Posted: Tue Sep 28, 2004 1:34 am
by Vindicator
Maybe the northwest was feelin left out from all the attention to hurricanes?
Fun trivia: Mount St. Helens blew 3 days before my brother was born in 1980.
Posted: Tue Sep 28, 2004 3:29 am
by Mobius
From my own web site:
The largest known eruption in the past 2000 years occurred in the Taupo volcanic centre. In 186 AD there was an eruption that, in nine individual spasms, ejected enough material to blanket almost all of the North Island, covering about 52,000 square kilometres of the land area with more than 100 mm of volcanic deposits - sufficient to kill almost all the vegetation. The material erupted was a magma froth that cooled to form ignimbrite, pouring out of the volcano at a speed sufficient to climb ridges 1000 metres higher than the crater out of which it spewed.
Taupo is a rhyolite caldera volcano, and a number of vents lie beneath the bed of the present lake. Most are, however, so deeply buried beneath the debris of later eruptions that only the last vent, towards the north-eastern end of the lake, is still recognizable. The volcano has been erupting intermittently for the last 330,000 years, and there has been a lake in the caldera for at least 100,000 years. The early eruptive history is poorly known, but we can reconstruct events of the past 22,000 years in some detail.
The eruption of 22,000 years ago was the first major ignimbrite eruption for more than 100,000 years. The presence of the lake greatly affected the course of events. The periodic introduction of large quantities of water into the vent caused enormous steam explosions which blew ash exceptionally high into the atmosphere so that, eventually, more than 10% of the Southern Hemisphere received some ash fall. Moreover, the violence of the explosions pulverised the the ejected material to a far smaller particle size than would otherwise have occurred, while the water cooled the pyroclastic flows so that, when solidified, they formed welded ignimbrite rock only immediately around the vent. The eruption was brief. In a matter of days more than 300 cubic kilometres of debris were produced and buried all of New Zealand in at least 1 cm of ash. The severity of this and subsequent explosions have earned Taupo the title of being the most violent currently active volcano on Earth.
Eruption in 186 AD. Apparently exhausted by its efforts in 20,000 BC, Taupo fell silent for 10,000 years. Then came a series of eight relatively small eruptions which built rhyolite domes and produced the occasional small pyroclastic flow. Today, these volcanoes form a line running across the eastern bed of the lake, then into the hills west of Taupo township and on to the north. Evidence suggests that all these eruptions occurred on dry land, so we must conclude that, at this time, the lake was confined to the western end of it's present domain. It was the ninth eruption which created Lake Taupo as we know it today. Although no eruption anywhere in the world has since come close to the violence of the 186 AD eruption, analysis of the particle size and layering of the ejected material enables us to make an educated guess at the course of events.
Initially, the eruption, while violent, affected only the immediate area. At first it seems that this vent, too, was above lake level, but subsidence soon occurred leading to violent explosions, as lake water flashed to steam, which threw up ash, scoria and lava, creating as impressive eruption cloud. The presence of so much water led to a very 'wet' eruption, and for some weeks the volcano showered the surrounding land with muddy ash, building up a layer many metres thick on the shores of the lake. So water-saturated was this deposit that run-off channels cut deeply into it as excess water drained back into the lake. Then the roof of the magma chamber subsided further, perhaps abruptly, and a huge volume of lake water rushed into the vent. The eruption immediately increased to an unprecedented level of violence.
In a period of no more than a day, more than 20 cubic kilometres of pumice, ash and rocks were ejected, forming a vast eruption cloud which probably reached a height of 50 kilometres. Prevailing winds carried this cloud far to the east to shed its load over the eastern North Island and the Pacific Ocean. Such was the fallout that, over the next few hours, 30,000 square kilometres were buried to a depth of at least 10 centimetres.
Then the violence of the eruption increased still further from the simply unbelievable to the almost inconceivable. In a matter of hours (One estimate suggests less than seven minutes!) 30 cubic kilometres of volcanic material were hurled skywards, forming a cloud so heavy that it almost immediately collapsed under its own weight. A shock wave exploded outwards at the speed of sound, flattening forests and killing wildlife, and in its wake came a boiling pyroclastic flow. Forced on by the collapsing cloud behind it and rendered almost friction-free by the amount of gas within it. This poured over the ground at an initial speed close to that of sound , covering mountains, hills and valleys alike to a distance of 90 kilometres from the volcano. The flow even reached Red Crater on Tongariro, 400 metres above present Lake Taupo, although it failed to top Ruapehu. Any remaining lake water was turned to steam and incorporated in the flow, and wood was instantly carbonised as it was engulfed. The amount of water in the lake meant the pyroclastic flow although over 300 degrees C, was too cool to form ignimbrite rock save near its source, but the partially welded material formed a mantle more than 100 metres deep near the lake and blanketed 20,000 square kilometres of the surrounding land.
This stupendous explosion marked the end of the eruption. More than 110 cubic klicks of material had been ejected and the roof of the magma chamber collapsed completely, forming a wide caldera. The fine ash thrown so high into the air would, in time, spread throughout the world and cause red sunsets that the Chinese and Romans reported. The volcanic plateau, meanwhile was left in peace to recover.
After the eruption the central North island must have been a desolate place indeed. Apart from the gentle hiss of steam escaping from the cooling layers of ignimbrite, not a sound would have broken the utter silence of the devastated landscape while, even beyond the reach of the ignimbrite, heavy falls of toxic, suffocating ash kiled forests, leaving many kilometres of dead wood through which vast fires, started by lightning strikes, swept unimpeded. But this eerie world was short lived for, even as close as 20 kilometres from the vent, some animal and plant life had survived, protected from the full force of the eruption by high hils.
Within relatively few years, tussock, bracken and scrub had invaded the desert, encouraging insects and birds which, in turn, brought the seeds of larger plants. Soon, taller secondary vegetation was well established, providing the shade essential for the growth of trees of a mature forest. Within 300 years, dense forest once more cloaked the shores of the lake, while, further out, the volcanic ash which had originally killed the trees now combined with the carbon from their burning to revitalise the soil, encouraging rapid regeneration.
In this way, Taupo has built up the Central plateau over millenia and become one of the best trout fishing areas in the world. Next time you're there, pray it doesn't explode again...
Posted: Tue Sep 28, 2004 8:40 am
by []V[]essenjah
Lovely, I live in Idaho, right between the two. :
I was in Yellowstone only like a year ago so this can't be a good sign.
Posted: Tue Sep 28, 2004 5:11 pm
by Beowulf
damn, talk about the ultimate cumshot.
