Gjálp () is a

hyaloclastite Hyaloclastite is a volcanoclastic accumulation or breccia consisting of glass (from the Greek ''hyalus'') fragments (clasts) formed by quench fragmentation of lava flow surfaces during submarine or subglacial extrusion. It occurs as thin margin ...

ridge (tindar) in

Iceland Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its s ...

under the

Vatnajökull Vatnajökull ( Icelandic pronunciation: , literally "Glacier of Lakes"; sometimes translated as Vatna Glacier in English) is the largest and most voluminous ice cap in Iceland, and the second largest in area in Europe after the Severny Island ice ...

glacier shield. It originated in an eruption series in 1996 and is probably part of the

Grímsvötn Grímsvötn (; ''vötn'' = "waters", singular: ) is a volcano with a (partially subglacial) fissure system located in Vatnajökull National Park, Iceland. The volcano itself is completely subglacial and located under the northwestern side of the Va ...

volcanic system,Snæbjörn Guðmundsson: ''Vegavísir um jarðfræði Íslands.'' Reykjavík 2015, p. 280-281 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 Subglacial eruptions, those of ice-covered volcanoes, result in the interaction of magma with ice and snow, leading to meltwater formation, jökulhlaups, and lahars. Flooding associated with meltwater is a significant hazard in some volcan ...

under a thick ice cover as well as the connected

jökulhlaup A jökulhlaup ( ) (literally "glacial run") is a type of glacial outburst flood. It is an Icelandic term that has been adopted in glaciological terminology in many languages. It originally referred to the well-known subglacial outburst flood ...

could be observed and analyzed by modern technique.

Geography

Eruption location

The subglacial

eruption fissure A fissure vent, also known as a volcanic fissure, eruption fissure or simply a fissure, is a linear volcanic vent through which lava erupts, usually without any explosive activity. The vent is often a few metres wide and may be many kilom ...

is to be found in the northwest corner of Vatnajökull ice cap more or less half way between the

central volcano Central is an adjective usually referring to being in the center of some place or (mathematical) object. Central may also refer to: Directions and generalised locations * Central Africa, a region in the centre of Africa continent, also known as ...

Bárðarbunga Bárðarbunga (), is a stratovolcano located under Vatnajökull in Vatnajökull National Park which is Iceland's most extensive glacier. The second highest mountain in Iceland, above sea level, Bárðarbunga is also part of a volcanic system th ...

and

.Helgi Björnsson, etal: ''Glacier-volcano interactions deduced by SAR-inferometry. Journal of Geology.'' Vol. 47, no. 156 (2001). Retrieved 8 August 2020.

Vatnajökull ice cap

The

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,P. Einarsson, Bryndis Brandsdottir, Magnus Tumi Gudmundsson, Helgi Bjornsson, Karl Gronvold and Freysteinn Sigmundsson: ''Center of Icelandic Hotspot experiences Volcanic Unrest. Eos'', Vol. 78, No. 35, September 2, 1997.
Retrieved 29 August 2020. but it is retreating and measured just 8.100 km2 in 2007.C. Pagli, etal.: ''Glacio-isostatic deformation around the Vatnajökull ice cap, Iceland, induced by recent climate warming: GPS observations and finit element modeling.'' JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, B08405, doi:10.1029/2006JB004421, 2007
Retrieved 8 August 2020. The glacier is

temperate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...

, lies in lower elevations and is therefore sensible to climatic changes. As a consequence it has been advancing and retreating since the

Weichselian The Weichselian glaciation was the last glacial period and its associated glaciation in northern parts of Europe. In the Alpine region it corresponds to the Würm glaciation. It was characterized by a large ice sheet (the Fenno-Scandian ice sheet) ...

glaciation A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betw ...

. Its last advance took place during the so called

Little Ice Age The Little Ice Age (LIA) was a period of regional cooling, particularly pronounced in the North Atlantic region. It was not a true ice age of global extent. The term was introduced into scientific literature by François E. Matthes in 1939. Ma ...

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 East or Orient is one of the four cardinal directions or points of the compass. It is the opposite direction from west and is the direction from which the Sun rises on the Earth. Etymology As in other languages, the word is formed from the f ...

(connected to

rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-grabe ...

ing at the

divergent plate In plate tectonics, a divergent boundary or divergent plate boundary (also known as a constructive boundary or an extensional boundary) is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent b ...

boundary in Iceland), responsible for the highest number of eruptions after deglaciation and with the

mantle plume A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hot ...

probably under

, ie. under Vatnajökull.Hildur María Friðriksdóttir: ''Landris á Vatnajökulssvæðinu metið með GPS landmælingum''. BS ritgerð. Jarðvísindadeild Háskóli Íslands. Leiðbeinendur Sigrún Hreinsdóttir, Erik Sturkell. (2017) "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

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 An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...

( M5+) had taken place in the central volcano

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 A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...

drop in Bárðarbunga central volcano and postulate a similar

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

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

. Another possibility is that Bárðarbunga magma entered in a smaller portion the magmatic system of Grímsvötn and started the eruption by this

intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

. Bárðarbunga is known for such tendencies, her magma mingled with

Torfajökull Torfajökull ( Icelandic for "Torfi's glacier"; ) is a rhyolitic stratovolcano, caldera (central volcano) and complex of subglacial volcanoes, located north of Mýrdalsjökull and south of Þórisvatn Lake, Iceland. Torfajökull last erupted in 1 ...

magma at least three times in the past which resulted in bimodal eruptions, eg. of the

Veiðivötn Veiðivötn (, "fishing lakes") is a volcanic lake region in the Highlands of central Iceland, where approximately 50 lakes fill two rows of fissure vents. Geologically, Veiðivötn is part of the Bárðarbunga volcanic system. In 6600 B.C. ...

and at

Landmannalaugar Landmannalaugar () is a location in Iceland's Fjallabak Nature Reserve in the Highlands. It is at the edge of the Laugahraun lava field, formed in an eruption in approximately 1477. It is known for its natural geothermal hot springs and ...

by the end of the 15th century.

Formation of the tindar volcano

The Gjálp

eruption Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are often ...

took place at a some kilometers long known

fissure A fissure is a long, narrow crack opening along the surface of Earth. The term is derived from the Latin word , which means 'cleft' or 'crack'. Fissures emerge in Earth's crust, on ice sheets and glaciers, and on volcanoes. Ground fissure A ...

under 550–700 m of

glacier A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its Ablation#Glaciology, ablation over many years, often Century, centuries. It acquires dis ...

ice within

. 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

. In the beginning, a 2–4 km long N–S trending depression was formed above the fissure, with time three

ice cauldron Ice cauldrons are ice formations within glaciers that cover some subglacial volcanoes. They can have circular to oblong forms. Their surface areas reach from some meters (as indentations or holes in the ice) to up to 1 or more kilometers (as bow ...

s 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 Meltwater is water released by the melting of snow or ice, including glacial ice, tabular icebergs and ice shelves over oceans. Meltwater is often found in the ablation zone of glaciers, where the rate of snow cover is reducing. Meltwater can be ...

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. Though the eruption was mostly

explosive An explosive (or explosive material) is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure. An expl ...

, the

ash Ash or ashes are the solid remnants of fires. Specifically, ''ash'' refers to all non-aqueous, non- gaseous residues that remain after something burns. In analytical chemistry, to analyse the mineral and metal content of chemical samples, ash ...

was not expelled far from the

vents Vent or vents may refer to: Science and technology Biology *Vent, the cloaca region of an animal *Vent DNA polymerase, a thermostable DNA polymerase Geology *Hydrothermal vent, a fissure in a planet's surface from which geothermally heated water ...

, but fell back into the canyon. The quantity of eruption products stayed more or less the same the whole time which was explained by

ice flow Ice sheet dynamics describe the motion within large bodies of ice, such those currently on Greenland and Antarctica. Ice motion is dominated by the movement of glaciers, whose gravity-driven activity is controlled by two main variable factors: ...

into the

crater Crater may refer to: Landforms *Impact crater, a depression caused by two celestial bodies impacting each other, such as a meteorite hitting a planet *Explosion crater, a hole formed in the ground produced by an explosion near or below the surfac ...

. During the two weeks of eruption, volcanic activity thawed no less than 3 km3 of ice, and this continued to a lesser extent for some time after the end of the eruption. The newly formed tindar disappeared again completely under the glacier ice about 1 year later.

Eruption products

The eruptive products consisted of

basaltic andesite Basaltic andesite is a volcanic rock that is intermediate in composition between basalt and andesite. It is composed predominantly of augite and plagioclase. Basaltic andesite can be found in volcanoes around the world, including in Central Ameri ...

which surprised the scientists as these more evolved rocks are neither typical for

nor for

, both more connected to

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

ic volcanism. Some scientists thought that Gjálp could be an independent volcano.

Jökulhlaup in 1996

In the beginning, scientists presumed that the eruption would be followed immediately by a big

(sort of a meltwater tsunami including large blocks of ice and a high quantity of sediment). But it took some time to fill the subglacial lake of Grímsvötn in such a manner that the ice wall holding it back would break. Not before some weeks had passed after the eruption was terminated, the expected jökulhlaup took place from 4 to 7 November 1996. The meltwater streamed mostly in subglacial channels and in the end under the

outlet glacier Glacier morphology, or the form a glacier takes, is influenced by temperature, precipitation, topography, and other factors. The goal of glacial morphology is to gain a better understanding of glaciated landscapes and the way they are shaped. Ty ...

Skeiðarárjökull. There, to everybody's surprise, the water masses streamed in such a quantity that the whole glacier was lifted up. In the end, the water sprang up from under the glacier edge and the flood covered most of

Skeiðarársandur Skeiðarársandur () is an Icelandic glacial outwash plain, a vast expanse of sand generated by the transport of debris by the Skeiðará and other rivers, whose flow is generated by the Skeiðarárjökull glacier and fed by the volcanic systems ...

glacial

outwash plain An outwash plain, also called a sandur (plural: ''sandurs''), sandr or sandar, is a plain formed of glaciofluvial deposits due to meltwater outwash at the terminus of a glacier. As it flows, the glacier grinds the underlying rock surface and ca ...

destroying on its way big parts of the main road

Hringvegur Route 1 or the Ring Road ( or ) is a national road in Iceland that circles the entire country. As a major trunk route, it is considered to be the most important piece of transport infrastructure in Iceland as it connects the majority of towns ...

including two bridges and some communication installations. Luckily, the road had been closed before so that nobody was injured. The volume of meltwater produced by this eruption was around 4 km3.M.T. Gudmundsson, G. Larsen, Á. Höskuldsson and Á.G. Gylfason: ''Volcanic hazards in Iceland. Jökull'' no. 58 (2008)
(PDF) Retrieved 8 August 2020. Over the sandur streamed up to 50–60.000 m3/sec. The first estimates had been somewhat lower.

Former eruption in 1938

At more or less the same place another eruption had taken place in the 1930s. It had also caused a jökulhlaup, but at the time, science could not yet analyze the events. This eruption stayed subglacial.

References

{{DEFAULTSORT:Gjalp, 1996 eruption 1996 in Iceland Bárðarbunga East Volcanic Zone of Iceland Grímsvötn Subglacial eruptions Subglacial volcanoes of Iceland VEI-3 eruptions Volcanic eruptions in Iceland