Hvitserkur

Hvítserkur is a 15-metre-high basalt stack on the eastern coast of the Vatnsnes peninsula in northwest Iceland. Two holes at its base make it look like a dragon drinking from the sea, and concrete has been added to the bottom to stop the waves from wearing away its foundation.

Local legend says Hvítserkur was once a troll that tried to destroy the monastery at Þingeyrar, but the rising sun turned it to stone. In the mid-20th century, farmers reinforced the base with concrete—partly to keep the sea from toppling it, and partly out of affection for the old story. Today, many visitors come to see it, but the lack of marked trails has led to damage to nearby bird nests and seal resting areas. Landowners, local authorities, and conservationists are now debating how to manage access, safety, and wildlife protection, a challenge made more urgent by the rock’s popularity.

Geologically, the Hvítserkur ignimbrite is a caldera-fill pyroclastic flow that forms one of the largest ignimbrite sheets in northeast Iceland, appearing as a pale-coloured mountain. This welded tuff is beige to pink and is cut by many basaltic dikes. In some places, it is covered by hyaloclastites and pillow-lava breccias, suggesting that after the explosive silicic eruption, a shallow lake formed inside the caldera, where underwater volcanism continued.

The Hvítserkur ignimbrite was deposited during the early Miocene, around 13 to 12 million years ago, along the Borgarfjörður Eystri–Loðmundarfjörður rift zone—a chain of extinct central volcanoes now buried under younger basaltic lava. Radiometric dates of 12.5 ± 0.6 and 13.1 ± 0.2 million years from nearby complexes bracket its age, making it one of the oldest well-preserved silicic rocks in Iceland. Ongoing studies aim to understand how such large volumes of felsic magma formed and survived in a region dominated by basalt.

Beneath the ignimbrite lies a trapdoor-style collapse caldera about 7.5 by 4.5 kilometres across. Measurements of the surrounding basalt flows—which dip steeply toward the caldera centre—and ignimbrite thicknesses of up to 410 metres on the northern wall, 277 metres to the west, and 180 metres to the south, reveal a broad bowl-shaped depression filled by a single explosive eruption.

After the ignimbrite accumulated, a shallow lake formed in the caldera, where up to 70 metres of sandstone, siltstone, and mudstone were deposited. These lake sediments often show small ripples and drying cracks, and are capped by a roughly 5-metre-thick ash-flow layer—evidence that the water level sometimes dropped, exposing the basin and allowing fresh eruptions to pour hot ash and pumice back into the lake.

Later, many olivine-basalt dikes and sills intruded the ignimbrite and lake beds; some of these fed hyaloclastite and pillow-lava breccias, recording ongoing underwater volcanism. The very top of Hvítserkur is covered by subaerial basalt flows, marking a return to mostly mafic eruptions. The small hill Hvítafjall, rising from the caldera floor, is made of strongly altered rhyolite breccias and is thought to be an intra-caldera lava dome or plug, representing late-stage magmatic resurgence. Three-dimensional reconstruction of the ignimbrite’s top and base surfaces suggests its original pyroclastic volume was about 4 cubic kilometres before erosion removed its upper parts.