Mount Spurr is only about 75 miles from Anchorage in Alaska. More than half the population of the northernmost state live in the Anchorage area. On top of that, a surprisingly large amount of air cargo traffic passes through its international airport not to mention the military aircraft from Ellsworth Air Force Base. So, any time a volcano close to such a vital hub begins to get restless, it is time to pay attention.

Spurr Gets Restless

Earlier this month, the Alaska Volcano Observatory raised the alert status at Spurr to Yellow for the first time in three decades. AVO is jointly run by the University of Alaska and the US Geological Survey and its keeps an eye on the many volcanoes of Alaska. In fact, the 1992-94 eruptions at Spurr were one of the first major eruptions managed by AVO after its founding in 1988.

The current information statement for Spurr reports that since April of this year, there has been a gradual increase in earthquakes recorded at the volcano. This, by itself, isn’t normally enough to raise the alert status. However, GNSS (similar to GPS) stations near Spurr have also recorded deformation that has been modeled as potential influx of magma or other fluids a few kilometers beneath the volcano. Combine that with satellite images that now show a small crater lake and steaming at the volcano’s summit, and you have the recipe for a restless volcano.

Telling People About Risk

Now, this is the tricky part for people who monitor volcanoes like AVO. Although all these signs flash “something new is happening!”, communicating that can be very challenging. In the USGS summary for the 1992-94 activity at Spurr, John Eichelberger and others comment

“[…]increasing levels of concern convey both increasing likelihood of eruption and increasing potential or actual size of eruption. No simple one-dimensional scale can adequately describe an impending or ongoing event. Yet, simplicity of communication is essential during an emergency.”

So, although there is a lot of nuance to the interpretation of the new unrest at Spurr, trying to express that in a simple way like a color-coded alert system is difficult. This is doubly important because if you prematurely use these alerts, the cost can both be public trust and real dollars.

To put it simply, there are really two options here: eruption or no eruption. At this point, it is not clear which is more likely beyond the fact that intrusions of magma without an eruption are relatively common at volcanoes. However, many of the signs that Spurr is exhibiting are similar to what it did prior to the 1992 reawakening. Volcanologists have to look at the signs of unrest (earthquakes, deformation, gas emissions, thermal changes) and fit them onto the past behavior of the volcano as well as what we have observed at other volcanoes like the one in question.

The Last Eruption

The 1992-94 eruptions at Spurr were a combined VEI 3-4, so significant but not huge. The economic impact in from those events was between $10-17 million (adjusted for inflation). That is relatively small compared to other eruptions like the 1988 eruption of Redoubt, another nearby Alaskan volcano. That eruption had an economic impact of at least $345 million (adjusted for inflation) but as the USGS report on Spurr points out, this is mainly coming from damage to 5 large aircraft from the ash of Redoubt’s eruption, with the KLM 747 that lost power to all four engines due to ash ingestion being the most famous.

Improved ash monitoring and predictions utilized during the 1992-94 Spurr activity helped keep the cost of the eruptions much lower. This drives home the point that active volcano monitoring, mitigation and communication more than pays for itself during any volcanic crisis. With the amount of air cargo passing through Anchorage each day, the potential for economic losses could be immense, but with appropriate ash monitoring and warning, some of that cost can be avoided.

The largest hazard from the 1992-94 eruptions at Mt. Spurr was all the ash that was emitted. Anchorage saw 3-5 millimeters of ash fall across multiple blasts from Spurr that sent ash and volcanic aerosols over 60,000 feet (19,000 meters) into the atmosphere. Ash fall was noted hundreds of miles from the volcano. The ash that fell closer to Spurr remained a hazard for years as it was picked up by strong winds.

Closer to the volcano, the eruptions produced pyroclastic flows, lahars (mudflows) and lots of volcanic bombs. These explosions threw 1-meter (~3.3 feet) long volcanic bombs over 10 kilometers (6 miles) from the vent of the eruption! The debris flows formed from volcanic debris melting part of a glacier ended up damming the Chakachatna River, briefly creating a new lake along the river valley.

What Might Happen

Right now, exactly what Spurr has planned is unclear. Volcanoes become restless and then go back to slumbering all the time. However, informing the public that signs are beginning to align that could mean a new eruption is a good idea. Clearly communicating the potential risk — and the potential outcomes — will help all people, agencies and companies make better decisions. If Spurr does erupt, we can expect disruptions of air travel across Alaska and possibly even across western North America. However, close monitoring will help reduce any chances of loss of life or massive economic damage.