International Mountain Day in 2025

The International Mountain Day (https://www.un.org/en/observances/mountain-day) is celebrated on the 11^th of December. Every year, to celebrat the day, the geopark publishes a short article about one of its favourite mountain, and in 2025 it was Mt. Sandfell in Skaftárhreppur Municipality.

Sandfell is a beautiful and well-shaped tuya in the northern part of Mýrdalssandur outwash plain. The mountain was formed in an eruption under a glacier and/or in the sea and such formations are common within the 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 shatters the magma into tephra and forms 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.

The upper two formations are seen in Sandfell, i.e. tuff layers and lava layers, along with some intrusions in the tuff. The tuff forms the lower part of Sandfell, and the tuff stack is thicker on the west side than on the east. The tuff is quite well layered and is also quite well vegetated in many places. A lava cap covers the upper part of Sandfell, and numerous lava layers can be seen there. The lava was analyzed in the mid-20th century and turned out to be trachybasalt. It is likely that there are as many as 12 layers of lava flows at the top of the mountain, each of which is between 10-20 meters thick, but the lava layer stack thickens from west to east. On top of the lava layers there are numerous younger ash layers, most of which probably originate from Katla volcano.

Sandfell originates from the Katla volcanic system and there is a small ridge that connects the mountain to Katla central volcano. The age of the mountain is unknown, but it cannot be very old, probably from the last cold period of the ice age, as erosion does not seem to have occurred on top of it and the top of the mountain is very flat.

The sides of the mountain are, however, heavily eroded, especially the western and southern slopes, and have been eroded both by glaciers and water flow from rivers and glacial outburst floods. Sandfellsjökull glacier reached to the western part of the mountain until the early 20th century, but since then the glacier has retreated and is now about 2 km west of the mountain.

The river Leirá flows on either sides of the mountain and used to merged together east of the mountain in the past, but in the last two decades or so, the southern part of Leirá has flowed southeast into the river Skálm instead. Several glacial outburst floods from Mýrdalsjökull, most of them very small, have flowed into southern Leirá and are believed to originate from a geothermal area in the caldera of Katla volcano, near the top of Sandfellsjökull glacier. During these repeated outbursts, the southern slope of Sandfell has been eroded and the river has also begun to dig itself in old sediments which has created high erosion scarps by the river.

The immediate vicinity of the mountain is spectacular and includes a glacial lagoon and a flood plain with many glacial potholes, a beautiful glacial valley area, glacial crests, moraines, and erosion scarps and terraces in the mountain itself and much more. The view over Sandfellsjökull is also very good, as well as over the eastern side of Mýrdalsjökull ice cap.