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.
Actually, the present lava dome at Mount St Helens is the third dome to form since the 1980 eruption, the previous two having been blasted away by the subsequent eruptions.
The current dome started growing after the volcano’s last explosive eruption on 17 October 1980.
Second, and most importantly, we know exactly when the lava dome formed.
This is one of the rare instances in which, to the question, ‘Were you there? ’ The dating method Dr Austin used at Mount St Helens was the potassium-argon method, which is widely used in geological circles.
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.
It is made of dacite, a fine-grained volcanic rock that contains a sprinkling of larger, visible crystals, like chopped fruit in a cake.