A lake fed by glacier water that has spent decades under the lava, four kinds of charr in one lake, and a rift that is still opening. Here is the natural history of Thingvellir, from the ice age to the birch woods, and which of the Thingvellir snorkeling tours put you on the shore or in the water.
Thingvellir lies on the Mid-Atlantic Ridge, where the North American and Eurasian plates are pulling apart. Iceland has a third plate in play as well: the small Hreppar microplate, Hreppafleki. Away from the boundaries the movement is about two centimetres a year.
The boundary is a zone, not a line. Silfra lies in a zone about five kilometres wide between the North American plate and the Hreppar microplate, one of about 100 fissures there. That is why you cannot stand with a hand on each plate: the park itself corrects the idea that you can touch both continents in Silfra, because the plates are some five kilometres apart.
The park sits in a graben, a sunken block seven kilometres wide. Its western edge is the Almannagjá fault and its eastern edge the Heiðargjá fault. Its floor is lava about 10,000 years old, from a crater south of Mount Hrafnabjörg.
Inside the rift, stress builds and then releases in bursts. The most recent swept over Þingvellir in the spring of 1789. In summer 2000 two earthquakes, centred 40–50 km to the south-east, dropped stones from the ravine walls and clouded the water of Flosagjá.
Silfra is at most 10 m wide and reaches 60 m deep. It was not formed in 1789; it has existed for more than a thousand years.
Þingvallavatn is Iceland’s largest natural lake, about 84 km². Its surface lies about 100 m above sea level, and its deepest point, at 114 m, lies below sea level. Its catchment covers about 1,300 km².
About 90% of the water arrives underground. From the Langjökull glacier it takes 20 to 30 years; rain falling on the lava takes two to four months. The only outflow is the river Sog. Silfra’s water, which stays at 2–4 °C all year, is part of that flow.
The shallows are crowded with life: up to 120,000 small animals per square metre on the shore, and 150 kinds of plant and 50 kinds of invertebrate in the lake. One groundwater amphipod, Crymostygius thingvallensis, lives nowhere else.
Three of Iceland’s five freshwater fish live in the lake: brown trout, the three-spined stickleback and Arctic charr. The charr have split into four forms: benthic, piscivorous, planktivorous and dwarf.
They have been isolated since the last ice age, and each form feeds in its own part of the lake. If you want to fish for them, the desk at Leirar Service Centre sells the permit.
The park holds 172 species of higher plants, about 40% of Iceland’s flora. Moss covers much of the lava, and birch woodland grows across it. The area’s old name, Bláskógar, means “Blue Woods”.
In autumn the birch, willow and heath plants turn, and the valley floor changes colour. If you walk the trails towards the old farms, you pass through the woods.
Fifty-two species of bird live by the lake, and about thirty more come and go. The great northern diver nests here; Iceland is the eastern edge of its range. Barrow’s goldeneye and the harlequin duck are here too.
The Arctic fox is the native mammal. Mink have lived here since escapes after 1931. Bring binoculars if you walk the lake shore, and keep to the paths.
The ice came first. In the last glacial period it lay more than 1,000 m thick over the area. The events below built the valley you walk through.
| When | What happened |
|---|---|
| Last glacial period | Ice more than 1,000 m thick covers the area |
| About 12,000 years ago | The first sign of the lake |
| About 10,000 years ago | Shield eruptions build Skjaldbreiður and the shield south of Hrafnabjörg |
| About 10,000 years ago | Their lava, Þingvallahraun, flows into the lake and floors the graben |
| Spring 1789 | The most recent rifting episode sweeps over Þingvellir |
| After 1931 | Mink arrive, after escapes |
| Summer 2000 | Two earthquakes drop stones from the walls and cloud Flosagjá |
| Every year | The plates move about two centimetres apart |
The fissures are the newest part of that story. The national park guide shows where each one lies.
Snorkel Silfra’s glacier water in a drysuit, with GoPro photos included and a group of up to six. You drive to the meeting point by P5; parking is extra.
From $149 per person
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Pickup in Reykjavík, then 40 minutes in the Silfra fissure in a wetsuit, with underwater photos. Bring your own swimwear and a towel.
From $219 per person
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Four hours in the park in a private vehicle: time for the lake shore and the birch woods as well as the gorge.
From $477 per person
Check AvailabilityThe natural history above is the park’s own account, published on its site.
The people who met in this valley are on the history page, the month to come on the weather page, and every tour on Thingvellir Tours.