Volcano enthusiasts know this by now. Most eruptions are announced well in advance, with signals that today, thanks to sophisticated geovolcanology research techniques, are soon intercepted. But even without much technology, the phenomenon of “magma knocking” is a clear sign that something inside a volcano is changing. Of course, the terms are much more technical and specific, but in order to illustrate what’s happening the “knocking” is a good example.
Magma rising
Magma is lava before it has released the gases that push it from underground to the surface. The magma is a very dense mass of molten rock that seeks a way to the surface, where it emerges and becomes lava flows. Magma rising can occur directly, as at Stromboli, or indirectly through a magma chamber that collects the rock rising from the source before pushing it upward. But magma doesn’t always finds its way to the volcano’s natural openings, the active craters. So it seeks an alternative.
What happens when magma “knocks”?
As it rises toward the surface, in the absence of a conduit or crater, magma literally digs new channels. It attempts to break through the ground wherever it tries to escape. In these cases, experts notice severe swelling of the ground, actual deformations: collapses, uplifts, fractures. The magma might also change direction, so nothing happens where it “knocked”. Sometimes, however, it cracks the ground and creates new eruptive craters.
Events in history
One of the most impressive events showing such a phenomenon was the swelling of the flank of the St. Helens volcano in the United States. It was 1980, volcanologists were able to predict in advance that a lateral eruption would occur; in fact, they warned local residents, evacuating most of the towns. Certainly, no one could have predicted the dramatic and enormous landslide that would split the entire flank of Mount St. Helens (video here), despite claiming several lives, including the young volcanologist David Johnston.
Mount Etna’s perfect eruption (2002-2003), which saw the volcano become active on both the north and south sides – splitting the ground in multiple places, was also monitored step by step thanks to studies of the ground’s swellings and collapses.
The eruptions of Grindavik, the Icelandic town literally obliterated by the lava of a not yet existent volcano—which found its initial exit point here—have offered unforgettable spectacles over the past decade. The severe deformations of the road and some private gardens signaled the presence of magma “knocking” in advance and allowed the town to be evacuated without injuries or casualties.
In the Philippines, studying ground swellings allowed the population of several villages to escape in time from the large ash eruptions and pyroclastic clouds of volcanoes such as Mount Kanlaon and Piñatubo. (PHOTO BY GRAZIA MUSUMECI)