Coming down in a volcano in Iceland: Thríhnúkagígur

In the Reykjanes Peninsula, in the Brennisteinsfjöll volcanic system located further east of the 2021-2022 eruptive sites, an interesting and unique composite spatter cone, discovered in 1974, has been open to tourism since 2012: Þríhnúkagígur (Thríhnúkagígur).

Direct observations of shallow magmatic plumbing systems of basalt fissures as well as associated, intact eruptive vents are rare.

Thríhnúkagígur presents an opportunity not only to see both the shallow intrusive structures and the extrusive components of a small volume basalt fissural eruption, but allows the study of the connection between the intruding magmas, the rock of the region (ie buried tephra) and eruptive products.

Descend into a volcano in Iceland: Thríhnúkagígur
Functional diagram of the formation of the caves during the eruption of Thríhnúkagígur - [C], the unconsolidated tephras were carried by the dike (dyke) and the flow was concentrated on two vents. Incorporating Tephra created a vacuum under the vent in the duct. At the end of the eruption, lava reflux left the south vent evacuated and open to the surface. Rockfall from the ceiling of the cave filled much of the area where the tephra had been incorporated and moved the cave upwards from its original location in the conduit to the overlying country rock [D] . -

Illustration by Natalie Renier, WHOI Creative Studio ©Woods Hole Oceanographic Institution.Rasbourg University PressPage 262

A scientific article, published in "Volcanica", and titled "Conduit formation and crustal microxenolith entrainment in a basaltic fissure eruption: Observations from Thríhnúkagígur Volcano, Iceland ", by MichaelR. Hudak & al.” (references in sources) explains the origin.

The excerpt below:

“The Thríhnúkagígur volcano in Iceland is a composite spatter cone and lava field characteristic of basalt fissure eruptions.

Lava drainage at the end of the eruption left about 60 m of evacuated conduit and a 40,000 m³ cave formed by the erosion of unconsolidated tephra by the feeder dyke.
Field relationships in the shallow plumbing system provide three-dimensional insight into conduit formation in fissural systems.
Both petrographic estimates and relative volumes of cave and erupted lava indicate that xenolithic tephra comprises 5–10% of the erupted volume, which cannot be reproduced by geochemical mixing models. Although the entrainment of crustal xenoliths is not geochemically significant, we postulate that this process may be common in the Icelandic crust.

The Thríhnúkagígur eruption illustrates how ubiquitous, poorly consolidated tephra or hyaloclastite can act as a mechanically weak pre-existing structure that provides a preferential pathway for magma ascent and can influence vent location."

An elevator, similar to those used in mining, has been installed and visitors are now allowed to take guided tours into its depths.
Its popularity comes not only from the uniqueness of the visit and the scale of the chamber, which could easily contain Reykjavik's Hallgrímskirkja church, but also from the incredible coloration of the mineral deposits, vivid shades of red, yellow , green and blue.

Tours only operate in the summer.

Reaching Þríhnúkagígur requires a short but uphill hike, which can be done by anyone with a reasonable level of fitness (see video).

Sources:

- Hudak, M. R., Feineman, M. D., LaFemina, P. C., Geirsson, H. and Agostini, S. (2022) “Conduit formation and crustal microxenolith entrainment in a basaltic fissure eruption: Observations from Thríhnúkagígur Volcano, Iceland”,

Volcanica, 5(2), pp. 249–270. doi: 10.30909/vol.05.02.249270.

- The websites: Inside the volcano and Guide to Iceland.