Cause of the Mount St. Helens earthquake revealed
The reason for the thousands of small tremors that have shaken Mount St. Helens since its renewed phase of activity in autumn 2004 has probably been deciphered: they form in the volcano's crater around a huge stone dome, which is caused by increased magmatic activity is pressed up.
As a group of geologists headed by Richard Iverson from the Cascades Volcano Observatory in Vancouver, USA, determined on the basis of measurements and subsequent modelling, these are very shallow tremors that are triggered by the stone dome as it rises and are reminiscent of drumbeats. The tough, low-gas magma inside the mountain pushes the rock upwards, which, however, does not happen without jerking and jerking – a movement known as stick-slip. Since the plug in the vent of Mount St. Helens is sometimes pushed up at a speed of three to six meters per day, the miniquakes also repeated every 30 to 300 seconds. With their seismometers, the scientists located the approximate depth at which they were formed about one kilometer below the cathedral.
In the absence of deeper earthquakes, the researchers suspect no profound changes in the volcano's magmatism that could indicate impending more violent eruptions. Rather, Mount St. Helens is in an ongoing, near-steady state, in which cooled and solidified magma is pushed up and eroded as it flows down. The extreme eruption in May 1980 - one of the strongest ever recorded - was due to a 2.5 cubic kilometer rockfall: This caused a sudden drop in pressure for magma with an enormously high content of volatile components, which led to a violent explosion.
