TheIceland hotspot is ahotspot that is partly responsible for the high volcanic activity that has formed theIceland Plateau and the island ofIceland. It contributes to understanding thegeological deformation of Iceland.
Iceland is one of the most activevolcanic regions in the world, with eruptions occurring on average roughly every three years (in the 20th and 21st century until 2010 there were 45 volcanic eruptions on and around Iceland).[1] About a third of thebasalticlavas erupted in recorded history have been produced by Icelandic eruptions. Notable eruptions have included that ofEldgjá, a fissure ofKatla, in 934 (the world's largest basaltic eruption ever witnessed),Laki in 1783 (the world's second largest),[2] and several eruptions beneathice caps, which have generated devastatingglacial bursts, most recently in 2010 after the eruption ofEyjafjallajökull.
Iceland's location astride theMid-Atlantic Ridge, where theEurasian andNorth American Plates are moving apart, is partly responsible for this intense volcanic activity, but an additional cause is necessary to explain why Iceland is a substantial island while the rest of the ridge mostly consists ofseamounts, with peaks belowsea level.
As well as being a region of higher temperature than the surroundingmantle, the hotspot is believed to have a higher concentration ofwater. The presence of water inmagma reduces the melting temperature, which may also play a role in enhancing Icelandic volcanism.
There is an ongoing discussion about whether the hotspot is caused by a deepmantle plume or originates at a much shallower depth.[3] Recently,seismic tomography studies have foundseismic wave speed anomalies under Iceland, consistent with a hot conduit 100 km (62 mi) across that extends to the lower mantle.[4]
Foulger et al. believe the Icelandic plume reaches only to the mantle transition layer and can therefore not come from the same source as Hawaii.[5] Bijwaard and Spakman, however, believe the Icelandic plume does reach to the mantle, and therefore comes from the same source as Hawaii.[6] While the Hawaiian island chain and theEmperor Seamounts show a clear time-progressive volcanic track caused by the movement of thePacific Plate over the Hawaiian hotspot, no such track can be seen at Iceland.
It is proposed that the line fromGrímsvötn volcano toSurtsey shows the movement of theEurasian Plate, and the line from Grímsvötn volcano to theReykjanes volcanic belt shows the movement of the North American Plate.[7]
TheIceland plume is a postulated upwelling of anomalously hot rock in the Earth'smantle beneathIceland. Its origin is thought to lie deep in the mantle, perhaps at thecore–mantle boundary at about 2,880 km (1,790 mi) depth. Opinions differ as to whether seismic studies have imaged such a structure.[8] In this framework, the volcanism of Iceland is attributed to this plume, according to the theory ofW. Jason Morgan.[9]
It is believed that amantle plume underlies Iceland, of which the hotspot is thought to be the surface expression, and that the presence of the plume enhances the volcanism already caused by plate separation. Additionally, flood basalts on thecontinental margins ofGreenland andNorway, the oblique orientation of theReykjanes Ridge segments to their spreading direction, and the enhanced igneous crustal thickness found along the southernAegir andKolbeinsey ridges may be results of interaction between the plume and theMid-Atlantic Ridge.[10] The plume stem is believed to be quite narrow, perhaps 100 km (62 mi) across and extending down to at least 400–650 km (250–400 mi) beneath the Earth's surface, and possibly down to thecore-mantle boundary, while the plume head may be greater than 1,000 km (620 mi) in diameter.[10][11]
It is suggested that the lack of a time-progressive track of seamounts is due to the location of the plume beneath the thick Greenlandcraton (Laurentia) for ~ 15Myr after continental breakup,[12] and the later entrenchment of the plume material into the northern Mid-Atlantic Ridge following its formation.[10]
According to theplume model, the source ofIcelandic volcanism lies deep beneath the center of the island. The earliest volcanic rocks attributed to the plume are found on both sides of the Atlantic. Their ages have been determined to lie between 64 and 58 million years.[13]: 73–74 This coincides with the opening of the north Atlantic in the latePaleocene and earlyEocene, which has led to suggestions that the arrival of the plume was linked to, and has perhaps contributed to, the breakup of[14]Laurasia. In the framework of the plume hypothesis, the volcanism was caused by the flow of hot plume material initially beneath thick continental lithosphere and then beneath the lithosphere of the growing ocean basin as rifting proceeded. The exact position of the plume at that time is a matter of disagreement between scientists,[15] as is whether the plume is thought to have ascended from the deep mantle only at that time or whether it is much older and also responsible for the old volcanism in northern Greenland, onEllesmere Island, and atAlpha Ridge in the Arctic.[16]
As the northern Atlantic opened to the east of Greenland during the Eocene, North America and Eurasia drifted apart; theMid-Atlantic Ridge formed as an oceanic spreading center and a part of the submarine volcanic system ofmid-oceanic ridges.[17] The initial plume head may have been several thousand kilometers in diameter, and it erupted volcanic rocks on both sides of the present ocean basin to produce theNorth Atlantic Igneous Province.[13]: 74 Upon further opening of the ocean and plate drift, the plume and the mid-Atlantic Ridge are postulated to have approached one another, and finally met. The excess magmatism that accompanied the transition from flood volcanism on Greenland, Ireland and Norway to present-day Icelandic activity was the result of ascent of the hot mantle source beneath progressively thinning lithosphere, according to the plume model, or a postulated unusually productive part of the mid-ocean ridge system.[18] Some geologists have suggested that the Iceland plume could have been responsible for thePaleogene uplift of theScandinavian Mountains by producing changes in the density of thelithosphere andasthenosphere during the opening of the North Atlantic.[19] To the south the Paleogene uplift of the English chalklands that resulted in the formation of theSub-Paleogene surface has also been attributed to the Iceland plume.[20]
An extinct ridge exists in western Iceland, leading to the theory that the plume has shifted east with time. The oldest crust of Iceland is more than 20 million years old and was formed at an old oceanic spreading center in theWestfjords (Vestfirðir) region.[13]: 74 The westward movement of the plates and the ridge above the plume and the strong thermal anomaly of the latter caused this old spreading center to cease 15 million years ago and lead to the formation of a new one in the area of today's peninsulas Skagi andSnæfellsnes; in the latter there is still some activity in the form of theSnæfellsjökull volcano. The spreading center, and hence the main activity, shifted eastward again 9 to 7 million years ago and formed the current volcanic zones in the south–west (Reykjanes,Hofsjökull) and north–east (Tjörnes). Presently, a slow decrease of the activity in the north–east takes place, while the volcanic zone in the south–east (Katla,Vatnajökull), which was initiated 3 million years ago, develops.[21] The reorganisation of the plate boundaries in Iceland has also been attributed to microplate tectonics,[18] and an independentHreppar microplate exists.
The weak visibility of the postulated plume in tomographic images of the lower mantle and the geochemical evidence foreclogite in the mantle source have led to the theory that Iceland is not underlain by a mantle plume at all, but that the volcanism there results from processes related toplate tectonics and is restricted to theupper mantle.[22][3]
According to one of those models, a large chunk of the subducted plate of a former ocean has survived in the uppermost mantle for several hundred million years, and its oceanic crust now causes excessive melt generation and the observed volcanism.[18] This model, however, is not backed by dynamical calculations, nor is it exclusively required by the data, and it also leaves unanswered questions concerning the dynamical and chemical stability of such a body over that long period or the thermal effect of such massive melting.
Another model proposes that the upwelling in the Iceland region is driven by lateral temperature gradients between the suboceanic mantle and the neighbouring Greenlandcraton and therefore also restricted to the upper 200–300 km (120–190 mi) of the mantle.[23] This convection mechanism is probably not strong enough under the conditions prevailing in the north Atlantic, with respect to the spreading rate, and it does not offer a simple explanation for the observed geoid anomaly.
Information about the structure of Earth's deep interior can be acquired only indirectly by geophysical and geochemical methods. For the investigation of postulated plumes,gravimetric,geoid and in particularseismological methods along with geochemical analyses of erupted lavas have proven especially useful. Numerical models of the geodynamical processes attempt to merge these observations into a consistent general picture.
An important method for imaging large-scale structures in Earth's interior is seismictomography, by which the area under consideration is "illuminated" from all sides with seismic waves fromearthquakes from as many different directions as possible; these waves are recorded with a network ofseismometers. The size of the network is crucial for the extent of the region that can be imaged reliably. For the investigation of the Iceland Plume, both global and regional tomography have been used; in the former, the whole mantle is imaged at relatively low resolution using data from stations all over the world, whereas in the latter, a denser network only on Iceland images the mantle down to 400–450 km (250–280 mi) depth with higher resolution.
Regional studies from the 1990s and 2000s show that there is a low seismic-wave-speed anomaly beneath Iceland, but opinion is divided as to whether it continues deeper than the mantle transition zone at roughly 600 km (370 mi) depth.[17][24][25] The velocities of seismic waves are reduced by up to 3% (P waves) and more than 4% (S waves), respectively. These values are consistent with a small percentage of partial melt, a high magnesium content of the mantle, or elevated temperature. It is not possible to unambiguously separate which effect causes the observed velocity reduction.
Numerous studies have addressed the geochemical signature of the lavas present on Iceland and in the north Atlantic. The resulting picture is consistent in several important respects. For instance, it is not contested that the source of the volcanism in the mantle is chemically andpetrologically heterogeneous: it contains not onlyperidotite, the principal mantle rock type, but also eclogite, a rock type that originates from the basalt insubducted slabs and is more easily fusible than peridotite.[26][27] The origin of the latter is assumed to be metamorphosed, very old oceanic crust that sank into the mantle several hundreds of millions of years ago during the subduction of an ocean, then upwelled from deep within the mantle.
Studies using the major and trace-element compositions of Icelandic volcanics showed that the source of present-day volcanism was about 100 °C (212 °F) greater than that of the source of mid-ocean ridge basalts.[28]
The variations in the concentrations of trace elements such ashelium,lead,strontium,neodymium, and others show clearly that Iceland is compositionally distinct from the rest of the north Atlantic. An example of this is seen in the ratio ofhelium-3 (3He) tohelium-4 (4He)isotopes. The ratio of helium-3 and helium-4 is a marker that indicates the origin of the mantle involved in eruptions. Helium-3 is captured during planetary accretion, thus is associated with relatively deeper or lower mantle. Helium-4 is created from the decay of uranium and thorium parent isotopes. A low ratio of3He to4He is strongly correlated with mid ocean ridge eruptions due to its shallow source of mantle, while high ratios of3He to4He are correlated with ocean island basalts due to its deeper source of mantle. Both high and low ratios of3He to4He are found on Iceland. High ratios are associated with the western portion of the island, while lower ratios are associated with the eastern part of the island.[29] These ratio trends correlate well with geophysical anomalies, and the decrease of this and other geochemical signatures with increasing distance from Iceland. Combined, they indicate that the extent of the compositional anomaly reaches about 1,500 km (930 mi) along theReykjanes Ridge and at least 300 km (190 mi) along theKolbeinsey Ridge. Depending on which elements are considered and how large the area covered is, one can identify up to six different mantle components, which are not all present in any single location.
Furthermore, some studies show that the amount of water dissolved in mantle minerals is two to six times higher in the Iceland region than in undisturbed parts of the mid-oceanic ridges, where it is regarded to lie at about 150 parts per million.[30][31] The presence of such a large amount of water in the source of the lavas would tend to lower its melting point and make it more productive for a given temperature. It would also produce the higher melt temperatures found, than typical of mid-ocean ridge basalts.[13]: 106
The north Atlantic is characterized by strong, large-scale anomalies of the gravity field and thegeoid. The geoid rises up to 70 m (230 ft) above the geodetic reference ellipsoid in an approximately circular area with a diameter of several hundred kilometers. In the context of the plume hypothesis, this has been explained by the dynamic effect of the upwelling plume that bulges up the surface of the Earth.[32] Furthermore, the plume and the thickened crust cause a positive gravity anomaly of about 60mGal (=0.0006 m/s²) (free-air).
Since the mid-1990s several attempts have been made to explain the observations with numerical geodynamical models ofmantle convection. The purpose of these calculations was, among other things, to resolve the paradox that a broad plume with a relatively low temperature anomaly is in better agreement with the observed crustal thickness, topography, and gravity than a thin, hot plume, which has been invoked to explain the seismological and geochemical observations.[33][34] The most recent models prefer a plume that is 180–200 °C (356–392 °F) hotter than the surrounding mantle and has a stem with a radius of about 100 km (62 mi).[13]: 71 Such temperatures have not yet been confirmed bypetrology, however.
Understanding how magma is transported from great depths near theMohorovičić discontinuity to the surface has implications for understanding the mechanics of magma movement under Iceland. A study on the Borgarhraun basalt flow helped to constrain the velocity of magma transport from great depths to the surface.[35] Geothermal barometry and statistical analysis ofaluminium within olivine crystals allowed the researchers to determine the depth that these crystals were formed in and how long it took them to reach the surface. In this case, the magma was originally at a depth of 24 km (15 mi). The resulting velocity of the magma ascension was calculated to be 0.02-0.1 m/s so that magma takes a mean of 10 days to reach the surface of Iceland from the Moho discontinuity, which is faster than previously thought.[35]
64°24′00″N17°18′00″W / 64.4000°N 17.3000°W /64.4000; -17.3000
