Since April 2024, Mount Spurr has shown increasing signs of activity. These are now at a level where an eruption is plausible. Nothing is ever guaranteed with Alaskan volcanoes: they can always decide to go back into the freezer. Eruptions are decided on the Spurr of the moment. (Yes, I had to get that one in. Do it early and get it done with.) But the build-up is there. We are all on hand to spurr it on. (Yes, that one was needed too.)

America tends not to go in for imaginative names. Things have simple names. That may be a legacy from the old homeland, where names as ‘Irish Sea’, ‘English Channel’ or ‘Mount Everest’ (named after someone who has never been there) also reveal a certain lack of nominative creativity. Often, mountains are named after people who no one remembers. That even applies to the entire continent, the only one named after a person (counting both Americas as one). Sometimes the naming goes a bit askew. The new-fangled ‘Gulf of America’ is presumably meant to be named after the country, not the continent and so should be called the ‘Gulf of the United States of America’. If the continent is meant, then the name should really be ‘Gulf of the Americas’, and if the national pastime was meant it should have been ‘Golf of America’. Historically, the name referred to the nation that owned it (or rather, owned the coast), and strictly speaking should have been the ‘Gulf of Spain and France’. I am now also awaiting the renaming of the Gulf Stream.

There are three locations in the US (to use the abbreviated name of the nation) or elsewhere named after Spurr: a town in Michigan, a crater on the Moon and a mountain in Alaska. It is the latter that is brewing trouble. The Michigan town is named after a local mountain which is called Spurr Mountain (so no confusion there) and was named after John Spurr, an early miner. The lunar crater is named after a geologist, Josiah Spurr, an early explorer who traveled extensively in Alaska and, amongst others, discovered the Valley of Ten Thousand Smokes’, albeit a decade before the Katmai eruption that turned it into smokes. He was known as an ‘economic geologist’, meaning his aim was to find resources suitable for mining. (The close link between geology and mining may be the reason why so many geology papers are still being published behind paywalls. The authors want the scientific credit without giving away any content.) It won’t come as a surprise that Mount Spurr is named after this same geologist explorer, even if it is an unmineable mountain. There is also actually an older, local name, K’idazq’eni, for the mountain.

Mount Spurr

Mount Spurr is not the best known of Alaska’s volcanoes. It is still a notable one. Part of this is from its location. It is located on the western side of the Cook inlet, and although 130 km away, is visible from the Anchorage on the opposite side. It is the closes volcano to Anchorage. At 3.4 km, it is also the tallest volcano in the region, 300 m taller than Redoubt. In fact, Mount Spurr is the highest volcano in the entire 3000 km stretch of the Aleutian Arc. But it is not as obvious as it should be: it is part of a smallish mountain range, the Tordillo Mountains, of which Mount Spurr is not the highest peak. The range is partly volcanic but Mount Torbert, its highest peak, is not. Mount Spurr is so tall because it got a headstart. It is standing on the shoulder of a giant – and that helps.

The second reason Mount Spurr is notable is the fact that it does decent eruptions. In living memory it has erupted twice, in 1953 and in 1992. In both cases, the VEI-4 eruptions deposited ash on Anchorage – this is a volcano with impact, while not as destructive as other volcanoes in the Aleutian arc! The local older name (‘burning mountain’) suggests a longer history of eruptions, perhaps more than are known. Josiah Spurr choose well.

Mount Spurr is at the eastern end of the Aleutian Arc. There is one volcano located 40 km further, Hayes Volcano: it is not as well known but possibly should be. It did several VEI-5 eruptions 3500 years ago but has not erupted in the past 1000 years. That sounds like a volcano worth monitoring.

Mount Spurr contains a summit dome which is centred on a large, 5-km wide caldera. The caldera has a gap on the southern side where a large collapse has occurred. A new eruptive centre has developed in this southern gap, called with a stretch of imagination, ‘Crater Peak’. The name refers to the crater on top of the peak, but is not something to distinguishes it from many other such mountains, including the summit of Mount Spurr itself! The mountain developed from 250,000 years ago with a combination of lava and pyroclastic flows. There was a large collapse and eruption (make that very large) some 15,000 years ago (the age is not well known) which formed the gapped caldera. The debris flow and an overlying pyroclastic tuff from this eruption can still be recognized in the landscape as far as 25 km away. But the tuff is not seen close to Mount Spurr, probably because of extensive ice cover.

Around 7000 years ago, the current Mount Spurr summit peak formed as a new vent at the centre of the original destruction, while the current Crater Peak formed within the southern gap of the caldera rim perhaps 6000 years ago. The original Mount Spurr was andesitic, while the current summit is more silicic and Crater Peak (and its predecessor) are a bit more mafic. Both centres can erupt but most eruptions have come from Crater Peak. There are some 30 tephra layers associated with Crater Peak, indicating eruptions about once per 200 years. There may have been more, of course. In contrast, the summit crater shows no clear signs of activity within the past 5000 years. Crater Peak also contains the remnant of an older cone, whether lost by collapse or by explosion is not known.

The mountain is heavily glaciated, and both the summit peak and Crater Peak have ice cover. Both peaks have craters. The summit crater was last active in 2004 with hydrothermal activity which did not lead to an eruption. This crater holds a frozen lake. Crater Peak has erupted twice in the last century. It used to hold a lake but not at the present time. Crater Peak is about a kilometer lower than the summit peak. The two cones are similar in size but Crater Peak starts from a much lower elevation.

The 1953 eruption

In the early morning of 9 July 1953, a large explosion took place on Mount Spurr. The eruption came from Crater Peak. The cloud reached a height of over 20 km. The westerly wind moved the ejecta towards Anchorage. The eruption had first been observed from two airforce jets on morning patrol. The crew provided a remarkably detailed report, aided by their on-board radar:

“At 5h05 Lieutenant Metzner noticed a column of smoke 60 mi ahead that was about 15,000 ft high and one-eight mile wide. As he approached the smoke, it was apparent that the eruption causing it was becoming increasingly severe with the smoke growing rapidly in height. At about 25 mi distance, the volcano was recognized as the 11,070 ft high Mt Spurr. Both planes approached the mountain about 15,000 ft and circled the volcano at about 05h25m. They noticed the continuing increase in the intensity and size of the column of smoke with lightning flashes through its core every 30 sec. Smoke issued from the volcano in violent billows at the 7000-ft level of the mountain caused by huge subterranean explosions. Tremors on the mountainsides were visible from the aircraft and were followed by snow slides on the mountain. The smoke had by now reached the 30,000-ft level, rolling upwards and assuming the shape of the atomic bomb mushroom. Clouds of smoke were every shade of grey from black at the crater to pure white at the top. By this time the width had increaed to about a mile at the base and 30 mi at its widest part.

About 05h40, Lieutenant Metzner climbed in order to estimate the height of the mushroom. The top of the stalk, or the borrom of the mushroom, was 30,000 ft and the top of the mushroom had climbed to 70,000 ft. Lightning was now flashing from top to bottom of the mushroom at three-second intervals.

At about 06h00m volcanic ash began falling from the mushroom on all sides and finally made the entire area hazy. A clear definition of the volcano and the mushroom rapidly faded and the patrol returned to its base.” (Juhle & Coulter 1955, https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/TR036i002p00199)

The report did not mention that one of the two planes flew through the ash cloud and suffered sandblasting of the plexiglass cockpit canopy!

The ash covered the glacier mainly to the west of the volcano. The cloud reached Anchorage by mid-day, where darkness fell and dark ash came down: the ash layer reached half a centimeter thickness in the city. The ash fall and darkness lasted several hours. Air traffic was disrupted: the airport closed for two days. The fallen ash and high winds led to a dust storm in Anchorage on July 15.

The eruption came from a single vent in the centre of the crater of Crater Peak. The crater rim was left largely in place, including its ice cover, apart from some erosion of the ice on the southern rim. The vent had a little activity during the days after the eruption, including one sharp explosion on July 13. No lava was observed at any time during the eruption: this was a purely explosive event. There were also no reports of sulphur smells. That may also be related to another aspect of the eruption: there was torrential rain in the vicinity. The ice melt and rain caused flash flooding along the Chakachatna river, with the river rising by as much as 15 meters. The floods carried large boulders and caused the river to dam, with a temporary lake flooding an 8-km long section of the river.

Mount Spurr had show some signs of activity over three decades before the eruption, with a vapour cloud visible at times above the summit. This vapour cloud had increased during the spring of 1953. But Crater Peak itself had shown no signs of activity: the first indication of trouble there was the eruption itself. This indicates that the two sites remain connected.

The 1992 eruption

The activity started in late August 1991, with a swarm of earthquakes directly underneath the centre of Crater Peak. These were volcano-tectonic (VT) earthquakes at a depth of 1-4 km, with either magma or hydrothermal activity involved. The swarm was followed by two months of quiescence, before a slow ramp-up of earthquake activity began. At this time, the earthquakes were clustered underneath Crater Peak, Mount Spurr summit and the north caldera rim. They remained shallow. Activity peaked on June 5 and then declined. On June 6, VT bursts were detected.

On June 8, the lake in Crater Peak turned from blue to gray and showed signs of heat upwelling.

Long-period events occurred on June 19, with renewed bursts of volcanic tremor on June 24 and 25. On midday June 26, the tremor became continuous. At this time, the lake had dried up, unable to cope with the heat from below. (Actually, water sources often dry up prior to an eruption, because of the inflation. The water table stays the same but the ground inflates, leaving the water behind.) This was also when the warning level was raised by AVO to yellow.

A final swarm of VT earthquakes occurred the following morning, June 27, at 3am local time. This was to be the last straw: at 7am the eruption occurred. Just as in 1953, it was first seen from aircraft, in this case a pilot of a commercial plane: it was too cloudy to see the eruption from the ground. Seismometers did detect the precise onset at 7:04am local time, but this was in hindsight. The warning level was raised to orange at 7:15 am, and to red at 9 am. The explosion plume reached almost 15 km – it was not as high as in 1953. The eruption lasted four hours.

Different from 1953, there was more to come. The earthquake activity had ended, apart from a new set of deep earthquakes at 20-40 km depth, and in July the volcano was set to ‘green’ again. Tis turned out to be too soon. In the afternoon of 18 August there was a small eruption, reported by a pilot who saw an ash cloud above the clouds. This was followed an hour later by another explosion, as strong as or a little stronger than the one in June. AVO had raised the warning level to yellow just minutes before this eruption, to orange five minuter after the onset and to red 11 minutes later. It is hard to keep up with this volcano! There had been no obvious warning signs. The eruption lasted 3 hours and the plume again reached 14 km. This time, the winds were in the direction of Anchorage which received a dusting, enough to close the airport.

This sequence repeated in September 16. This time there was a 3-hour tremor burst before the eruption, which again was a double, starting with a small one followed by a much bigger bang 15 hours later. The bigger-bang eruption was again of the same size as the previous two. AVO scientists had visited the crater just hours before the eruption and had not noticed anything unusual – they were lucky! After this escape, AVO became justifiably very cautious about visiting the site. The seismometer the scientists had installed on the crater rim detected the onset of the eruption, and this time code red was raised 3 minutes before the eruption. It shows the importance of such instruments. After the eruption, there was a very strong burst of VT earthquakes at 10 km depth.

Lightning was detected in all three eruptions, but at much lower levels than in 1953. That may be related to visibility: the aircraft in 1953 was superbly placed to see lighttning.

There were two further swarms of VT earthquakes in November and December. These did not lead to an eruption and are seen as failed attempts (or successful intrusions, if you prefer). During 1993, the activity reduced to background levels and Mount Spurr went back to its beauty sleep.

The activity has been used to map out the likely magma reservoirs and conduits underneath Crater Peak. This is shown below.

The 2004 non-eruption

During 2004, the summit crater again began to show indications for a high heat flow, with melting ice on the crater flank and a cauldron forming in the ice inside the crater opening a view to a warming crater lake. This continued for two years, with boiling water and a fumarole dubbed Jumbo Jet. It was all in vain. Magma reached shallow depths but failed to break through. In 2006, the heat began to disappear and the magma stayed where it was. This event had some similarity to 1953, with as difference that Crater Peak did not become involved.

2025

Earthquake activity picked up again in April 2024. AVO increased the warning level to yellow in October. In recent weeks, increased gas emissions have been reported. A GPS in the region has shown 5 cm of uplift. AVO states that an intrusion is on-going. The chances of an eruption are put at 50% or more, depending on who you ask. The similar burst of activity around 2004 did not lead to an eruption, so nothing is guaranteed. But it is best to take precautions. Up to date information can be found at https://avo.alaska.edu/volcano/spurr/activity (hopefully this site will remain on-line). That site also has links to webcams.

The increasing rates of earthquake activity shows that the events of 1953 and 1992 are repeating themselves. Instruments are much more sensitive now, and leave little doubt about the daily tally of shallow, volcanic earthquakes. In addition, gas emission have been detected, showing that new magma has arrived and has started to outgas. The most recent (March 7) measured rate is 450 tons of SO2 per day. Steaming has been seen, and both inflation and expansion has been detected.

The earthquake activity is distributed beween the north caldera rim, Mount Spurr summit and Crater Peak, just as it did in 1992. The centre of inflation is fairly close to Mount Spurr summit (a few km west). The summit crater again has a visible crater lake and fumaroles. But as in 1992, activity is shifting to Crater Peak which over the past weeks has shown signs of snow melt and new fumaroles. It is reasonable, based on experience and history, that an eruption would come from Crater Peak. But the summit crater cannot be ruled out.

Still, the 2004 events show that an eruption is not a certainty. On the other hand, the 1992 events show that VEI4 explosions can happen here with little warning. On the other hand if the previous two eruptions removed an old plug, than new eruptions may not be as explosive. AVO expects that there will be precursor signs if an eruption were to occur, but these may only give hours of warning. This would be a good time not to be too close to he mountain, and aircraft will be rerouted as needed very quickly if events accelerate.

An eruption could happen within weeks. Or not at all. Uncertainty is what volcanoes are good at.

Albert, March 2025