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The current dome started growing after the volcano’s last explosive eruption on 17 October 1980.
During 17 so-called dome-building eruptions, from 18 October 1980 to 26 October 1986, thick pasty lava oozed out of the volcanic vent like toothpaste from a tube.1 Dacite lava is too thick to flow very far, so it simply piled up around the vent, forming the mountain-like dome, which now plugs the volcanic orifice.
Contrary to what is generally believed, it is not just a matter of measuring the amount of potassium-40 and argon-40 in a volcanic rock sample of unknown age, and calculating a date.
Unfortunately, before that can be done, we need to know the history of the rock.
For example, the rapid outflow from the volcano caused massive amounts of sediment to fill in the entire valley adjacent to the mountain.
And a 1982 dam breach of the snow-melt lake that had formed in the mountain's crater caused a catastrophic flood that tore a gash through those fresh deposits from two years earlier.
Experts at the time of the 1980 eruption predicted that the area would take perhaps hundreds of years to rebound.
Yet after only 20 years, biologists noted the speedy recovery of plants and animals on what had been a vast moonscape.
It sits directly over the volcanic vent at the south end of the huge horseshoe-shaped crater that was blasted out of the mountain by the spectacular eruption on .1 From the crater, the dome appears as a huge steaming mound of dark, block-like rubble.Many logs were transported by the blast to nearby Spirit Lake.As they jostled and shifted in the water, the bark rubbed off and sank to the bottom of the lake to form a sheet of waterlogged bark.To this day, the resulting steep-sided canyon walls can be seen, showing that horizontal sediment layers hundreds of feet thick were formed within hours during the eruption.This sparks the question: what other layered sedimentary rocks in earth's crust were formed rapidly?
The lava dome at Mount St Helens provides a rare opportunity for putting radioisotope dating to the test.