THE STORY
Contents
- Kerið, Iceland: Why the Red Crater Caved Inward, the Horse with Backward Hooves, and the Volcano We Drive On
- How a Mountain Rusted in Mid-Air and Swallowed Its Own Summit 6,500 Years Ago
- The Open Window into the Aquifer and the Horse with Backward Hooves
- Musicians on Floating Boats, the 2013 Tollbooth War, and the Sibling Volcano Under Your Tires
- Quick Facts: Kerið Crater & The Grímsnes Volcanic System
- Frequently Asked Questions (FAQ)
- Related Iceland Explained Episodes
Kerið, Iceland: Why the Red Crater Caved Inward, the Horse with Backward Hooves, and the Volcano We Drive On
Pick up a fist-sized stone along the upper rim of Kerið, fifteen kilometers north of Selfoss on Route 35, and your hand expects the dead weight of Icelandic iron. Across most of the country, black basalt is dense enough to ring under a boot heel. The clinker in your palm at Kerið feels almost hollow: riddled with frozen gas bubbles, jagged as broken bottle glass, and coated in a dry, brick-red powder that stains your thumb like rusted cast iron. Out here on the flat lava plains of Grímsnes, the wind sweeps off the heath with nothing to slow it down, snapping at your jacket collar while fifty-five meters below your boots an oval sheet of opaque aquamarine water lies completely still.
Tour buses pulling off the southern leg of the Golden Circle call this place "The Eye of the World." Local farmers, who had to live alongside it for a thousand years, named it Kerið, which simply means "The Tub." For decades, even the printed guidebooks got the tub backwards, telling visitors they were looking into a three-thousand-year-old explosion crater (maar) blasted open when rising magma hit groundwater, filled today with trapped rain. Once you look at the tephra layers, the water table, and the political battles fought over its rim, every single detail of that postcard story turns inside out.
How a Mountain Rusted in Mid-Air and Swallowed Its Own Summit 6,500 Years Ago
To see why the explosion story falls apart, look at the ground outside the fence. When magma meets shallow groundwater and blasts a true phreatomagmatic maar into the bedrock, the steam explosion shatters the surrounding country rock and throws a wide, low-angle apron of grey base-surge ash across the plain. Kerið has no explosion ash ring at all. Its walls are built entirely of stacked vesicular scoria, welded spatter, and volcanic bombs erupted along the eight-hundred-meter-long Tjarnarhólar fissure.
Tephrochronology, reading the ash layers from prehistoric eruptions of neighboring Hekla like barcode stripes in the soil, shows that the Grímsnes Volcanic System is Iceland's smallest and youngest distinct Holocene volcanic field. Covering barely fifty-four square kilometers without a central stratovolcano, the system opened roughly twelve short northeast–southwest fissures that poured out 1.3 cubic kilometers of lava after the Ice Age ice sheet retreated. The sibling craters of Seyðishólar just to the northeast erupted first around 9,500 years ago; the vegetated sister cone of Kerhóll followed around 7,050 years ago; and the Tjarnarhólar fissure tore open around 6,500 years ago: more than double the three-thousand-year estimate still repeated on tourist signs.
Instead of exploding outward in a single steam blast, Kerið rose as a roaring curtain of Strombolian and Hawaiian fire fountains. Usually, when basaltic lava flows across Iceland as a thick, insulated river, its iron stays locked in a reduced ferrous state (FeO) and cools into charcoal-black stone. At Kerið, expanding gases shredded the molten rock into frothy, gas-pocked cinders and hurled them high into an oxygen-rich sky saturated with superheated steam. With that huge surface area exposed to air while still glowing near a thousand degrees Celsius, the ferrous iron inside every bubble wall flash-oxidized into ferric iron oxide: crystalline hematite (Fe2O3). The mountain rusted crimson in mid-flight before the cinders even hit the ground.
Then the plumbing underneath gave out. As the eruption wound down, the shallow magma pocket directly beneath the cone drained away: either feeding lateral lava flows across the Grímsnes plain or retreating deeper into the crust. Deprived of hydraulic support from below, the summit and core of the cinder cone could no longer hold up their own weight. The center of the mountain slumped straight downward into the evacuated chamber, leaving an elliptical collapse bowl two hundred seventy meters long, one hundred seventy meters wide, and fifty-five meters deep. Because the steep southern and western inner walls stand at roughly thirty-five degrees, the natural angle of repose for loose gravel, gravity keeps shedding cinders downward, preventing soil from forming and keeping the 6,500-year-old red hematite raw and exposed.
The Open Window into the Aquifer and the Horse with Backward Hooves
Walk down toward the bottom of the bowl and the second optical illusion appears. The opaque turquoise oval at the base of the red walls looks like a rainwater pond sealed inside a clay cup, yet no stream flows into Kerið and no river drains out. If it were merely stagnant rain, dry summers and wind evaporation would shrink it into a mudflat.
Instead, when the cone's core collapsed 6,500 years ago, its floor dropped below the saturation horizon of the porous Grímsnes lava field. The lake is a natural piezometer: an open cross-section of the regional groundwater table. When snowmelt and autumn rain recharge the surrounding basalt aquifer, the lake rises with the water table; when the regional aquifer drops, the lake subsides, its level rising and falling with the surrounding groundwater. Dissolved silica and iron-magnesium minerals leached from the volcanic glass, combined with fine silicate and clay particles washed off the red walls, scatter short blue and green wavelengths of sunlight back to your eyes. Down inside that cold, nutrient-poor water column, completely cut off from Iceland's salmon and trout rivers, limnologists have found a self-contained biological island of ten zooplankton taxa, including five rotifer taxa: wheel-animalcules whose resting eggs can reach isolated waters on the wind or on visiting ducks.
Centuries before anyone used the word piezometer, the farm families of Grímsnes noticed that the water in "The Tub" rose and fell on its own schedule without waiting for local rain, and they mapped that invisible hydrology onto a predator. According to local folklore preserved across Árnessýsla, the bottom of Kerið is connected by a dark subterranean tunnel to a small mountain pond atop Búrfell í Grímsnesi five kilometers to the northwest: whenever the water rises in Kerið, it drops in Búrfell, and swimming back and forth between the two basins is the Nykur.
In Icelandic tradition, the Nykur (water kelpie) walks out of deep crater lakes looking like a calm, handsome grey farm horse standing quietly by the bank, waiting for a tired traveler or a curious child to climb onto its back. Only one physical clue betrays it: its hooves are turned backward (hófar snúa aftur). The instant a hand touches its withers or a leg swings over its spine, the horse's hide turns sticky as pine pitch, gluing the rider fast as the creature gallops straight down into the freezing water. The only way to break the bond before drowning is to shout its true name (Nykur!) aloud. Strip away the supernatural coat, and the folk tale is two pieces of field observation welded together: an accurate pre-scientific grasp that Kerið's water moves through underground lava channels, and a memorable piece of rural child-safety engineering designed to keep farm children away from a thirty-five-degree sliding gravel pit that drops into deep, icy water.
Musicians on Floating Boats, the 2013 Tollbooth War, and the Sibling Volcano Under Your Tires
Step onto the one-hundred-thirty-nine timber stairs descending the gentler, moss-covered northern flank, and you cross a physical boundary that cannot be seen in photographs. Up on the crest, the lowland gale buffets your ears; farther down the staircase, the rim can shelter you while the wind sweeps overhead. The fifty-five-meter inverted cone of porous scoria works as both a wind baffle and a parabolic acoustic mirror. Down on the shoreline bench, the ambient noise floor drops so low that you can hear a single piece of red cinder skitter down the opposite scree face and your own breath inside your collar, while a voice spoken on the water projects straight up the angled walls to the rim like a Greek theater.
In August 1987, long before fleets of tour coaches queued along Route 35, musicians including singer Björgvin Halldórsson and tenor Kristján Jóhannsson performed from boats on the cold lake at a concert organized by the regional association Héraðssambandið Skarphéðinn, with the audience seated on the crater slopes. It was an unforgettable setting for the listeners sitting on the bank, and a bruising afternoon of manual labor for whoever had to lug the amplifiers, drums, and cables down and up fifty-five meters of volcanic gravel.
Those 139 timber steps, and the fact that any green moss still survives on the northern wall, are the product of one of twenty-first-century Iceland's sharpest legal battles over who owns the landscape. Since the Viking Commonwealth lawbook Grágás, Icelandic law has protected almannaréttur: the ancient public right to walk freely across uncultivated private land. That custom worked for a thousand years when a few dozen horsemen passed by each summer. By 2013, however, hundreds of thousands of Golden Circle visitors were scrambling up and down Kerið's loose inner walls every season while the state contributed nothing toward upkeep.
On the sheltered northern slope, the crater's green skin consists of woolly fringe-moss (Racomitrium lanuginosum, locally called gamburmosi), which drinks moisture directly from the fog and grows barely a few millimeters a year. When dry, its cell structure shatters under the twist of a single boot sole. Uncontrolled foot traffic was grinding decades of moss into black mud, carving deep erosion gullies into the red hematite, and shoveling tons of loose scoria straight into the lake. When the private landowner association Kerfélagið put up a small wooden booth in 2013 and began charging visitors a few hundred krónur to fund paths and stairs, it set off a national uproar: complete with denunciations from then-Interior Minister Ögmundur Jónasson and angry headlines accusing the owners of putting a tollbooth in front of a volcano.
Yet if you want to see the alternative fate for a red scoria cone in Grímsnes, you only have to walk to Kerið's northern rim and look two kilometers northeast across the lava field. Rising to 214 meters is Seyðishólar, Kerið's 9,500-year-old older sibling, built of the exact same flash-rusted crimson hematite and porous black cinder. Nobody argues about entrance tickets or takes holiday portraits at Seyðishólar, because South Iceland is already driving on it. For decades, heavy excavators have chewed open Seyðishólar's red volcanic flanks in an industrial quarry, hauling away its oxidized cinders to ballast highway sub-bases and insulate house foundations across the region. Two cones born from the same magma plumbing: one was turned into road gravel, and the other survived intact because a fence and a timber staircase lifted human boots off the moss.
Come back to Kerið in January, when the sun barely clears the southern horizon and never touches the bottom of the pit, and the crater drops its summer colors altogether. The red hematite walls darken into a heavy, matte maroon bruise against the snow, the aquamarine window freezes into a flat white plate of ice, and freezing rain turns the thirty-five-degree slopes and wooden steps into a glazed chute where smooth-soled city boots will deliver you to the bottom far faster than you planned without steel microspikes (broddar). Down at the water's edge, out of reach of the wind roaring fifty-five meters overhead, the red dust on your glove is still 6,500-year-old airborne rust, and the silent ice sheet under your boots is still rising and falling with the hidden water table beneath the plain.
A STORY. NOT A GUIDE.
ICELAND. TOLD.
Quick Facts: Kerið Crater & The Grímsnes Volcanic System
| Feature / Attribute | Verified Field Detail |
|---|---|
| Location & Coordinates | Grímsnes district, South Iceland (Route 35, 15 km north of Selfoss); 64.0411, -20.8850 (~175 m a.s.l. at the rim). |
| True Age & Formation | ~6,500 years old (mid-Holocene, more than double the old 3,000-year guidebook myth); formed at the northern end of the 800 m Tjarnarhólar fissure when a Strombolian/Hawaiian scoria cone drained its magma chamber and collapsed inward. |
| Dimensions & Scale | 55 meters deep, 270 meters long, and 170 meters wide; inner collapse walls rest at a ~30–35° angle of repose. |
| Rock & Water Chemistry | Walls: Vesicular tholeiitic basalt scoria flash-oxidized in flight into crimson ferric hematite (Fe2O3). Lake: Natural piezometer (groundwater window) rising and falling with the Grímsnes aquifer (historical surveys measured a maximum depth of 8.8 m); inhabited by 10 zooplankton taxa, including five rotifer taxa. |
| Culture, Folklore & Law | Folklore of the backward-hoofed Nykur and the underground tunnel to Búrfell; site of the documented 1987 concert on boats; center of the 2013 almannaréttur vs. Kerfélagið conservation fee dispute; paired with the quarried sibling cone Seyðishólar (~9,500 BP). |
| Primary Field Hazard | Winter glaze ice on the 139 timber steps and 35° scoria slopes (bring steel microspikes / broddar Oct–Apr); summer ball-bearing slip on loose rim gravel and fragile Racomitrium moss (gamburmosi). |
Frequently Asked Questions (FAQ)
Is Kerið an explosion crater, and is it really 3,000 years old?
Neither claim is true. Modern tephrochronology and radiocarbon dating show that Kerið erupted roughly 6,500 years ago along the Tjarnarhólar fissure in the Grímsnes volcanic system. Because there is no phreatomagmatic base-surge ash ring around the rim, geologists established that Kerið did not form as a steam-explosion crater (maar); instead, it was a fire-fountaining scoria cone whose underlying magma reservoir drained away, causing the center of the cone to collapse inward under gravity.
Why does the water level in Kerið crater lake change without any rivers flowing in or out?
When the center of the scoria cone collapsed 6,500 years ago, the crater floor dropped below the saturation level of the porous Grímsnes lava field. The lake is not trapped rainwater; it is an open window into the regional groundwater table (a natural piezometer) that rises and falls as the surrounding aquifer fluctuates, while suspended silicate minerals scatter sunlight into an opaque aquamarine hue.
Why are Kerið's crater walls bright red instead of black like most Icelandic lava?
During Kerið's fire-fountaining eruption, expanding volcanic gases shredded the basaltic magma into porous, frothy cinders and hurled them high into an oxygen- and steam-rich atmosphere. While airborne at extreme temperatures, the ferrous iron (FeO) in the cinders rapidly oxidized into ferric iron oxide, hematite (Fe2O3), rusting the rock bright red before it landed.
Why is there an entrance fee at Kerið when Iceland has the right to roam (almannaréttur)?
Kerið sits on private land managed by the landowner association Kerfélagið. By 2013, hundreds of thousands of visitors scrambling down the steep slopes were destroying the slow-growing Racomitrium moss and causing severe scoria landslides into the lake without state conservation funding. The landowners introduced a small maintenance fee to build and maintain the rim paths, viewing decks, and the 139-step timber staircase that protects the crater walls from erosion.
Related Iceland Explained Episodes
- Selfoss, Iceland | The Suspension-Bridge Town on a Fault: Drive fifteen kilometers south of Kerið to the Ölfusá river crossing where imported Newcastle bridge steel, Bobby Fischer's grave, and strike-slip faults meet.
- Brúarfoss, Iceland | The Blue Fissure Waterfall, Head north up the Golden Circle to see how subterranean lava filtration turns another spring-fed system electric cyan, and why Skálholt broke its natural stone bridge in 1602.
- Laugarvatn, Iceland | Hot Sand, Móberg, and Geothermal Bread (coming later): Follow Route 365/37 past Kerið to the boiling lakeshore that baptized the Vikings in 1000 AD.
