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Posted:
January 30, 2015
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FloatingRocks-Canaries

The most recent eruption on the Canary Islands – at El Hierro in 2011 – produced spectacularly enigmatic white “floating rocks” that originated from the layers of oceanic sedimentary rock underneath the island. Despite being violently transported through the volcano, some of these rocks contain microscopic fossils of delicate single-celled marine organisms, making the survival of these fossils all the more extraordinary. A new study published in Scientific Reports, an open access journal of the Nature Publishing Group, by a team of scientists which includes Prof Valentin Troll and Dr Fiona Meade (formally of Trinity College Dublin), uses these fossil time-travellers to date the sedimentary layers beneath El Hierro and, in turn, shed new light on the long-standing puzzle about the origin of the Canary Islands.

The origin and life cycle of oceanic volcanoes, such as the Canary Islands, has long been a source of debate among natural scientists. There are two competing models for the origin of the Canaries – one in which ocean floor fractures control the location of volcanic activity, and another in which an anomalously hot plume of molten rock from the Earth’s mantle feeds island growth from below. A cornerstone of the debate concerns the validity of an age-progression along the island chain. A fixed mantle plume under the roughly eastwards moving African tectonic plate would cause the islands and the pre-volcanic ocean sediments underlying them to become progressively younger towards the westernmost island of El Hierro. The fracture model, in turn, would give rise to randomly distributed island ages.

Fossils and volcanoes are not usually compatible with each other, which is what makes these samples so special. The newly published study in Scientific Reports by a research group led by Prof. Valentin Troll from Uppsala University, Sweden, offers a unique perspective on the plume versus fracture model debate for the origin of the Canary Islands. The fossils are de facto witnesses of the pre-island environment. Researchers can now place constraints on the ages of the sedimentary strata present before island-building and, indeed, on the initiation of island-building itself. In combination with known sediment ages from the east of the archipelago, it is now clear that the oceanic sediments become younger towards the west of the island chain, thus verifying an age-progression among the islands. These findings are in strong agreement with the mantle plume model for the origin of the Canary Islands and thus contribute to our wider understanding of ocean island volcano genesis.

For more information please contact Prof. Valentin R. Troll, Chair in Petrology at Uppsala University, valentin.troll@geo.uu.se

Zaczek, K., Troll, V. R., Cachao, M., Ferreira, J., Deegan, F.M., Carracedo, J.C., Soler, V., Meade, F.C., Burchardt, S. 2015. Nannofossils in 2011 El Hierro eruptive products reinstate plume model for Canary Islands. Scientific Reports 5:7945. DOI 10.1038/srep07945.

This project was initiated by Prof. Valentin Troll (Uppsala University, Sweden), Dr. Mario Cachao (University of Lisbon, Portugal), and Prof. Juan Carlos Carracedo (University of Las Palmas de Gran Canaria, Spain) and forms part of the doctoral thesis of Kirsten Zaczek at Uppsala University. The research was supported by an international team of co-workers from institutions in Spain and Portugal and by the Royal Swedish Academy of Sciences (KVA), the Center for Natural Disaster Sciences (CNDS) at Uppsala University and through the Swedish Science Foundation (VR).

Posted:
June 20, 2014
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Volcano

The Carlingford Igneous Centre, NE Ireland, erupted 60 million years ago, but a new study published in Nature Communications reveals it has much to teach us about currently active volcanoes.

Since the geological expedition of R.W. Bunsen to Iceland in the mid 19th century, scientists have been puzzled by the frequent co-occurrence of basalt and rhyolite at many volcanoes. Bunsen, who also invented of the Bunsen burner, was the first to describe this phenomenon of “bimodal volcanism”, but these fundamentally different lava types have by now been found together at sites across the planet. Crucially, the mixing of basalt and rhyolite in a volcano’s magma chamber is a major cause of violently explosive eruptions, but in the 160 years since Bunsen’s observations, no consensus has been reached on how bimodal volcanism actually originates. A new article in “Nature Communications” now re-ignites the debate and offers a fresh perspective on bimodal volcanism at continental volcanoes. Using detailed chemical analyses of rocks from the Carlingford Igneous Centre, the roots of a large, extinct volcano in northeast Ireland, an international team of scientists suggests that the key control on bimodal volcanism could, in fact, be the crustal rocks that lie below the erupting volcano.

Sixty million years ago, the North Atlantic Ocean was only beginning to form and America and Europe were slowly breaking apart. This process was exacerbated by an increased flow of molten rock from the Earth’s mantle, known as a mantle plume, which caused extensive volcanism throughout northeast Ireland, Greenland and western Scotland. Fissure-fed basaltic lava, as seen at the Giant’s Causeway in Northern Ireland, was the most common type of activity, but a number of large volcanoes also formed. A key feature of these volcanoes was that they were short-lived and bimodal, producing significant amounts of light-coloured rhyolite and granite, as well as dark basalt. One such volcano was the Carlingford Igneous Centre, Co. Louth, Ireland. As the hot basaltic magma (>1200 °C) beneath Carlingford made its way from the mantle to the surface, it passed through the Earth’s continental crust, which is 30 km thick in this part of Ireland. “Luckily rocks from the crust and rocks from the mantle have characteristic chemical compositions, like geological DNA”, explains Dr Fiona Meade, the principal author of the article, “By using cutting-edge isotope analyses on the volcanic rocks from Carlingford, we can detect that the crust began to melt and that these melts were incorporated into the ascending magmas, transforming the basalt into rhyolite and granite”.

Significantly, the team’s work has shown that the continental crust was most strongly involved during the early stages of activity at Carlingford. It appears that while a first flush of crustal melt was easy to extract, melting became increasingly difficult and granite formation quickly stalled. This is because not all minerals in crustal rocks melt at the same temperature, and while some components are readily incorporated into the magma, others are left behind and will never melt. “This research suggests that crustal melts are vital for the formation of rhyolite/granite magmas in continental volcanic systems, and that once the crust can no longer produce such melts, the volcanoes rapidly return to producing basalt – forming a bimodal rock suite” added Prof Valentin Troll, the team leader and chair of petrology at Uppsala University (Sweden). “Evidence of basalt-rhyolite magma mixing is preserved at Carlingford, indicating that violent eruptions are likely to have been triggered early in the lifetime of the volcano, and while Carlingford has not posed any danger for 60 million years, it gives us a major insight into the processes that drive currently active volcanoes”, he concludes.

This project was initiated at Trinity College Dublin by Prof Valentin Troll and Dr Fiona Meade, who are now based at Uppsala University (Sweden), and was supported by an international team of co-workers from institutions in the UK, Italy and the Netherlands. The research was funded by Science Foundation Ireland (SFI), the Irish Research Council for Science, Engineering and Technology (IRCSET) and the TEKNAT faculty at Uppsala University.

For more information please contact Prof Valentin Troll (valentin.troll@geo.uu.se) or Dr Fiona Meade (meade.fiona@gmail.com).