Kerið’s crater lake reveals the restless volcanic underpinnings of south Iceland

Amid the undulating lava fields of south Iceland, Kerið draws attention for its striking, oval-shaped depression and a luminous pool of water at the bottom. Found in Grímsnes along the well-known Golden Circle route, this crater lake exemplifies how Iceland’s volatile geology leaves lasting imprints on the landscape.

A caldera formed by ancient fire

Kerið stands out among other features of the Western Volcanic Zone, a region that stretches from the Reykjanes peninsula north to the Langjökull Glacier. Scientists believe the crater’s origins can be traced back six to nine thousand years, although there is some debate over the exact date. It was formed when volcanic explosions tore open the earth along what is now an 800-metre fissure called Tjarnarhólar, part of a line of four craters. The site’s deep basin, lined with layers of basalt and scoria, speaks to the intense forces that shaped it.

Measured from rim to base, Kerið plunges 55 metres. Its shape is distinctly elliptical, reaching 270 metres across and 170 metres wide. The walls sweep steeply down to the centre except in one area, softened by a gentle slope and dense moss, providing both a contrast in colour and texture to the exposed volcanic rock.

An ever-changing pool below

At the crater’s heart lies a small lake whose depth shifts with the surrounding groundwater, usually between six and fourteen metres. The water’s pure blue-green hue is influenced by mineral runoff from the iron-rich volcanic soil, while the red tones of oxidised scoria encircle it above. Life has also made a quiet home here: surveys have recorded ten different types of zooplankton, mainly from the group known as Rotifera, inhabiting the lake.

Unlike most of the crater’s rim, the gently sloping bank is cloaked in moss, a rare sight amid the largely bare and vertical sides. Vegetation clings here more stubbornly, marking a slow reclaiming of land shaped by an ancient eruption.

Kerið is managed by a local landowner who oversees visitor access, reflecting the balance between preservation and the regular flow of guests drawn to its dramatic form. As one of the area’s best-preserved calderas, Kerið stands as a reminder of the complex volcanic activity beneath much of Iceland’s surface.

Topicscrater lakeGolden CircleGrímsnesKeriðvolcano