GEOLOGY
The Katla Volcano and Its Glacier System
A blue cave, a vast ice cap and one of Iceland's most powerful volcanoes are the same story. Here's how Mýrdalsjökull, Kötlujökull and Katla fit together.
Check dates and availability ↗Mýrdalsjökull — the ice cap on top
The cave begins with the ice above it. Mýrdalsjökull is one of Iceland's largest ice caps, put by recent surveys at somewhere around 520 to 595 square kilometres, with an average ice thickness of roughly 230 metres. It sits over the south coast, catching the moisture that rolls in off the North Atlantic, which is part of why this stretch of Iceland is so wet and why the glacier is fed so heavily. This is not a thin skin of ice but a deep, slow-moving mass, and its sheer volume is what makes the meltwater caves possible in the first place — there is simply an enormous amount of ancient, compressed ice for water to carve its way through.
Kötlujökull — the outlet tongue
Ice caps don't sit still; they push outward and drain downhill through outlet glaciers, and Kötlujökull is the tongue that matters here. It is the outlet down which most of the ice cap's meltwater floods have historically drained, flowing south onto the black plain of Mýrdalssandur. The Katla ice cave forms inside this outlet glacier, near its broken, sandy front. Because Kötlujökull is actively moving — flowing slowly downhill and remelting from within — it is constantly reshaping itself, which is exactly why the cave is never the same two seasons running. When guides talk about 'the glacier,' this outlet tongue is the specific ice they mean, and its restlessness is the engine behind everything you see inside.
The caldera hidden under the ice
Beneath all that ice lies Katla itself, one of Iceland's largest and most powerful volcanoes. Its summit reaches about 1,512 metres, and its caldera — some 10 kilometres across — lies buried under an estimated 200 to 700 metres of ice. That buried caldera is central to why Katla is so watched: a volcano capped by hundreds of metres of glacier is a volcano with a vast store of ice sitting directly over its heat. You cannot see the caldera from the surface; it reveals itself only indirectly, through the ash the glacier carries and, historically, through the floods its eruptions unleash. Standing in the cave, you are on the ice above a caldera you will never lay eyes on directly.
Jökulhlaups — when the volcano melts the glacier
The most dramatic link between volcano and glacier is the jökulhlaup, or glacial outburst flood. When Katla erupts, its heat melts vast quantities of ice at once, and that water bursts out from under the glacier and surges down Kötlujökull and across Mýrdalssandur in a matter of hours. These floods are part of what built the black outwash plain the super-jeeps cross to reach the cave; the 1918 eruption produced an especially enormous discharge. It's a useful thing to understand while standing on the ice: the flat black sands around the glacier are not a beach but the deposited record of past floods, and the whole landscape is shaped by the interplay of fire and ice rather than by ordinary rivers.
The ash record in the walls
All of this geology becomes visible inside the cave as the dark bands running through the blue. Over centuries the glacier has absorbed layer after layer of volcanic ash from Katla's eruptions, and as the ice flows those layers are carried along and surface in the cave walls as stripes and smears. In effect the glacier keeps a diary of the volcano beneath it, and the cave is a cross-section of that diary. Each dark line marks ash that fell in some past eruption and was then buried by fresh snow that compressed into ice. It is one of the few places where you can read a volcano's history not from a textbook but from the frozen material directly around you.
A long quiet, closely watched
Katla has erupted at least 23 times since the year 920, with major events in 1918, 1860, 1823, 1755 and 1721, and it has historically tended to erupt every few decades. Since 1918 it has stayed quiet — a dormancy that is among the longest in its recorded history — and it is monitored closely by Iceland's scientists. A 2019 assessment estimated a high probability of a significant eruption within the following decades, but volcanologists are careful with the word 'overdue,' and nothing is erupting or imminent as you read this. For a visitor, the takeaway is simple: the volcano beneath is part of the story rather than a hazard you're walking into unannounced, and the everyday risks on the trip are glacier risks, managed by your guide.