Huge blocks of ice constantly break off the glacier, Breiðamerkurjökull, and large icebergs float on the lagoon. The lagoon is not very wide but it is over 250 meters deep which makes it the deepest lake in Iceland.
Breiðamerkurjökull is an outlet of the Vatnajökull glacier. The icebergs that calve from the glacier edge move towards the river mouth and get entrenched at the bottom. The movement of the icebergs fluctuates with the tide currents, as well as being affected by wind. However, they start floating as icebergs when their size is small enough to drift to the sea. These icebergs are seen in two shades: milky white and bright blue, which depends on the air trapped within the ice and is an interplay of light and ice crystals
The first settlers arrived in Iceland around AD 870, when the edge of the tongue of Breiðamerkurjökull glacier was about 20 km (12 mi) further north of its present location. During the Little Ice Age between 1600 and 1900, with lower temperatures prevailing in these latitudes, the glacier had grown by up to about 1 km (0.62 mi) from the coast at Jokulsá River, by about 1890.
When the temperatures rose between 1920 and 1965, the Breiðamerkurjökull glacier tongue rapidly retreated, continually creating icebergs of varying size, thus creating a lagoon in its wake around 1934–35.
The lake is over 200 m (660 ft) deep where the glacier snout originally existed. Glacial moraines became exposed on both sides of the lake.
In 1975, the lake was about 8 km2 (3.1 sq mi) in area and now it reportedly stands at 18 km2 (6.9 sq mi) at the edge of the glacier tongue.
In spite of being a rather recent formation, Jökulsárlón is the deepest lake in the country, with depths of 248 metres (814 feet). With a surface area of 18 square kilometres (7 square miles), it is also growing to be one of the largest.