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(Above) Eventually the volcano sent material high into the sky. This is a Plinian type of eruption.
the north of the volcano, levelling everything over an area of 550 square kilometres. Close to the volcano
the force of the blast was so great that the trees were simply blown away. Beyond this was a zone where everything was knocked down. In total some ten million trees were felled by the explosion.
The Pelean eruption was, however, only the first part of the total eruption. It had taken off the top of the mountain and created a huge crater in the north flank from which a new eruption then occurred.
As soon as a new vent was established, the gases and pyroclastic materials were able to blow upwards, creating a Plinian type of eruption. This ash rose up into the stratosphere and was carried by the winds eastwards. Ash was then deposited over a wide area that included the town of Yakima.
By the time the eruption was over, the volcano
had been reduced in height from 2900 metres to 2500 metres, and some 2.7 cubic kilometres of material had been moved, of which about half a cubic kilometre was new magma. The rest was old cone material blown away in the first blasts.
In the following days the Plinian eruption subsided, but instead a very sticky lava began to flow into the new crater, building up a lava dome on the crater floor. This dome, which caps the new vent, remains the final feature of the eruption. It continued to grow until 1986.
But devastating as these eruptions were, the violence of the Mount Saint Helens eruption spread much further than the area blasted away or buried by ash. This was because the glaciers and snowfields on the mountain had been turned into steam, and they were now coming back to the ground in the form of torrential rainfall. The rain swept much of the ash into the nearby rivers, causing widespread flooding and at the same time silting them up.
Mount Saint Helens was not a large eruption. There have been some far more devastating eruptions, such as when the Indonesian island of Krakatoa blew apart in 1883, expelling 100 cubic kilometres of material and creating a sound that could be heard across the world.
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