How Dettifoss and Its Canyon Were Made

Dettifoss is not a permanent feature. It is the working face of an erosion event that is still running, and the canyon behind it is the record of where the falls used to be.

The river that does the cutting

The Jökulsá á Fjöllum — the glacial river of the mountains — rises at the northern edge of Vatnajökull and drains a large part of northeast Iceland on its way to the Arctic. It is the second longest river in the country. At Dettifoss it carries an average of 193 cubic metres a second, and the significant word is average: a glacier-fed river runs low in winter and high through the summer melt, and higher still when something happens under the ice.

The floods that made the canyon

Jökulsárgljúfur was not cut gradually. It was cut by jökulhlaup: catastrophic glacial outburst floods. Volcanic activity beneath an ice cap melts a reservoir of water, and that water leaves all at once. About eight thousand years ago fissures of the Askja system erupted directly beneath the river and the ice. The floods that followed moved enough rock to open a canyon.

Vatnajökull National Park records at least two such events shaping Ásbyrgi at the canyon's northern end: one eight to ten thousand years ago and another about three thousand. The scale is hard to hold in the head. A single flood left a horseshoe 3.5 kilometres long, cliffs a hundred metres high, and a rock island 250 metres wide standing in the middle of it.

Why the water is grey

Glacial rivers carry rock flour: fine sediment ground off the bedrock by the weight of moving ice. The Jökulsá á Fjöllum carries a great deal of it, and that is why Dettifoss is brown-grey rather than the blue or white of a spring-fed Icelandic waterfall. Visitors who have seen photographs of Gullfoss or Selálandsfoss are routinely surprised by it.

The silt is also part of the mechanism. Water alone does not cut basalt quickly; water loaded with abrasive sediment and dropping 44 metres does.

The basalt

The canyon walls are layered basalt lava flows. That is why they break into the vertical columns you see on both rims, and into the flat pavement the west path crosses on the way to Selfoss. Basalt fractures into columns as it cools slowly and evenly. The joints between those columns are the weakness the river exploits: it does not grind the rock away so much as pull it apart, block by block.

Downstream at Hafragilsfoss the rock takes on red tones where iron in the basalt has oxidised, and the same colouring gives Rauðhólar, further down the canyon, its name.

A waterfall that is moving

Because the falls undercut and then collapse the columns at the lip, Dettifoss retreats upstream. Vatnajökull National Park gives the rate as about half a metre a year. Over a human lifetime that is forty metres; over the eight thousand years since the floods it is the whole canyon.

It also means the three waterfalls on this stretch of river are one process at three stages. Selfoss, a kilometre upstream, is wide and low because the river there spreads across a broad ledge. Dettifoss is concentrated and deep. Hafragilsfoss sits where the falls were long ago, and the canyon has had time to widen around it.

Ásbyrgi, and the hoofprint

The northern end of the canyon is a flat-floored horseshoe closed by cliffs, with a pool called Botnstjörn beneath the highest of them. Geology calls it a plunge pool. A waterfall stood here briefly during the floods, then vanished when the river changed course. Norse tradition calls it the mark left when Sleipnir, Odin's eight-legged horse, put down a hoof.

The national park publishes both, in that order, without adjudicating. That is the right way round: the flood explanation is what happened, and the hoofprint is what a people who could see the shape of it made of the fact.

Where this sits in the park

The canyon has been protected, under one name or another, for over fifty years.

The 2019 inscription cites the interaction of ice and volcanic activity. That is precisely the process that made this canyon, which is a neat piece of circularity: the park is a World Heritage Site because of the thing you are standing in.

The park is not a museum of a finished landscape. Ten central volcanoes sit under the ice cap, eight of them subglacial. The next jökulhlaup is a question of when. The canyon is what the last ones left.