Iceland for Rock Nerds

From Petra's it's a bit over two hours inland and uphill, trading the fjord for the broad Jökuldalur valley, and the payoff is a textbook wall of hexagonal basalt columns with a turquoise glacial river running along the foot of it. Same physics that built the Giant's Causeway (cooling-contraction joints in a thick lava flow, cracking into those tidy hexagons as it slowly set), except here the whole thing is half-drowned in meltwater. Here's the nerd twist, and it's the good part: these columns sat underwater and unseen until the Kárahnjúkar hydropower project diverted the Jökla river in 2009, which makes the entire exposure you're admiring an industrial accident nobody planned as a view. Budget a couple of hours; the best angle is down on the canyon floor, a roughly five-kilometre round trip from the Stuðlafoss-side parking, following the river in and back out again. Admire the river; do not get intimate with it. It's a powerful glacial river, loud enough to feel in your chest and closed to entry for reasons that become entirely convincing the moment you're standing next to it, and the wet basalt underfoot has no interest whatsoever in your traction.

The drive up from Stuðlagil is another two hours or so, and it deposits you at Lake Mývatn in the long evening light, which is exactly where you want to be, because the whole of the next day is spent inside a single caldera and you've earned a night's sleep first. Start that Mývatn day at Krafla, at the top of the system, where the ground has form: this caldera physically tore itself apart during the Krafla Fires of 1975 to 1984. A borehole here later punched straight into rhyolite magma at about 2.1 km down, one of the vanishingly few times anyone has reached magma directly (they were drilling for steam and hit the thing steam comes from). The Víti explosion crater and the softly steaming boreholes of the power station are all right there from the public car park, no permit and no guide, just a short walk to the working edge of the plumbing. You could walk the crater rim, watch the boreholes vent and be back at the car within two hours. Keep to the marked paths: the ground here is both hot and unconvinced of your right to stand on it, hissing and venting steam a few steps off the boardwalk.

Eleven minutes down the road from Krafla, Hverir is the same geothermal system with its shirt off. Boiling mud pots, fumaroles roaring like something has gone wrong at a boiler, ground crusted in sulfur and precipitated minerals, and not one scrap of vegetation politely covering any of it. What you're actually watching is hydrothermal alteration in real time: solid basalt being cooked into pale, soft clay directly under your boots, the surface sitting somewhere between 80 and 100°C, which is the entire reason you stay on the boardwalk. One loop of the boardwalk is the whole visit, roughly forty-five minutes with the boiling mud and the fumarole roar for company. It also smells accordingly, so this is the natural moment to drop back into Reykjahlíð, the little village on the lake, for lunch and for getting the sulfur out of your hair before the afternoon.

Fed and de-sulfured, it's twenty minutes to Hverfjall, one of the largest and most symmetrical tephra explosion craters on the planet, which is an oddly specific superlative to hold, and it holds it well. It's a near-perfect ash ring about a kilometre across, thrown up around 2,500 years ago in a single violent phreatomagmatic eruption (rising magma met a shallow arm of Lake Mývatn, the groundwater objected, and everyone in the vicinity lost). The short, sharp climb from the north-west car park to the rim takes fifteen minutes at most, and then the whole geometry of that one very bad afternoon is laid out beneath your feet on the roughly hour-long loop around the edge. Watch your footing on the way up: it's loose ash and scree the whole way, sliding out from under you, with nothing between the rim path and the drop and nothing up top to break the wind.

Barely nine minutes on (or a longer walk, if you take the connector trail straight off Hverfjall's flank) is Dimmuborgir, a collapsed lava lake left as a maze of pillars, arches and hollow chambers, and the name, 'the dark castles,' is doing honest work. The rock froze hard around rising steam vents while the molten lake underneath quietly drained away, so what you're wandering through is essentially the negative space that lava lake left behind when it failed, formed some 2,300 years ago. Few places on Earth read that particular failure this clearly, which is worth the ninety minutes it takes to get properly lost in it, the paths ducking between pillars into cool, still, shaded air after the open wind of the rim. Icelandic folklore files the place under 'entrance to the underworld,' and it's also the namesake of a Norwegian black-metal band, but those are two separate obsessions and only one of them is geology.

Skútustaðagígar is about 14 minutes from Dimmuborgir and makes an easy final stop around Mývatn. Its grassy cones are pseudocraters, formed by steam explosions when lava flowed over wet lakeside ground about 2,300 years ago. No magma rose through the cones themselves. A mostly level 1.5-kilometre loop gives clear views across the formations and the lake, with plentiful birdlife along the way.

Day three is mainly a driving day: nearly 500 km and about 7.5 hours on the Ring Road from Mývatn to the Golden Circle. Break the journey for a meal in Akureyri, then reach Kerið in the late afternoon. This roughly 6,000-year-old crater exposes clear layers of red oxidised scoria and black basaltic ash. It is about 170 metres across, has a caldera lake at the bottom, and costs 700 ISK to enter. Allow about 45 minutes to walk the rim, where strong wind is common, then stay in Hveragerði before the final day.

Þríhnúkagígur is less than an hour from Kerið, near Bláfjöll. A guided tour includes a 3-kilometre hike each way and an open lift that descends 120 metres into the empty magma chamber of a dormant volcano. Tours run from May to October, last 5 to 6 hours, and cost about 51,000 ISK per person. Visitors spend about 30 minutes inside, where oxidised minerals colour the chamber walls red and gold. The chamber last held magma more than 4,000 years ago. Book ahead and reserve most of the day.

The trip ends on the Reykjanes Peninsula at Fagradalshraun, formed by the 2021 eruption near Fagradalsfjall after roughly 800 quiet years in the area. Later eruptions occurred elsewhere on the peninsula in 2022 and 2023, so this is not Iceland's newest lava, but it remains one of the country's youngest accessible lava fields. The route crosses exposed volcanic terrain with no shelter, so check official access and gas updates before leaving and allow most of a day. Here you can compare glassy pāhoehoe and rough ʻaʻā only a few years old with the ancient basalt and crystals seen at the start of the guide.