An explosion crater from the settlement period

Hnausapollur formed during the Vatnaöldur eruption, part of fissure activity associated with the Bárðarbunga volcanic system. The eruption occurred around the time of Iceland’s settlement. It was traditionally connected with the year 871, based on the Settlement tephra used to date early archaeological remains, although revised dating places that tephra at approximately 877 ± 2.

The eruption opened a long series of vents across the central Highlands. At Hnausapollur, rising basaltic magma encountered groundwater. The resulting interaction caused violent explosions that fragmented the magma and surrounding rock, excavating a broad crater into the landscape. Craters produced in this way are commonly described as maars.

Water later occupied the depression, creating the lake seen today. Hnausapollur is therefore not simply a collapsed volcanic cone or a basin left by a lava flow. Its rounded form was created by explosive interaction below and at the surface, while the lake represents a later stage in the crater’s history.

The crater walls remain steep enough to make the formation immediately legible from above. Dark and reddish volcanic material encloses the blue water, while erosion, vegetation and weathering have softened parts of the rim without erasing its overall shape.

How Hnausapollur became a crater lake

The force that formed Hnausapollur came partly from water. When magma rises into wet ground, groundwater can heat and expand almost instantly. This sudden conversion to steam increases the violence of the eruption and breaks both new volcanic material and older ground into fragments. Repeated explosions enlarge the vent into a broad depression rather than building a high, symmetrical cone.

After eruptive activity ended, the crater became a basin in which water could collect. There is no glacial river entering Hnausapollur, and its colour should not be explained through glacial sediment. Nor is there enough evidence to state that suspended volcanic sediment alone produces the familiar turquoise appearance.

The lake is better understood through what can actually be observed. Its colour changes with reflected sky, direct sunlight, cloud cover, surface movement, water clarity and the angle from which it is viewed. Bright overhead light can illuminate more of the basin and strengthen the separation between blue water and dark crater walls. Under overcast skies, the lake may become subdued, grey-blue or green. Wind replaces clean reflection with texture and can make different parts of the surface appear to carry different tones.

This variation is part of Hnausapollur rather than a departure from its supposed “true” colour. The intense blue seen in many photographs is one expression of the lake under particular light, not a permanent property that every visitor should expect to find.

Hnausapollur and Ljótipollur

Hnausapollur and Ljótipollur lie close together north of Landmannalaugar. Both are crater lakes, both are reached from the F208 road network, and both place coloured water inside dark volcanic landforms. Their proximity makes it easy to assume that they were created during the same eruption.

They were not. Hnausapollur formed during the settlement-period Vatnaöldur eruption, around 877. Ljótipollur formed during the much later Veiðivötn eruption of 1477, part of the connected Veiðivötn–Torfajökull volcanic episode. The two craters therefore preserve events separated by roughly six hundred years.

Their visible character also differs. Hnausapollur is known particularly for the blue or turquoise appearance of its lake and the comparatively open terrain around it. Ljótipollur is defined more strongly by red and dark crater walls, with the lake held within a larger and more varied landform. Neither needs to be treated as a substitute for the other. Seen together, they reveal how similar volcanic processes can recur in the same region at very different points in history and produce distinct landscapes.

Nearby Frostastaðavatn provides a third useful comparison. It is a broad, exceptionally shallow, lava-impounded lake rather than an explosion crater. Within a short journey, visitors can therefore see three bodies of water whose similar highland setting conceals very different origins.

Reaching Hnausapollur from F208

Hnausapollur lies beside F208, a few kilometres north of Frostastaðavatn and Landmannalaugar. Most visitors use the parking area at the foot of the crater and walk up to the rim. The climb is short, and the lake becomes visible from above without a long hike.

An older one-way track also climbs towards the rim and returns to F208 at the other end. It remains in use, but it is now known mainly to people already familiar with the site. The rim is consequently less used for routine parking than older descriptions suggest. The lower parking area and short walk should be treated as the normal visitor approach rather than directing every vehicle onto the crater.

The parking area has room for approximately five or six vehicles. When it is full, drivers should not create new parking on the surrounding ground or obstruct the road. There are no dependable visitor services at Hnausapollur itself.

The northern approach on F208 normally reaches the crater without river crossings. Travellers arriving by F225 from the west or F208 from the south face different road conditions, and the southern route includes several river crossings before reaching the Landmannalaugar area. A description of Hnausapollur as an easy roadside stop must therefore not be mistaken for a description of the complete journey.

A four-wheel-drive vehicle is recommended for highland travel here, and drivers must follow the conditions attached to their vehicle and rental agreement. The road is seasonal, but fixed opening months are unreliable. Snow, thaw, water levels and road damage determine when each route becomes passable. Current road and weather information should always govern the journey.

The rim, fishing and visitor conditions

The lake can be viewed soon after reaching the upper rim, and short walks along its stable sections provide several compositions and viewing angles. A complete circuit is not a sensible visitor walk. Parts of the rim are steep, loose or unstable.

There is no barrier following the crater edge. Loose volcanic material can shift underfoot, and an edge that appears firm from above may be undercut or less stable than expected. This matters especially when people are concentrating on a camera, stepping backwards to widen a composition or placing a tripod close to the drop. Remaining well back rarely harms the view; the elevated position already reveals the lake and crater clearly. The steep ground between parts of the rim and lake also means that shoreline access should not be assumed to be easy everywhere.

Hnausapollur lies within Fjallabak Nature Reserve. Visitors should remain on existing roads and paths, avoid driving onto undisturbed ground and leave the crater as they find it. Its closeness to the road makes the lake accessible; it does not make the volcanic surface around it expendable.

• • Hnausapollur is also recorded under the alternative name Bláhylur, meaning “Blue Pool,” although Hnausapollur is the name commonly used locally.
  • The crater formed during the settlement-period Vatnaöldur eruption, traditionally dated to 871 but now placed at approximately 877 ± 2.
  • Rising magma interacted explosively with groundwater to excavate the crater.
  • Hnausapollur is commonly described as a maar or explosion crater.
  • Nearby Ljótipollur formed during the 1477 eruption, approximately six centuries later.
  • Frostastaðavatn, also nearby, is a lava-impounded lake rather than a crater lake.
  • Fishing occurs in Hnausapollur and requires a permit.
  • Most visitors now park below the crater and walk to the rim.
  • The rim cannot safely or sensibly be walked as a complete circuit.