Gjálp () is a hyaloclastite ridge (tindar) in Iceland under the Vatnajökull glacier shield. It originated in an eruption series in 1996 and is probably part of the Grímsvötn volcanic system, though not all the scientists involved are of this opinion.
Importance
The eruption was of imminent importance, because it was for the first time that a subglacial eruption under a thick ice cover as well as the connected jökulhlaup could be observed and analyzed by modern technique.
Geography
Eruption location
The subglacial eruption fissure is to be found in the northwest corner of Vatnajökull ice cap more or less half way between the central volcanoes Bárðarbunga and Grímsvötn.
Vatnajökull ice cap
The Vatnajökull glacier which covered the location at time of eruption had a thickness of 500–600 m. In other places the glacier shield can have thicknesses of up to 900 m. Vatnajökull covered an area of 8.200 km2 in 1996, but it is retreating and measured just 8.100 km2 in 2007. The glacier is temperate, lies in lower elevations and is therefore sensible to climatic changes. As a consequence it has been advancing and retreating since the Weichselian glaciation. Its last advance took place during the so called Little Ice Age from the 13th to the end of the 19th century and since then it is retreating.
Parts of two volcanic zones of Iceland are placed under Vatnajökull, ie. the very active East Volcanic Zone (connected to rifting at the divergent plate boundary in Iceland), responsible for the highest number of eruptions after deglaciation and with the mantle plume probably under Bárðarbunga, ie. under Vatnajökull. "More than 80 eruptions occurred during the last 800 years in Vatnajökull." There is also the much less active Öraefi Volcanic Belt, a flank zone mostly under the eastern part of Vatnajökull. It is thought that due to climate change, Vatnajökull has lost about 10% of its mass since the end of the 19th century. Measurements showed an accentuated and even accelerating rate of glacio-isostatic uplift. This could lead to increased magma production (so called decompression melt production), because the "pot lid" formed by the glaciers and their weight will be absent in the future, and eruption frequency could increase as a consequence.
The region of the Gjálp fissures is part of this active East Volcanic Zone under Vatnajökull.
Geology
The Gjálp eruption formed in about two weeks a subglacial hyaloclastite ridge, also called tindar by some geologists, in a zone of known former eruptions.
The eruption in 1996
Precursors and possible connection between volcanic systems
Some heavy earthquakes (M5+) had taken place in the central volcano Bárðarbunga before and proved to be precursors of the eruptive events. New seismological studies (2019) see a parallel to the 2014–2015 eruptions and to the caldera drop in Bárðarbunga central volcano and postulate a similar magma migration to the eruption site though on a smaller scale. This could mean that the volcano is part of the fissure system of Bárðarbunga, not Grímsvötn.
Another possibility is that Bárðarbunga magma entered in a smaller portion the magmatic system of Grímsvötn and started the eruption by this intrusion. Bárðarbunga is known for such tendencies, her magma mingled with Torfajökull magma at least three times in the past which resulted in bimodal eruptions, eg. of the Veiðivötn and at Landmannalaugar by the end of the 15th century.
Formation of the tindar volcano
The Gjálp eruption took place at a some kilometers long known fissure under 550–700 m of glacier ice within Vatnajökull. The eruption in October 1996 could push through this ice in about 30 hours and took place from 30 September to 13 October 1996. The eruption fissure had a length of 6–7 km.
The location is some kilometers to the north of Grímsvötn caldera.
In the beginning, a 2–4 km long N–S trending depression was formed above the fissure, with time three ice cauldrons were built at each end and in the middle, but the eruption concentrated later on one of them where a 200–300 m wide crater came to light. After some time, an open ice canyon was built above the fissure. It had a length of about 3,5 km and was up to 500 m in width.
The meltwater drained first through the ice canyon and then disappeared into subglacial channels and run from there to the subglacial caldera lake of Grímsvötn. The subglacial channels were easily recognized, because continuous melting caused by the hot water from the eruption site initiated the formation of depressions on the ice surface. And so the scientists followed the melting path down to Grímsvötn caldera.