11. December 2022

The International Mountain Day

Today is the International Mountain Day, so we here at Katla Geopark wanted to share with you some information on one of our favourite mountains, which is called Þórólfsfell and is east of Fljótshlíð and south of Tindfjöll mountains. The mountain is a tuya and has an area of 8 km2, volume of 2,2 km3, and reaches up to 570 m.a.s.l. but the mountain itself is 450 meters high. It is the largest tuya near Tindfjallajökull, which is the closest volcanic system.

Þórólfsfell was probably formed in an eruption at the end of the glaciation in Iceland, about 11.500 years ago. The tuya has not been heavily eroded, which suggest a fairly young age since if it was older, it would be more eroded. The top of the tuya has glacial striations, indicating that it was formed before the end of the glaciation and that it was once covered by glaciers. The glacial river of Markárfljót has also eroded the mountain on its southern side. Þórólfsfell is mentioned in Njála, Njál‘s saga, one of Iceland‘s most famous saga. Njáll, one of the main characters, had another farm there but the remains of the farm have never been found and it is likely that the river has eroded it away.

Tuyas are formed in eruptions under glaciers or in the sea and are common within Katla Geopark. Pillow basalt normally forms at the bottom of them when there is still deep water (meltwater from the glacier) or sea above the eruption vent. As the eruption carries on the depth of the water or the sea decreases, allowing explosive eruption to start which forms palagonite tuff. If the eruption continues and water is no longer able to get to the eruption vent, lava will start to flow and covers the upper part of the tuya. A good example of such eruption is the 1963 eruption of Surtsey and an example of an „ordinary“ tuya is the mountain Pétursey on the South Coast. Þórólfsfell tuya has a slightly different formation history but is still considered a tuya. The eruption that formed the mountain was underneath a glacier, but it is believed that the incline of the bedrock underneath the volcano, which is about 12°, prevented meltwater to build up above the eruption vent and flowed away. With little to no water above the eruption vent, there was no explosive eruption phase. This resulted in that most of the tuya was built up by successive lava layers, not palagonite tuff, but has the same shape as a normal tuya. Þórólfsfell is not unique in this, as another tuya called Bláfell and is not far from Þórólfsfell was created by the same process.

The lava in Þórólfsfell has been chemically analysed to try to determine form which volcanic system it came from. The lava is very similar to the chemical make up of the Tindfjallajökull volcanic system, but it is not possible to determine if the tuya is part of the system or was formed unrelated to the nearby volcanic systems, and hopefully future research will be able to determine it.

There is a hiking path leading up to the tuya, and you can find a map here (https://www.katlageopark.com/about-katla/maps/hiking-maps/), where the view is magnificent as Tindfjallajökull is to the north, Þórsmörk and Mýrdalsjökull glacier to the east, Eyjafjalljökull volcano to the south and Markarfljót river and Fljótshlíð to the vest. The hike takes about 4-5 hours back and forth and the distance is about 6,5 kilometres. It is also worth it to hike to the nearby canyon of Mögugil and have a look at the cave of Mögugilshellir. The cave is one of the more unique geological formations in Iceland and it is a shame it has not been researched more.

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