The devastating eruption of Mount St. Helens on May 18, 1980, was the most powerful eruption in the last 100 years on the contiguous United States. Successfully being able to determine when it – or any of the arguably more deadly volcanoes nearby – will erupt next would be one of the greatest achievements in modern science.
With this in mind, a team of researchers have come across a specialized feature in the crystals found within the magma of Mount St. Helens that brings the world one step closer to achieving this feat. Presenting their findings at the Goldschmidt conference in Yokohama, Japan, the researchers suggested that the story of Mount St. Helens’ recent cataclysm appears to be contained in a mineralogical feature of its volcanic rocks called “crystal zoning.”
This is a common feature of crystals within extrusive (lava-based) or intrusive (magma-based) igneous rocks. Essentially, they are like tree rings: They are chemically distinct lines that build up on the edges of crystals that record the temperature, pressure and chemical conditions of the magmatic source at the time it began to cool.
Ultimately, this means that volcanologists can use crystal zoning to determine how the magma “evolved” and moved through the underground plumbing system. If, for example, a batch of magma kept rising and falling within a magma chamber, it would experience fluctuating temperatures and pressures. This would be recorded in the crystals as a pattern called “oscillatory zoning,” where a light band would grow next to a darker band, which would grow next to a lighter band, and so on.
By looking at Mount St. Helens’ volcanic rocks and the crystals within them, they have spotted distinct zoning patterns that indicate that, three years prior to the huge eruption, a vast batch of magma moved from a chamber around 12 kilometers (7.5 miles) deep to a far shallower one about 4 kilometers (2.5 miles) deep. Analysis of crystal zoning has always been fairly approximate, however, but it appears that this team may have significantly improved the precision of this technique.
“We have found a way of correlating the crystal composition to where they came from,” Professor Jon Blundy, an expert in the generation of magma at the University of Bristol and the team’s lead researcher, said in a statement. “Rapid upwards movement of magma at depths of several kilometers is a pretty good indication that something significant is happening.”
Image in text: Oscillatory zoning seen within a crystal. RSM Rock Library/Imperial College London