Hallmundur and Hallmundarhraun

Hallmundur and Hallmundarhraun

By Jóhannes M. Jóhannesson

The Cave Víðgelmir is located in a lava field called Hallmundarhraun, which is the largest lava field in Borgarfjörður. The field covers about 200 (or 242) square kilometres with an estimated volume of at least two to three cubic kilometres. The total length is about 50-55 kilometers while the field is about 7 kilometers wide at the widest point. The lava field covers a large area between the glaciers Eiríksjökull and Langjökull (Fig. 1.), and as the field kept growing it reached the mountain Strútur and then flowed down Hvítarsíða valley and the end of the field can be seen from Hraunfossar (i.e. The Lava Waterfalls) (Kristján Sæmundsson,1966; Sinton et al., 2005).

Both the exact starting time and duration of the eruption are not known, but it has been estimated that the eruption lasted 1-4 years and the start of the eruption is usually set at around 900 AD. The age of the lava field has been estimated by carbon dating both soil and tephra layers that are found underneath the lave field itself. The dating of the soil layer gave the age 1190±100 B.P or 760±100 AD, while the tephra layer was likely the so-called Settlement layer (i.i. Landnámslagið) and has been dated to be formed in the middle to late 9th century. Therefore, it has been estimated that the eruption began around 900 A.D. (Haukur Jóhannesson, 1989; Kristján Sæmundsson, 1966; A.E. Sveinbjörnsdóttir et al., 2004).

The eruption started out as a fissure eruption, with three possible vents feeding the eruption that formed at least two craters. It is likely that these two craters, near the northwest margin of the glacier (Fig. 1.), produced most of the lava that makes up the lava field, while there are likely two more craters underneath Langjökull. Most of the lava field is of Pahoehoe type, while there is some A‘A‘ lava to be found in the field, especially on the surface near the two exposed craters (Sinton et al., 2005; Haukur Jóhannesson, 1989; Kristján Sæmundsson, 1966). To get a better feel for eruptions of this type, and how these kinds of field grow, we recommend taking a look at the eruption that is going on in Hawaii now from Kilauea volcano, as the two eruptions have many things in common.

Four of the five largest known caves in Iceland are located in Hallmundarhraun, and many other smaller caves are scattered throughout the lava field. Archaeological remains are found within many of these caves, along with Víðgelmir, but sadly very little is known about most of these remains (Björn Hróarsson and Sigurður S. Jónsson, NY).

The lava field itself, Hallmundarhraun, is named after the troll Hallmundur, who is mentioned in Grettis saga (one of the Icelandic sagas). In the saga, Hallmundur helps out the main “hero”, Grettir, and then offers him to stay with him and his daughter. Hallmundur lived in a cave near the glacier Balljökull, but that name is not used anymore and is an older name of either the glacier Eiríksjökull, or the north-west part of the glacier Langjökull (Svavar Sigmundsson, 2003). The name of the cave itself is however never mentioned, but we know (from bitter experience) that he now resides within the Cave Víðgelmir.

Fig. 1. Overview of the lava fields around the glaciers Langjökull and Eiríksjökull. Hallmundarhraun
shown in red along with the location of the craters formed during the eruption. From Sinton et al, 2005.



References and further reading:

A. E. Sveinbjörnsdóttir et al (2004). 14 C dating of the settlement of Iceland. Radiocarbon, 46:387-394

Björn Hróarsson and Sigurður S. Jónsson (NY). Lava Caves in the Hallmundarhraun Lava Flow,
Western Iceland. 6th International symposium on vulcanospeleology, 85-88

Haukur Jóhannesson (1989). Aldur Hallmundarhrauns í Borgarfirði. Fjölrit Náttúrufræðistofnunar nr. 9

Kristján Sæmundsson (1966). Zwei neue C14 - Datierungen Isländischer Vulkanausbrüche. Eiszeitalter und Gegenwart, 17:85-86

Sinton, J., K. Grönvold, and K. Sæmundsson (2005). Postglacial eruptive history of the Western Volcanic Zone, Iceland. Geochem. Geophys. Geosyst. 6:1-34

Svavar Sigmundsson. „Hvenær og af hverju varð Baldjökull að Langjökli?“ Vísindavefurinn, 9. september 2003. Sótt 27. júní 2018. http://visindavefur.is/svar.php?id=3718.

Grettis saga
https://www.snerpa.is/net/isl/grettir.htm - á íslensku
http://sagadb.org/grettis_saga.en - in English

Information on the 2018 Kilauea volcano eruption