Violent eruptions

Press, 25 August 1988, Page 21

Violent eruptions

One of the Haast Hall of Geology’s most popular exhibits, is the large revolving globe which few tourists pass without tracing their route around the world.

So many people have pointed at Christchurch with the words, “And we are here,” that recently the worn lettering had to be replaced. During April, the globe had its film debut when the National Film Unit used it for the opening shots of a documentary on the Taupo Eruption of 186 A.D.

This eruption is regarded as one of the world’s largest in recent time, so large that it was recorded by Chinese and Roman historians. It was a violently explosive eruption that tore out the depression now filled by Lake Taupo, spewing out vast amounts of material that blanketed the land surface and

reached as far as North Canterbury and as far east as the Chatham Islands. Should such an eruption occur today the effect on the North Island would be disastrous.

Taupo forms part of the Taupo Volcanic Zone, an area which stretches from Mt Ruapehu in the south to White Island, Bay of Plenty, in the north. This zone has been volcanically active for many millions of years, and owes its existence to the melting at depth of the Pacific plate which is being steadily moved westward below the North Island.

Material melted at depth rises to the surface as volcanic lava. Some of this lava is silica rich and viscous (rhyolite), and the associated volcanism tends to be highly explosive because vents become easily blocked — the congealed lava acting like a cork in a bottle of champagne.

The most violent eruption in New Zealand’s recorded history was of Mount Tarawera on June 10, 1886, a rhyolite dome that had been regarded as extinct. Canterbury Museum has several items in its geological collections related to this eruption, including samples of volcanic ash, a map produced one month after the event showing new craters that had formed, and a bound volume containing the official report of the eruption as well as photographs and newspaper cuttings of personal accounts by survivors. The main eruption occurred at 2.10 a.m. on a calm, clear night. The inhabitants of Rotorua were woken by a strong earthquake, a great roar, and the sight of Mt Tarawera “belching out fire and lava to a great height.” A growing cloud of dense smoke began to block out the light of the stars, and there was a rushed evacua-

tion of the town by most people. The Rotorua telegraphist “who manfully and bravely stuck to his instrument in the face of the most dreadful danger” reported that dense ash had begun falling in the town at 4.00 a.m. “accompanied by a suffocating smell,” and that hundreds of boiling springs had appeared, some in the middle of roads. Ash also covered Tauranga which found itself still in total darkness at 9.00 in the morning, causing the Mayor to appeal to his counterpart in Auckland for help evacuating residents. The light of the eruption could be seen as far away as Gisborne, Wanganui, and Auckland, while the explosions were plainly heard in Blenheim, where it was thought to be a large gun firing in distress.

Later it was discovered that an enormous amount of ash had fallen close to Mount Tarawera, killing many people. Eye witness accounts from the few survivors record violent hailstorms, and a rain of heavy volcanic stones and ash balls on to the buildings, which resulted in their eventual collapse. Apart from giving an insight into the horrors of a big eruption, the accounts also gave scientists a chronology of events during the eruption.

In hindsight, we can now see that there had been several

indications of local volcanic unrest during the previous year. In 1885 there had been increased geothermal activity at Lake Rotomahana, where tourists flocked to see the world famous Pink and White Terraces, and in November the White Terrace geyser rose to a height of 45

metres — the highest it had ever been.

On the day of the eruption strong earthquakes began half an hour after midnight, increasing in intensity so that the ground was. shaking almost continuously by the time the first eruption occurred. Unbeknown to the inhabitants of Rotorua, the eruption had already started at Wahanga Dome in the north, with new explosion centres progressively appearing along a 17km long line to the south-west. The large explosion which woke everybody at 2.10 a.m., tore out a series of explosion craters along the summit of Mount Tarawera, producing a rift that is still visible today. Flares reached 300 m into the sky, and a steam cloud estimated to have been almost 10km high, could be seen from Gisborne. At 6.00 a.m. in the morning the Lake Rotomahana crater exploded, to produced a dense black cloud of steam and ash, reputedly higher than that of Tarawera. The roar of escaping steam, combined with the thunder of an associated electrical storm, forced Rotorua people to have to shout to make themselves heard by others only two metres away. It was this explo-

sion that greatly enlarged Lake Rotomahana and destroyed the Pink and White Terraces.

The central part of the North Island can expect more eruptions in the future, but scientists keep a careful eye open for tell-tale signs of imminent volcanism. Far more warning will be given than for those unfortunate enough to have been near Mount Tarawera on June 10, 1886.

By I

MARGARET BRADSHAW