GLACIER SCIENCE
How Jökulsárlón Formed and Why It Keeps Growing
A lagoon younger than living memory: how a retreating tongue of Vatnajökull carved Jökulsárlón, why it is Iceland's deepest lake, and why it keeps growing.
Check dates and availability ↗The glacier that reached the sea
Breiðamerkurjökull is one of the largest outlet tongues of Vatnajökull, Europe's biggest ice cap by volume. For most of recorded settlement it flowed almost the whole way to the Atlantic, leaving only a thin strip of black sand between its snout and the surf. As it advanced and retreated over centuries it behaved like a slow bulldozer, grinding the ground beneath it and pushing debris ahead into ridges. Crucially, because the ice was thick, heavy and moving, it cut downward as well as forward, carving a basin in the bedrock that would later hold the lagoon. Nothing about Jökulsárlón makes sense without starting here: the lake is the shape the glacier left behind when it withdrew.
A trench gouged below sea level
The detail that turns this from an ordinary meltwater pond into something remarkable is depth. As Breiðamerkurjökull ground forward it excavated a trough reaching well below sea level — the reason Jökulsárlón is now, by common account, Iceland's deepest lake, with published soundings ranging from roughly 248 to over 284 metres. That spread is not sloppy surveying so much as a moving target: the basin keeps enlarging. Because the floor sits below the level of the sea, the lagoon connects to the ocean rather than perching above it, and that single fact drives almost everything else about the place — its tides, its salinity, and the fact that it does not freeze solid even in the depths of an Icelandic winter.
Born in the 1930s
Jökulsárlón is one of very few world-class natural sights in Europe younger than living memory. Until the early twentieth century Breiðamerkurjökull still reached nearly to the shore, and there was no lagoon at all. In the warming decades after the late nineteenth century the glacier began to pull back off the basin it had carved, and around 1934–35 that hollow started filling with meltwater. Within a few decades a pond had become a lake. People who grew up on farms along this coast can, in principle, remember a time before Jökulsárlón existed — which is a genuinely strange thing to be able to say about a place now on the front of guidebooks.
Why it keeps growing
The process that made the lagoon has not stopped. Breiðamerkurjökull's snout continues to retreat — on the order of 200 metres a year in recent times, though the rate varies — and every metre it withdraws hands more of that deep bedrock trench over to open water. The lagoon has grown roughly fourfold since the 1970s, from around 8 km² to somewhere in the region of 18 to 25 km² depending on whose survey you read. Warmer summers accelerate melt at the calving front and across the surface, and the tidal connection to the sea helps eat at the ice from below. In practical terms, the Jökulsárlón you photograph is measurably larger than the one earlier visitors saw.
What calving is, and why the ice is blue
The icebergs come from calving — the glacier's front breaking apart where it meets deep water. Buoyancy and the pull of the tide undermine the ice until slabs shear off, sometimes with a crack you can hear across the lagoon, and float free. That ice is old: it began as snow that fell on Vatnajökull's high plateau and was buried, compressed and driven slowly downhill over centuries. The compression is why so much of it glows deep electric blue — the trapped air bubbles are squeezed out until the dense ice absorbs every colour but blue. White bergs are younger or more aerated; black stripes are volcanic ash swallowed during past eruptions and carried down inside the ice. Reading those colours tells you the ice's history.
A lake still writing its own future
Jökulsárlón's short channel to the sea is slowly widening and deepening, and observers have long flagged that the surf is working back toward the Ring Road bridge over the outlet — the shoreline there has been reinforced more than once. Look far enough ahead and the lagoon's fate is tied to the glacier's: as Breiðamerkurjökull continues to retreat inland and thin, the basin will keep filling, and some researchers expect the bay to eventually open more fully to the Atlantic. None of that is settled, and the timescales are long and uncertain. But it underlines the essential character of the place — this is not a finished landscape but one visibly, actively under construction.