Feature

A Blast from the Past

The huge Taupo volcanic eruption landscaped the central North Island.

Janet Wilmshurst

Hidden under Lake Taupo, in the central North Island of New Zealand, lies one of the most geologically active and violent volcanoes known. Although the Taupo volcano has been dormant for the past 1,900 years, its last blast was larger than any other eruption in recorded history.

If the total volume of ejectmenta produced by the Taupo eruption landed on Manhattan, it would have covered the city from Wall Street to the Empire State building, and stood twelve times higher than the World Trade Centre. Even recent eruptions from Mt Pinutabu, Mt St Helens or Tambora look pocket-size in comparison to the sheer violence of the Taupo eruption.

Although Polynesians did not colonise New Zealand until about 850 years ago, they would certainly have heard the thundering explosions from the Taupo eruption on their neighbouring Polynesian islands. The smaller Tambora eruption in 1815 was heard over 2,000 kilometres from the volcano, and the infamous 1883 Krakatau eruption, over 5,000 kilometres away.

During the Taupo eruption, airfall deposits, called tephra, were dispersed over 30,000 km² of land east of the vent, and spread at least 800 kilometres off the east coast of the North Island. During the final phase of the eruption, a ground-hugging pyroclastic flow consisting of ash, pumice and hot gases travelling at 600-900 kilometres per hour, smothered a roughly circular area 70-90 kilometres out from the vent. The rock formed by this deposit is known as the Taupo Ignimbrite and everything in its path was buried. Despite its relatively small volume, the Taupo Ignimbrite covered 20,000 km², the largest documented area ever covered by a pyroclastic flow.

Although the Taupo eruption has been studied intensively by vulcanologists, little is known about the effect of the eruption on central North Island forests at the time of the blast. For example, how much destruction had one of the world's largest volcanic eruptions caused, and how long did the vegetation take to recover from such an extensive and large-scale disturbance?

Pollen-Based Time Machine

Because of the relatively recent settlement of New Zealand by Polynesians, a unique opportunity existed to measure the impact of a major volcanic eruption on vegetation unmodified by humans, and compare this with the effects of human settlement. I unravelled some of these prehistoric mysteries using a technique that some consider the next best thing to a time machine (although not quite as exciting) by counting pollen grains contained within peat and lake sediment deposits immediately overlying Taupo Tephra at sites around the central North Island.

Most flowering plant species, or at least taxonomic family groups, have uniquely sculptured pollen grains. Thus the counted pollen assemblages in a sample from a peat or lake sediment core can represent the source plants that were present on the landscape at a certain point in time. Pollen grains preserve well in peat and lake sediments, and because these microfossils accumulate in the sediments year after year, they remain entombed in chronological order. The sediments can be collected intact by retrieving long cores from peat bogs or the bottom of lakes, and the pollen counts can be read like the pages of a book.

It's like having 2,000 years of city newspapers stacked in chronological order, removing one paper from the stack at five- r ten-year intervals, and reconstructing the city's history using the information in the sampled newspapers. Instead of newsprint for clues, pollen grains provide the information for the vegetation history, with modern pollen/vegetation relationships as guiding analogues. With Matt McGlone, a colleague at Landcare Research, I looked at pollen assemblages in peat and lake sediment cores taken from sites at different distances from the Taupo vent.

Making sense of all these pollen profiles has been like putting the pieces of a jigsaw puzzle together, but without a picture on the box for guidance. The images that emerged from our completed puzzle allowed us to understand the events that followed the enormous Taupo eruption. Vegetation within an 80-km radius around the vent was smothered by the Taupo Ignimbrite and completely devastated. Unlike the ash-covered vegetation, nothing survived burial by this searingly hot deposit, which reached temperatures of 800-1,000^oC and consumed everything in its firey path. In fact, the remains of charred tree trunks and vast amounts of charcoal can still be seen trapped within the Taupo Ignimbrite in roadside cuttings. Pollen records from sites that were inundated by the flow reflect this destruction by the sudden and rapid decline of pollen from the tall forest giants such as rimu, matai, totara and miro, and a host of smaller canopy and understory trees and shrubs.

As the Taupo Ignimbrite steam-rollered its way over the landscape, it covered hillsides and filled in valley bottoms creating deposits in some areas up to 30 metres thick. Even mountain ranges over 1,500 metres high did not cause the pyroclastic flow to be diverted. It swept across all but one of the mountains in its pathway, and only came to a halt when it ran out of material.

Pollen records from the upland ranges south-east of the vent show a rapid decline of beech pollen immediately above the Taupo ash layer, reflecting the devastation of the beech dominated forests that grew on these ranges. Pollen profiles from areas that were only affected by airfall deposits at least 160 kilometres due east of the vent also show there was significant vegetation disturbance after the eruption.

Although pollen profiles cannot reveal the exact cause of plant death, we can make inferences from observations of damage caused by volcanic eruptions this century. For example, branches would have been weighed down by ash, low growing plants buried, leaves smothered and plants poisoned by toxic chemicals.

Lightning strike fires were also an important cause of forest destruction during the eruption. Electrical discharges from ash- aden clouds would have ignited fires as the plume travelled eastwards from the vent. Intense lightning episodes were observed during the 1886 Tarawera eruption in the central North Island, with electric flashes frequently ending in fire-balls that caused small forest fires in the Tarawera district. There was also intense lightning during the 1980 Mount St Helens eruption which caused hundreds of small fires.

Fires would have broken out for as much as a century after the Taupo eruption, as vast amounts of decaying vegetation from damaged forests and bracken fern provided a readily combustible fuel supply. Exposed, stripped crowns and uppermost branches of tall emergent trees that were still standing after the eruption may also have acted as lightning conductors and increased the chances of lightning-strike fires. Evidence of these fires can be found as charcoal fragments sandwiched in the tephra deposits and in the overlying sediments, as far away as 200 kilomtres from the vent.

The landscape probably looked moth-eaten and war-torn after the eruption; an untidy mosaic of mutilated and recovering forest. In small forest clearings the main plants to prosper were those capable of rapid invasion and establishment including bracken and grasses. The pollen evidence shows that although destruction was widespread, it was also very variable, with ash thickness, drainage, topography and climate all influencing the degree of damage. One of the most surprising discoveries from the pollen evidence was the relatively short time it took for the forests to recover after such a large eruption. Within about 200 years, tall forest had reestablished even at sites that were devastated by ignimbrite.

Human Eruption

How did the Taupo eruption compare with the effects of human settlement in the same area? The first indication of permanent human settlement in New Zealand is clearly visible in the pollen profiles. Throughout the country, these profiles show the same pattern of change that suggests widespread deforestation by fire. There was a rapid and sustained decline of pollen from forest species together with a massive increase of charcoal fragments. Bracken and shrubs replaced the forest and remained the dominant type of vegetation until Europeans arrived in the 1800s, by which time over 50% of the forest had been destroyed.

Clearance by European settlement further reduced the amount of total forest cover in New Zealand, which now stands at less than 25%. Radiocarbon dates from sites throughout the country indicate clearance began about 850 years ago, making New Zealand one of the last large landmasses to be colonised by humans. The timing of this widespread event coincides with radiocarbon dates for numerous faunal extinctions and the oldest recovered archaeological artefacts.

Although the type of vegetation changes recorded after the Taupo eruption were basically the same as those recorded after anthropogenic deforestation, the contrasts are far more striking. Even considering the enormity of the Taupo eruption, the impact of human settlement on New Zealand forests was much more destructive -- like comparing a fire cracker with a stick of dynamite. The reason for human settlement creating such large and quasi-permanent change was mainly a result of the sustained and ongoing nature of human clearance and land development which prevented the chances of regeneration and recovery.

The Taupo eruption was one of the largest to affect the central North Island postglacial forests, but it was just one of many eruptions that caused devastating changes to the landscape and vegetation throughout the last 10,000 years. In addition to volcanic eruptions, rain, wind, erosion and earthquakes have all contributed to the ongoing remodelling of the environment.

Perhaps what is so remarkable about these repeated natural disturbances, including ones the size of the Taupo eruption, is that the vegetation always recovered after each disturbance. However, the volcanic violence of the past 10,000 years pales in comparison to the widespread and sustained environmental damage caused by human settlement over the last 850 years.

This may be slightly ironic, because if a similar sized eruption to the last Taupo eruption occurred today, about 200,000 people would be at risk from pyroclastic flows, heavy ashfalls, lightning strikes, fires and flooding.

Although we have modified our environment beyond the extent of most natural disturbances, we still remain extremely vulnerable to unannounced live performances from dormant monsters like the Taupo volcano.

Volcano Cam

If you're halfway around the world and you want to take a look at what Mount Ruapehu is up to, you can courtesy of VolcanoCam, an Internet-linked videocamera operated by Victoria University.

The videocamera is set up in the manager's lounge at the Chateau, looking out the window at the nearby volcano. A laptop computer takes the video image and coverts it to a still image for transmission over a cellphone to NetLink, the university's commercial Internet group. From there, it's put on the VolcanoCam World Wide Web page for all to see (at http://www.actrix.gen.nz/ruapehu).

The image is updated about once a minute, about as much as the Telcom-donated cellphone link can handle effectively. Information on the volcano, the best image captured to date and a host of related material can be accessed through various links on the Web.

Julian Meadow, of TeleConsultants, was one of the team who put the system together, and enthuses about the role such "hyper-live-reality" can play.

"I [can] now go home, open up my laptop, dial-in over two loosely bount, prehistoric copper cables and get live pictures in my living room of a mountain four hours drive away."

Since announcing the availability of the images, Meadow has has been startled and gratified by the response. A school in Denmark has checked it out as part of their earth sciences studies, Kiwis overseas have been on-line to see if reports of North Island evacuations were correct, and one visitor noted that the idea was in the spirit of Maurice and Kattia Kraft, a French volcanologist couple who lost their lives documenting a volcanic eruption in Japan.

A view rating, based on weather forecasts, local knowledge and the state of the equipment, provides an indication of how good the viewing opportunity may be. It ranges from 0 or Zilch chance (noted as likely to occur with equipment faults or a really large eruption) through to 10, rated as Frontpage material.

Most of the time recently, there hasn't been a lot of detail -- cloudy days and ashy haze, along with some equipment problems, have conspired to produce a nice picture of the trees next to the Chateau but not a lot else. One British Web surfer was puzzled to find the image pitch black, until the 12-hour time difference was pointed out to him.

Dr Janet Wilmshurst is a palaeoecologist with Landcare Research in Lincoln.