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Sub-Glacial Eruptions


Subglacial eruptions; these do not occur in many places due to the fact that there aren’t many glaciers around at the present time. This means that locations susceptible to this style of eruption must meet two very obvious requirements: it must be glaciated and it must be volcanically active. Finding locations at which one of these requirements are met is not very difficult, but finding the two in tandem is quite a bit tougher.

Now, almost all very tall mountains have glacial caps at their peaks, but this is not what we’ll be talking about because these glaciers are relatively thin and melt almost instantanly at the outset of an eruption. This means that the glacier, other than creating massive lahars, does not affect the course of the eruption significantly; thus the eruption is not truly subglacial. By ruling out this style, we find that the occurrence of active subglacial volcanism is actually quite rare. We find it mostly in two places: Iceland and offshore Antarctica (Deception Island). One might ask why this is important to look at then; there are two answers: they are there and the people who live near them (Iceland) can be in extreme danger during an eruption.

This article is going to cover a number of points and skip over some that are discussed in other articles. To begin, the dangers of subglacial eruptions will be looked at as well as examples of extinct volcanoes of this type and how they appear. This will be followed by investigating how these volcanoes form, what it is that makes them so dangerous, and how an eruption can progress through various stages (and what this means for people around it as well).


There is only one very real danger to people around active subglacial volcanoes, but it is a very significant one. It is called a johkulhlaup (yo-kul-yop). Sounds like kind of a strange name? Well, that’s because it’s Icelandic…the only place in the world right now that really has to worry about these things. It’s not a huge issue elsewhere, but it is in Iceland where jokulhlaups can absolutely devastate the land they cross. So, what are these things? Massive floods. How are they different from lahars? Lahars are volcanic floods that are triggered by a number of mechanisms: flash-melting of a summit glacier, displacement of a lake due to a volcanic mudslide or pyroclastic flow, fluidization of landslides consisting of altered slope material, etc… A johkulhlaup is different, however; it is known as a glacial burst: during an eruption, water within the glacier is melted by the lava and trapped by the surrounding ice. This body of water can be thought of as a giant trapped lake or, more accurately, as a massive dam. The water continues to accumulate throughout the course of the eruption while the ice containing it is progressively melted. This ice melting weakens the “dam” and the process continues until this icy wall fails and catastrophically unleashes the entire glacial lake in one fell swoop. The volumes of water are enormous and carve out the countryside, destroying everything (and anyone) in its path.

Fortunately for the Icelanders, these events can now be predicted. Unfortunately, however, difficulties arise concerning specifics such as: volume of water to be unleashed, rate at which it will flow, where it will flow, and the actual day and time (or even week) the event will occur. So how are these dangers monitored and mitigated? How are the people kept safe?

Iceland has been, and remains, home to some of the best volcanologists in the world. The country maintains a network of geologists to monitor its many volcanoes and report any changes in activity. These changes can be difficult to identify in subglacial settings for a reason that may be very obvious to you: they are under ice. Typically, the first obvious clue is subsidence and faulting of the glacial surface (yes, ice can form faults). The reason for this is that ice cannot handle tensile stress very well. The ice at the top of a glacier appears extremely solid and durable, but this is only because it is being supported by hundreds or thousands of feet of ice underneath it that runs straight to the ground. Once this ice begins to be removed, the overlying ice loses its support and becomes weak. Eventually, it begins to fail structurally and begins the process of collapsing. This is where we see the subsidence; this is usually in the form of a relatively small (several hundred yards) conical depression in the glacier surface that may be heavily crevassed and faulted. These crevasses are roughly circular arcs that may trace the general shape of the underlying body of water (though the water body underneath is much larger than the surface collapse). These surficial warnings are spotted during aerial surveillance flights and are reported immediately. An additional warning, which may not always be present, is increased flow of meltwater from the glacier and contaminants in that water. Once an eruption is identified, it is heavily monitored. The eruption can continue beneath the water or punch its way through the ice. Scientists then look at maps depicting glacier edges and thickness to determine the most likely place for the johkulhlaup to occur. Once the most likely exit point is found, a path for the flood is mapped based on topography and the expected volume of water. Residents are evacuated and the waiting game begins…waiting for the inevitable disaster to strike. The wait can sometimes be weeks. Frequent mapping of surface deformation, water-vault estimates (how large is the underlying lake), thickness and weight of ice walls, and estimated eruption rates are used in creating and adjusting eruption warnings and forecasts.


Iceland, 1996: Vatnajokull is Iceland’s largest glacier and on September 29 a magnitude 5.4 earthquake is detected beneath it, followed by a low-intensity earthquake swarm. This seismic activity was followed by lower-amplitude volcanic tremor; the eruption of the new subglacial volcano Gjalp is estimated to have begun between 10 and 11pm September 30. Fifteen hours later, an overflight of the glacier photographed a beautifully crevassed conical section displaying near-perfect circular faults in the ice angling inward to the center. The depression was 2km in diameter and 200-300m deep. Moreover, additional depressions formed along a straight line indicating that this eruption was along a fissure that was 4-5 km long. The rate of glacial melt was estimated at five-thousand cubic meters every second and at 5am the following day (Oct 2) the eruption finally punched through the surface of the ice. The plume was originally composed of white steam, but progressed to black ash that eventually rose to 9km and was punctuated by intermittent explosions. On Oct 4, the eruption was estimated to be occurring beneath at least 50m of ice; on Oct 13, the eruption ended and what remained was an island of hot ash and an ice canyon 500m wide and 150m deep. Throughout the eruption, volcanologists carefully watched the water level under the ice, which quickly rose to levels not seen during this century. The johkulhlaup didn’t occur during the eruption, however. In fact, the glacial burst began more than a month after the eruption started; on November 5th the flood began by releasing 6,000 cubic m/s which increased to 25,000 cubic m/s by that afternoon. The flood peaked that night at 45,000 cubic m/s and finally ended on November 7. For comparison, the Mississippi River has a discharge of ~17,000 cubic m/s; the Nile has a discharge of 1,580 cubic meters/second. 2,000,000 pound icebergs were carried downstream and anything in the water’s path was destroyed; where the water reached the ocean, the coastline was extended by 800m.

Characteristics and Styles

Subglacial volcanoes look and behave differently depending on a number of characteristics, including: temperature and composition of magma, volume of magma, and thickness of the overlying ice. In addition to the obvious influences, permeability of the ice and temperature are also important influencing factors. As the glacier forms, layers of snow are compressed to ice and this is an important process to keep in mind. A vital distinction lies between snow and ice. Snow is loose and uncompacted; it contains plenty of pore space through which water can flow with relative ease. Ice, on the other hand, is dense and does not allow for fluid water to flow through with any sort of rapidity. Snow is deposited atop the glaciers and is eventually compacted to ice; during the intermediate stages of this compaction, however, we have layers that remain permeable, but don’t quite meet the density requirements to be called snow or ice. These layers are thus referred to as firn. Firn remains very permeable and, thus, allows water to escape with relative ease. Also facilitating water movement are fissures or cracks in the ice, which typically form through faulting of the glacier.

Without delving too deeply into the subject of hydrology, it is important to note that the flow of water, within the ice, follows the surface of the glacier. Thus, if the surface is dipping to the north, then the water within the ice will flow to the north. It does not depend on the slope of the ground at the base of the glacier except in extreme situations. This is another important aspect to keep in mind.

While eruptions occur beneath thin glaciers with notable frequency, these do not stray tremendously from eruption styles described elsewhere in this site. The thin glacier is mostly composed of snow and firn, and thus allows the melted water to escape until the overlying glacier has been removed from the erupting volcano. Due to the presence of meltwater, the eruption is almost always explosive, but can be come more effusive later once the glacier has retreated to some distance away.

Thick Glacial Volcanism

The behavior of large glacial volcanoes mirror, to some degree, the formation of volcanic islands as discussed elsewhere in this site. The eruption typically begins under a very large and heavy sheet of ice, which is adequate to suppress any explosive activity and allow the creation of pillow lavas. The lava, as you might guess, quickly melts the ice it contacts. As the lava continues to enter the base of the glacier, water is melted in volumes roughly ten times greater than the volume of erupted lava, causing the formation of a large subglacial lake. Liquid water, being denser than ice, causes the surface to collapse under the tensile stresses (the ice’ support is being removed). This collapse restricts the flow of water from the vault as it means the glacier surface now dips toward the volcano. From here, the physical structure of glaciers dramatically comes into play.

Where water barely, or simply does not, leak at the base of the volcano, the water may accumulate to the point where it overflows its chamber and floods the surface of the glacier. More commonly, however, water does not reach that level in the ice vault. Where a glacier has a thick layer of snow and firn capping it, the water may build to that point and escape. This layer has to be extremely thick so that the water can escape before building enough to float the glacier (remember that liquid water is more dense…picture ice cubes in a glass of water). The melt water in the vault builds to the point at which it reaches the base of this firn layer and begins to escape. The water then creates a channel, through thermal erosion, over the course of hours or days. You can think of the water as simply melting a drainage into the ice as it passes over it. The channel will continue to drop and drain the water until the vault is effectively drained, allowing the eruption to progress as though there was no surrounding ice (though flow of lava will still be heavily influenced). Water may also escape through pre-existing cracks in the ice, which result in a similar type of situation.

Where there is no such thick layer of firn at the surface and no sizeable faults, the water continues to build and approach the surface. The water’s only escape is along the base of the glacier, where it meets with the underlying sediment. Eventually, the pressure of the water is enough to literally lift the overlying glacier and float it. This allows for a sudden and massive release of water called a johkulhlaup. The johkulhlaup is curtailed when the escaping water level lowers enough for the weight of the glacier to overcome it and collapse back down. The flood is not completely stopped, however, as water continues to flow from the base through channels that were eroded during the main release of water. As mentioned earlier, these are the most dangerous and destructive events possible during a subglacial eruption. Eventually, the channels will become plugged and the volcano will, once again, become subaqueous.

Regardless of the type of glacier, however, the actual eruption of lava behaves much like the emerging island scenario. The pillow lavas build themselves up to a point where they approach the surface of the ice. Here, a shoaling-stage style eruption takes place, which may involve the eruption of sizeable dark ash plumes as well as explosive ash jetting through a vent punched through the ice. The timing and duration of these stages depend heavily on the drainage of the ice vault in which the volcano resides. Once the vault has been drained, the eruption will proceed as though there was no ice (most subglacial volcanoes today are basaltic in composition and thus erupt in the Hawaiian or Strombolian styles during late stages).

Throughout the eruption, the physical characteristics of the lava continually change. At the start of the eruption the lava is erupted as pillow basalts, but as it grows upward the lava becomes more vesicular as gas has more opportunity to exsolve. Next, the lava begins to shatter, forming hyaloclastites and tephras. If the eruption continues and is energetic enough, it may explode with enough energy to punch through the ice and form tephra that falls back into the water and settles out to the surface of the volcano. Finally, if the water drains away, you may again have effusive activity.

Extinct subglacial volcanoes are usually easy to identify and are very common in British Colombia, Canada. The ease in identifying them comes from their shape. As the pillow lavas flow, they melt their way through the ice. The ice, however, causes the pillows to quickly harden and stop, meaning that the lava must be diverted to new pillows forming overtop of the old. This means the boundaries of the volcano are likely to be very steep with a fairly flat top. These extinct volcanoes are called table mountains, or tuyas in British Colombia. At the top of a table mountain, you can find layers of ash and hyaloclastite. In situations where the vault was completely drained, you may find a small shield volcano and even cinder cones and a lava delta. Frequently you find breccias (deposits of shattered material) where the confining ice wall has melted away, causing the side of the volcano to collapse.


Subglacial volcanoes are easy to find, if you don’t mind counting extinct examples. For active subjects, the field narrows significantly to Iceland and Antarctica. While many eruptions occur atop glacier-capped peaks, a truly “subglacial” eruption requires the glacier to be thick enough to significantly influence the eruption (beyond lahar production). We’ve looked at the dangers posed at these volcanoes, as well as the manner in which they erupt. Though relatively uncommon on a global scale, the subject can be incredibly interesting to study, and extremely important for those living nearby (Iceland). To find examples to see yourself, you need only visit British Colombia. I encourage you to continue looking at this site and finding aspects that interest you. You can never know too much.