō Volcanic Zone have erupted far more recently, however, notably a violent VEI-5 eruption of Mount Tarawera in 1886, and frequent activity of Whakaari/White Island, which erupted most recently in December 2019.
Rhyolitic eruptions
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at:-9999 shift: text: Recent eruptions of Taupō caldera
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The Taupō Volcano erupts rhyolite, a viscous magma, with a high silica content.
If the magma does not contain much gas, rhyolite tends to just form a lava dome. However, when mixed with gas or steam, rhyolitic eruptions can be extremely violent. The magma froths to form pumice and ash, which is thrown out with great force.
If the volcano creates a stable plume, high in the atmosphere, the pumice and ash is blown sideways, and eventually falls to the ground, draping the landscape like snow.
If the material thrown out cools too rapidly, and becomes denser than the air, it cannot rise as high, and suddenly collapses back to the ground, forming a pyroclastic flow, hitting the surface like water from a waterfall, and spreading sideways across the land at enormous speed. When the pumice and ash settles, it is sufficiently hot to stick together as a rock called ignimbrite. Pyroclastic flows can travel hundreds of kilometres an hour.
Earlier eruptions
Earlier ignimbrite eruptions occurred further north than Taupō. Some of these were enormous, and two eruptions around 1.25 and 1.0 million years ago were big enough to generate an ignimbrite sheet that covered the North Island from Auckland to Napier.While Taupō has been active for 300,000 years, explosive eruptions became more common 65,000 years ago.
Oruanui eruption
The Oruanui eruption of the Taupō Volcano was the world's largest known eruption in the past 70,000 years, with a Volcanic Explosivity Index of 8. It occurred around 26,500 years ago and generated approximately 430 km³ of pyroclastic fall deposits, 320 km³ of pyroclastic density current deposits and 420 km³ of primary intracaldera material, equivalent to 530 km³ of magma.Modern Lake Taupō partly fills the caldera generated during this eruption.
Tephra from the eruption covered much of the central North Island with ignimbrite up to 200 metres deep. Most of New Zealand was affected by ashfall, with even an 18 cm ash layer left on the Chatham Islands, 1,000 km away. Later erosion and sedimentation had long-lasting effects on the landscape, and caused the Waikato River to shift from the Hauraki Plains to its current course through the Waikato to the Tasman Sea.
Hatepe eruption
The Taupō eruption represents the most recent major eruption of the Taupō Volcano, and occurred about 1,800 years ago. It represents the most violent eruption in the world in the last 5,000 years.Stages of eruption
The eruption went through several stages.- A minor eruption occurred beneath the ancestral Lake Taupō.
- A dramatic increase in activity produced a high eruption column from a second vent, and pumice was deposited over a wide area.
- Water entered the first vent, and mixed with the magma, producing a white ash-rich pumice fall.
- A new vent formed, and produced a dark ash- and obsidian-rich fall deposit.
- A larger eruption ensued, producing pumice over a huge area, and a small ignimbrite deposit.
- The most destructive part of the eruption then occurred. Part of the vent area collapsed, unleashing about 30 km³ of material, that formed a fast moving pyroclastic flow.
- Rhyolitic lava domes were extruded some years later, forming the Horomatangi reefs and Waitahanui bank.
The power of the pyroclastic flow was so strong that in some places it eroded more material off the ground surface than it replaced with ignimbrite. Valleys were filled with ignimbrite, evening out the shape of the land.
All vegetation within the area was flattened. Loose pumice and ash deposits formed lahars down all the main rivers.
The eruption further expanded the lake, which had formed after the much larger Oruanui eruption. The previous outlet was blocked, raising the lake 35 metres above its present level until it broke out in a huge flood, flowing for more than a week at roughly 200 times the Waikato River's current rate.
Dating the Taupō eruption
Many dates have been given for the Taupō eruption. One estimated date is 181 CE from ice cores in Greenland and Antarctica. It is possible that the meteorological phenomena described by Fan Ye in China and by Herodian in Rome were due to this eruption, which would give a date of exactly 186. However, ash from volcanic activity does not normally cross hemispheres, and recent radiocarbon dating by R. Sparks has put the date at 233 CE +/- 13.There was no local population at this time, so the nearest humans would have been those in Australia, more than 2000 km to the west.