Earthquakes Beneath Mount St. Helens Indicate Magma Chamber Is Recharging

Mount St . Helens erupts again in July 1980, two months after the unprecedented May 1980 eruption. Bettmann/UPI JO/MAX GUTIERREZ/Getty Images

The infamous 1980 eruption of Mount St. Helens was as deadly as it was unusual. This fairly active stratovolcano, located in Washington state in the U.S. released at least 100 million billion joules of energy as the magma chamber erupted sideways out of the mountain, killing 57 people in the process. This total energy release is comparable to the detonation of the largest nuclear warhead ever designed, the Tsar Bomba.

New data emerging from the site brings with it good news, bad news, and very bad news. The bad news, as reported by the United States Geological Survey (USGS), is that lots of small earthquakes are indicating that magma is moving upwards through the crust. There have been around 130 since mid-March.

Although little information has been given on these new tremors, they appear to be occurring at a depth of 2 to 7 kilometers (1.2 to 4.3 miles). The USGS thinks that these tremors, which are registering as earthquakes as high as magnitude 1.3, are being produced as the result of magma inflowing into new voids in the upper crust, perhaps recharging a magma chamber there.

“Earthquake rates have been steadily increasing since March, reaching nearly 40 located earthquakes per week,” the USGS wrote on their Facebook page.

The current swarm isn’t dissimilar from signals preceding eruptive activity, but it’s more likely that the upper magma chamber is recharging in this case. USGS

The good news is that, as Mount St. Helens erupted cataclysmically just 36 years ago, it’s likely that any subsequent near-future eruption will not be anywhere near as dangerous. That huge pressure release means that any eruption in 2016 – if indeed there is one – will almost certainly be less destructive. Its minor eruptive activity in 2004, featuring steam and ash explosions, also contributed to the depressurization of the volcano.

The peculiar eruption mechanism of the 1980 event meant that it was even more powerful than it would have been had it erupted from its peak, and it’s incredibly unlikely that this will be repeated.

But the very bad news is that Mount St. Helens is just one of over 160 volcanic edifices along the Cascade Volcanic Arc, and it’s these neighbors that have spent a long time in dormancy. The longer these silent harbingers of doom stand silent, the worse their eventual – and perhaps inevitable – eruptions will be.

The plumbing system beneath Mount St. Helens is complex, as it often is beneath stratovolcanoes. A recent initiative, which uses thousands of seismometers to image the movement of magma beneath the surface, has already revealed that there isn’t just one, but actually two magma chambers present.

The relatively sizeable upper chamber, at a depth of 5 to 12 kilometers (3.1 to 7.5 miles), is being fed fresh magma by an even larger one, 12 to 40 kilometers (7.5 to 25 miles) below the surface. A series of ascending tremors prior to the 1980 eruption are now thought to have been magma forcing its way upwards from the lower chamber to the shallower one, where it pressurized to the point of eruption.

Activity of the volcano on October 1, 2004. Any new eruption is more likely to be along these lines than that of the 1980 catastrophe. USGS

These new tremors aren’t too dissimilar from these older ones, or indeed, the ones preceding the 2004 activity. Their depths correlate with the position of the upper magma chamber, so magma is definitely edging ever closer to the surface. However, as the USGS point out, there shouldn’t be too much cause for concern just yet.

“The current pattern of seismicity is similar to swarms seen at Mount St. Helens in 2013 and 2014; recharge swarms in the 1990s had much higher earthquake rates and energy release,” it adds. This comparatively weaker seismic swarm, along with the lack of any other early warning triggers, suggests that an eruption of any kind is not imminent.

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