Iceland plume

For alternative theory, see Iceland hotspot.

The Iceland plume is a postulated upwelling of anomalously hot rock in the Earth's mantle beneath Iceland. Its origin is thought to lie deep in the mantle, perhaps at the boundary between the core and the mantle at ca. 2880 km depth. Opinions differ as to whether seismic studies have imaged such a structure.[1] In this framework, the volcanism of Iceland is attributed to this plume, according to the theory of W. Jason Morgan.[2]

Geological history

According to the plume model, the source of Icelandic 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 58 and 64 million years. This coincides with the opening of the north Atlantic in the late Paleocene and early Eocene, which has led to suggestions that the arrival of the plume was linked to, and has perhaps contributed to the breakup of the North Atlantic continent. 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,[3][4] but was probably beneath central Greenland;, 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, the Ellesmere Island Volcanics on Ellesmere Island, and in the Arctic Ocean (Alpha Ridge).[5]

As the northern Atlantic opened to the east of Greenland during the Eocene, North America and Eurasia drifted apart; the Mid-Atlantic Ridge formed as an oceanic spreading center and a part of the submarine volcanic system of mid-oceanic ridges. 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 the North Atlantic Igneous Province. 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.[6]

An extinct ridge exists in western Iceland, leading to the theory that the plume has shifted east with time. The oldest crust of Iceland itself is more than 20 million years old and was formed at an old, now extinct oceanic spreading center in the western fjord (Vestfirðir) region. 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 starve 15 million years ago and lead to the formation of a new one in the area of today's peninsulas Skagi and Snæfellsnes; in the latter there is still some activity in the form of the Snæfellsjökull volcano. The spreading center, and hence the main activity, have shifted eastward again 7–9 million years ago, however, and formed the current volcanic zones in the southwest (WVZ; Reykjanes–Hofsjökull–Vatnajökull) and northeast (NVZ; Vatnajökull–Tjörnes). Presently, a slow decrease of the activity in the WVZ takes place, while the volcanic zone in the southeast (Katla–Vatnajökull), which was initiated 3 million years ago, develops.[7] The reorganisation of the plate boundaries in Iceland has also been attributed to microplate tectonics.[6]

Topography/bathymetry of the north Atlantic around Iceland

Geophysical and geochemical observations

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 particular seismological 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.

Seismology

An important method for imaging large-scale structures in Earth's interior is seismic tomography, by which the area under consideration is "illuminated" from all sides with seismic waves from earthquakes from as many different directions as possible; these waves are recorded with a network of seismometers. The size of the network is crucial for the extent of the region which 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 depth with higher resolution.

Regional studies from the 1990s and 2000s (ICEMELT,[8] HOTSPOT[9][10]) 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 depth. 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 Mg content of the mantle, or elevated temperature. Unfortunately it is not possible to unambiguously separate out which effect causes the observed velocity reduction.

Geochemistry

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 and petrologically heterogeneous: it contains not only normal peridotite, but also eclogite — a fertile rock originating from subducted slabs — contribute to the melts.[11][12] The origin of the latter is assumed to be metamorphosed, very old oceanic crust which sank into the mantle several hundreds of millions of years ago during the subduction of an ocean, then upwelled from deep in the mantle.

Using the major and trace-element compositions of Icelandic volcanics [13] showed that the source of present-day volcanism was about 100°C greater than that of the source of mid-ocean ridge basalts.

The variations in the concentrations of trace elements such as helium, lead, strontium, neodymium, and others show clearly that Iceland is compositionally distinct from the rest of the north Atlantic. For instance, the ratio of He-3 and He-4 has a pronounced maximum on Iceland, which correlates well with geophysical anomalies, and the decrease of this and other geochemical signatures with increasing distance from Iceland indicate that the extent of the compositional anomaly reaches about 1500 km along the Reykjanes Ridge and at least 300 km along the Kolbeinsey Ridge.[14] Depending on which elements are considered and how large the area covered is, one can identify up to six different mantle components, which, however, 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 ppm.[15][16] 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 of magma for a given temperature.

Gravimetry/Geoid

The north Atlantic is characterized by strong, large-scale anomalies of the gravity field and the geoid. The geoid rises up to 70 m 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 which bulges up the surface of the Earth.[17] Furthermore, the plume and the thickened crust cause a positive gravity anomaly of about 60 mGal (=0.0006 m/s²) (free-air).

Free-air gravity anomalies in the north Atlantic around Iceland. For better representation the color scale was limited to anomalies up to +80 mGal (+0.8 mm/s²).

Geodynamics

Since the mid-1990s several attempts have been made to explain the observations with numerical geodynamical models of mantle 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 observed crustal thickness, topography, and gravity, whereas a thin hot plume which has been invoked to explain seismological and geochemical observations.[18][19] The most recent models prefer a model for a plume that is 180–200°C hotter than the surrounding mantle and has a stem with a radius of ca. 100 km. Such temperatures have not yet been confirmed by petrology, however.

Challenges to the plume model

The weak visibility of the postulated plume in tomographic images of the lower mantle and the geochemical evidence for eclogite 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 to plate tectonics, and is restricted to the upper mantle.[20][21]

Subducted ocean plate

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.[22] 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.

Upper mantle convection

Another model proposes that the upwelling in the Iceland region is driven by lateral temperature gradients between the suboceanic mantle and the neighbouring Greenland craton and therefore also restricted to the upper 200–300 km of the mantle.[23] However, this convection mechanism is probably not strong enough under the conditions prevailing in the north Atlantic, e.g. with respect to the spreading rate, and it does not offer a simple explanation for the observed geoid anomaly.

See also

References

  1. ↑ Ritsema, J; Van Heijst, HJ; Woodhouse, JH (1999). "Complex shear wave velocity structure imaged beneath Africa and Iceland". Science 286 (5446): 1925–1928. doi:10.1126/science.286.5446.1925. PMID 10583949.
  2. ↑ W.J. Morgan (1971): Convection plumes in the lower mantle, Nature 230,42-43
  3. ↑ L.A. Lawver, R.D. Müller (1994):Iceland hotspot track, Geology 22,311-314
  4. ↑ R.S. White, D.P. McKenzie (1989): Magmatism at rift zones: The generation of volcanic continental margins and flood basalts, J. Geophys. Res. 94(B6),7685-7729
  5. ↑ D.A. Forsyth, P. Morel-a-l'Huisser, I. Asudeth, A.G. Green (1986): Alpha Ridge and Iceland - products of the same plume?, J. Geodyn. 6, 197-214
  6. ↑ 6.0 6.1 Foulger, G.R.; Anderson, D.L. (2005). "A cool model for the Iceland hotspot". J. Volc. Geotherm. Res. 141: 1–22.
  7. ↑ K. Sæmundsson (1979): Outline of the geology of Iceland, Jökull 29,7-28
  8. ↑ C.J. Wolfe, I.Þ. Bjarnason, J.C. VanDecar, S.C. Solomon (1997): Seismic structure of the Iceland mantle plume, Nature 385(5727), 245-247
  9. ↑ R.M. Allen, G. Nolet, W.J. Morgan, K. Vogfjörd, B.H. Bergsson, P. Erlendsson, G.R. Foulger, S. Jakobsdóttir, B.R. Julian, M. Pritchard, S. Ragnarsson, R. Stefánsson (2002): Imaging the mantle beneath Iceland using integrated seismological techniques, J. Geophys. Res. 107(B12),2325,doi:10.1029/2001JB000595
  10. ↑ Foulger, G.R. et al. (2001). "Seismic tomography shows that upwelling beneath Iceland is confined to the upper mantle". Geophys. J. Int. 146: 504–530. doi:10.1046/j.0956-540x.2001.01470.x.
  11. ↑ M.F. Thirlwall (1995): Generation of the Pb isotopic characteristics of the Iceland plume, J. Geol. Soc. Lond. 152, 991-996
  12. ↑ B.J. Murton, R.N. Taylor, M.F. Thirlwall (2002): Plume-ridge interaction: a geochemical perspective from the Reykjanes Ridge, J. Petrol. 43(11), 1987-2012
  13. ↑ Herzberg, C., Asimow, P.D. Arndt, N.T., Nui, Y., Lesher, C.M., Fitton, J.G., Cheadle, M.J., Saunders, A.D. (2006). Temperatures in ambient mantle and plumes: constraints from basalts, picrites and komatiites. Geochem. Geophys. Geosyst. 8, Q02006, doi:10.1029/2006GC001390
  14. ↑ K. Breddam, M.D. Kurz, M. Storey (2000): Mapping out the conduit of the Iceland mantle plume with helium isotopes, Earth Planet. Sci. Lett. 176, 45-55
  15. ↑ B. Jamtveit, R. Brooker, K. Brooks, L. Melchior Larsen}, T. Pedersen (2001): The water content of olivines from the North Atlantic Volcanic Province, Earth Planet. Sci. Lett. 186, 401-415
  16. ↑ A.R.L. Nichols, M.R. Carroll, Á. Höskuldsson (2002): Is the Iceland hot spot also wet? Evidence from the water contents of undegassed submarine and subglacial pillow basalts, Earth Planet. Sci. Lett. 202(1), 77-87
  17. ↑ e.g. G. Marquart, G. (1991): On the geometry of mantle flow beneath drifting lithospheric plates, Geophys. J. Int. 144(2), 356-372
  18. ↑ N.M. Ribe, U.R. Christensen, J. Theißing (1995): The dynamics of plume-ridge interaction, 1: Ridge-centered plumes, Earth Planet. Sci. Lett. 134, 155-168
  19. ↑ G. Ito, G. J. Lin, C.W. Gable (1996): Dynamics of mantle flow and melting at a ridge-centered hotspot: Iceland and the mid-Atlantic ridge, Earth Planet. Sci. Lett. 144, 53-74
  20. ↑ Foulger, G.R. (2010). Plates vs. Plumes: A Geological Controversy. Wiley-Blackwell. ISBN 978-1-4051-6148-0.
  21. ↑ Foulger, Gillian R. (2005-02-08). "Iceland & the North Atlantic Igneous Province". Retrieved 2008-03-22.
  22. ↑ G.R. Foulger, D.L. Anderson (2005): A cool model for the Iceland hotspot, J. Volc. Geotherm. Res. 141(1-2), 1-22
  23. ↑ S.D. King, D.L. Anderson (1995): An alternative mechanism of flood basalt formation, Earth Planet. Sci. Lett. 136, 269-279

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