It is a truth universally acknowledged that any Icelandic volcano in possession of magma must be in want of an eruption. However little known the feelings and innards of such a volcano may be, any volcano in the neighbourhood, no matter how inactive looking, is considered the rightful property of VC. What a fine thing for our readers and commenters!

But volcanoes are not to be trifled with. Who would have thought that Fagradalsfjall, an ancient and eroded plateau on a peninsula that had not erupted for 800 years, would do this? That the shades of Reykjanes would be thus polluted? And that the eruption would occupy not the plateau but a small hill within a central valley? And that it would become a major tourist attraction? (That was actually predictable, in hindsight.) Most surprising perhaps, to all but the Icelanders and the Dutch, is that the love-hate relation would develop into a dam building exercise by the Icelanders which turned parts of their country into deep polders within seas of lava. In Iceland’s dispute between people’s pride and volcanoes’ prejudice, and in spite of some set-backs (Laki comes to mind), the people remain ahead.

As volcanoes have no compassion for poor nerves, IMO provides them with a request to provide adequate warning, an admonition paid no regard only by Hekla, leaving IMO most seriously displeased. The Eyjafjallajökull eruption of 2010 was a proper pre-advertised event. There were four weeks of warnings (and in hindsight, almost a year), followed by a small and brief fissure eruption, complete with 200-meter tall lava fountains. So far, so good and on an Icelandic scale hardly worth mentioning.

25 days later the eruption resumed and this time it did not go to plan. It wasn’t really the fault of the volcano. All it did was a medium-size explosion. But it became a VEI-4 eruption (0.18 km³ dense-rock-equivalent) which closed Atlantic and European airspace for 6 days. Whereas the first fissure eruption was out in the open, the second event (14 April 2010) was underneath a glacier. And that made a major difference.

In hindsight, three things happened together to make the eruption so impactful. The first one was that the eruption melted the glacier, and rather than flowing away, the meltwater collected in the crater. The eruption became phreatomagmatic, the melt water vaporised, and water vapour takes up a thousand times for volume than the water – hence the powerful explosion. The second thing was that vaporising water takes up a lot of heat. The ash cooled instantly and became as sharp and abrasive as Lady Catherine – not the kind of ash you want inside your jet engine. The third thing is that the 9-km tall explosion column reached the jet stream – and this was in a state where it happily transported the ash at high speed towards Western Europe, in various directions, covering all the major air routes in the region. Not quite knowing where and when the ash could turn up, there was little choice but to stop flying; many airports were closed and re-opened and re-closed repeatedly. The two European countries least affected by the ash were Portugal – and Iceland!

Not all things glacial were bad. Eyjafjallajökull is a known fluoride hazard: previous eruptions have led to deaths among animals from grazing the ash-covered vegetation. The 2010 eruption avoided this, probably because at the lower temperature of the phreato-magmatic phase, fluoride does not attach to the ash surface as efficiently. The subsequent magmatic eruptions did contain fluorides but these eruptions were much smaller.

Eyjafjallajökull showed us the need for much better mapping of volcanic ash clouds. But there was another lesson: glaciers and volcanoes are like the proverbial ice and fire. They amplify each other’s impact. Think Baked Alaska, or the Asian version Flame on the Iceberg.

Iceland has several glaciers which overlie active volcanoes: Vatnajökull (covering Grimsvotn and Bardarbunga, amongst others), Mýrdalsjökull (Katla and Eyjafjallajökull). There are several more which cover less active or (perhaps) extinct volcanoes. Mýrdalsjökull with Katla is considered the most dangerous. It hasn’t suffered significant eruptions for over a century, but it is not to be trusted. This is a high-risk volcano-glacier which could put Eyjafjallajökull to shame.

Risks

But what are the risks such a combination entails? Can it do more than create abrasive, high altitude ash clouds? This post will tell. It closely follows a paper by Barr et al, Volcanic impacts on modern glaciers: A global synthesis, published in 2018.

A tour of Iceland quickly reveals the risks. The region southwest of Öræfajökull was once a fertile plain, with twenty or more farms and several churches called Litla-Hérað . To the southeast was another wealthy region; the landscape was prairie-like. But in the eruption of 1362, it was destroyed and lay fallow for 40 years. The region never fully recovered: parts remain a sand desert even today. One of the causes of this were the massive jökulhlaups which variously removed and covered the fertile soil. It was not the only cause: the massive ash layers from the 10km³ eruption did not help (and gave ammunition to the floods). We have posted on this ancient disaster before.

Glaciers can turn an eruption into a disaster. But eruptions can turn a glacier into a catastrophe. Volcanoes and glaciers affect each other.

Location

Why do volcanoes and glaciers so often go together? There is an obvious reason: volcanoes can grow tall and are often the largest, stand-out mountain in the region. The height means low temperatures near the summit, and the isolation provides a focal point for orographic precipitation from the air rising up against the cone. Enough precipitation at low enough temperatures causes glaciers to form. In Iceland, the best conditions are found near the south coast where the volcanic bulges of Vatnajökull and Mýrdalsjökull provide the height, and the winds from the south bring the moisture. Öræfajökull, at the edge of Vatnajökull, has the highest amount of snowfall in Iceland. In the north of Iceland, glaciers are much less common, not only because the volcanoes are not as tall but also because the north is drier.

Glaciated volcanoes are found in many places. The Pacific coast of Alaska, Canada and the US has many. So does Chile. In between, glaciated volcanoes do exist but are less common because the climate is a bit too warm and dry. But even Equador has some, most notably on Cotopaxi. Mexico has three: Pico de Orizaba, Iztaccíhuatl and of course its connected twin Popocatépetl. The Aleutian islands have several glaciated volcanoes, although there are also many without. The image below shows Umnak Island with the (patchy) glaciers of Mount Vsevidof (2100 meters) but the non-glaciated Okmok caldera (1000 meters). Height matters.

There are many glaciers on Kamchatka’s volcanoes, not surprisingly giving the local climate. But the Pacific Ring of Fire then has a large gap of volcanic glaciers until Mount Ruapehu, 2800 meters, on New Zealand’s North Island. (Japan has 7 known small glaciers on Mount Shirouma, but the mountain is not volcanic.) Elsewhere, Iceland has already been mentioned, and its volcanic glaciers are supplemented by Beerenberg on Jan Mayen.

Finally, the volcanoes of the Caucasus (Elbrus, Kazbek) should be mentioned. And of course the final mention should be of this most contradictory of examples, Africa’s Kilimanjaro. But if you want to experience its glaciers, haste is recommended because their remnants are melting rapidly. Whilst Cotopaxi is four times closer to the equator than Kilimanjaro and is 2 meters(!) taller, Kilimanjaro retains the ‘wow’ factor.

This list is not exhaustive, but it shows where to look for glaciated volcanoes.

A definition of what counts as a glacier may be useful. It is not just a snow field that can survives the summer sun. To be a glacier, it also has to flow. The glaciers in Japan were originally classified as snow fields, before they were found to move by tens of meters over the year. Strictly speaking, Antarctica and Greenland have ice sheets (or ice caps), which form glaciers where they flow through the mountains. Therefore, an ice-covered volcano deep down in Antarctica is not strictly speaking a ‘glaciated’ volcano – but this quickly becomes semantics.

What does a volcano do to a glacier? What is the effect?

Cauldrons

Volcanoes are very good at producing heat and may do so even when not erupting. Ice and heat don’t go together too well: the heat melts the ice from below and the water forms a lake underneath the ice. Once this lake begins to drain (typically when it gets so large that it physically lifts the ice barrier), the ice above is no longer supported well and begins to sink. A sink hole in a glacier may be the second indication of volcanic activity, the first being the melt water coming out from the glacier.

A good example comes from the Skaftá cauldrons on Vatnajökull. There are two, called the eastern and the western cauldron. The eastern one was discovered in 1938. The western one did not exist at that time but was first seen in the 1960’s. Their meltwater lakes cause jökulhlaups in the Skaftá river: from the volume of these, the geothermal area below the cauldrons is found to be among the most powerful in Iceland, similar in power to Grimsvötn.

The melt can affect the glacier in different ways. If a sufficient amount of ice melts, then the glacier may stall, retreat (as seen at Volcán Iztaccíhuatl) or (in rare cases) could even disappear. More common is the opposite effect: as melt water gets underneath the glacier, it acts as a lubricant and the glacier accelerates. This causes the glacier to advance. It can happen after earthquakes, if existing meltwater becomes released through new cracks (as happened at Mount Wrangell after the 1964 Alaska mega-earthquake) but is often caused by volcanic heat.

Several volcanoes have shown acceleration months or years before an eruption. An example is Volcán Michinmahuida in Chile where glaciers showed sudden advances in the year before the 2008 eruption of nearby Volcán Chaitén. (In this case, tephra fall on the glacier may have added to the acceleration.) Mount Kazbek is also an example of this effect. However, it is difficult to establish geothermal melt as the main cause of each advance. The evidence for the cause-and-effect is often indirect. An indication of volcanic heat as the cause is that the movement is not seasonal, as is the case when the advance comes from summer melt.

Ice domes

Volcanic activity can result in the growth of lava domes. If this happens underneath a glacier, then the ice can also deform. This was seen at St Helens (in hindsight) before the 1980 eruption, and again (after the glacier re-established itself) around 2005.

The lava dome may end up sticking out above the ice. There are few cases known of this, but it happened at Great Sitkin in 1945.

Destruction

An eruption underneath the ice can be devastating to a glacier. It is very common because so many volcanoes are covered by glaciers: the effects are well known. At Eyjafjallajökull, a large crater formed within the glacier which destroyed about 10% of the ice in the caldera. This was in effect an oversized cauldron, with no ice left in the centre, a cauldron worthy of Edgar Allen Poe. It did not form the concentric ice rings as were seen at the Skaftá cauldrons, because the ice was not thick enough for this: such rings have been found at Volcán Hudson.

Instead of a crater, a fissure may form in the ice. This may show the path of the melt water. These fissure can be a kilometer or more long. A glacier can also fracture. Fractures are prone to further erosion, and can form pinnacles of ice, vaguely similar to the penitentes seen in snow on Mount Rainier. After the 1953 eruption of Mount Spurr, it was reported that “The southern flank of the continuous ice rim was partly breached, with castellated ice pinnacles standing in sharp relief within the breached zone”  (Juhle and Courter 1995).

An extreme case of destruction is glacier beheading. This happens when the feed area for a glacier is destroyed, leaving a glacier without its head. Because the formation region of a glacier tends to be close to the summit, this is not unusual. A prime example is the Novarupta eruption of 1912, when the entire summit of Mount Katmai collapsed into a caldera, with two glaciers losing their heads. Over time (decades), such glaciers will thin and may disappear. The tail end of glaciers may also suffer destruction, but this is rare since the chance of an eruption happening there is much smaller. It has been seen at Klyuchevskoy 1987.

The complete destruction of a glacier by an eruption is exceptional. This has only been seen at St Helens.

Similar to geothermal melt, an eruption can cause a glacier to advance because of meltwater lubrication. This again is fairly common. Glaciers at Klyuchevskoy advanced after eruptions in 1953, 1980 and 2010. The advance ceases after a few years, and over time the glacier (if undamaged) will return to its previous location, awaiting the next eruption.

Lava

Lava and ice are like Darcy and Elizabeth, a conflict waiting to happen. But surprisingly, the two can co-exist and even get along. Lava flowing below ice causes melt, which can be explosive and can create deep channels in the glacier. In other cases deep depressions form but the lava remains hidden below the ice, perhaps because the lava flows in a lava tube.

Lava can also flow (surprisingly) on top of the ice. This sounds like a recipe for disaster but the combination survives remarkably well. The lava can flow for a kilometer or more, as was seen for instance at Beerenberg in the 1970 eruption. There is melt – that is inevitable – and a channel forms that can be tens of meters deep. Lava can even pond on the glacier. What is protecting the ice, apart from avoiding cases with high lava flow rates? There are three possibilities. The first is a tephra fall (or other debris) from the eruption, with the lava flowing on this. But when the lava stands out against the white glacier, the glacier is clearly not covered by tephra. The second possible shield is snow. Snow is very good at insulation due to its porous structure. A deep snow field can protect the ice below well. The third shield is the lava itself. When it solidifies, it can become a good insulator and so the bottom of the lava flow becomes a solid shield over which the lava flows, away from the snow. This may be even more effective if it solidifies in the presence of water (ice/snow) which gives rise to pumice – with lots of pores which stops the heat from flowing across. Who knew.

There are some spectacular images of lava on snow fields in Iceland. I have not found anything similar for glaciers: lava on glaciers is seen mainly from satellite images (Beerenberg, Mount Melbourne).

Pyroclastics density currents

Amazingly, pyroclastic flows can be more destructive to glaciers than lava. They can scour deep channels into a glacier, sometimes down to the bedrock. Mount Redoubt scoured a section of Drift Glacier to bedrock during the 1966 eruption and again in 1990, effectively separating the glacier from is head twice. This effect is strongest on the steep, upper parts of glaciers. On the shallower lower part of the valleys (the piedmont part) the pyroclastic density currents incise channels but don’t scour as deep: this happened on the Drift Glacier (him again) in 2009.

Tephra

Volcanic eruptions have a tendency to deposit tephra, and being nearby, this can affect in particular any glaciers on a volcano. What does this do a glacier?

In most cases, not a lot! If the tephra is hot, it will melt some ice. If it is thin (and basaltic, i.e. black), it will later absorb sunlight, and begin to melt the ice below, at least until it becomes covered in snow. If it is thick, it will instead insulate the ice. In most cases, the tephra becomes a dark layer embedded in the glacial ice. There may be several to many such layers in the glacier, one for each eruption. They can often be recognized decades to centuries later, looking delightful like a yummy frozen desert. Baked Iceland, anyone?

Lahars and jökulhlaups

Early in the post, the damage caused by jökulhlaups was mentioned. This is in effect an indirect result, where the volcano affects the glacier and the glacier in return threatens its surroundings. Perhaps another post, another day.

For now, let’s celebrate the resilience of ice against the pride and prejudice of fire.

Albert, September 2026

(With apologies to Jane Austen: any similarity to her Pride and Prejudice is entirely intentional. The book was published two years before Tambora and Austen died in the dark and cold years that followed the eruption, so a certain volcanic connection is not to be dismissed.)

Fire and Ice

Some say the world will end in fire,
Some say in ice.
From what I’ve tasted of desire
I hold with those who favour fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice.

(Robert Frost)