RHUM-RUM (Réunion Hotspot and Upper Mantle & Réunions Unterer Mantel): Seismological imaging of a mantle plume under La Réunion, western Indian Ocean
Final Report Abstract
This grant funded a large geophysical experiment named "RHUM-RUM", aimed at illuminating the earth's deep subsurface structure under the "hotspot-type" volcano island of La Réunion in the western Indian Ocean. This is one of the most active volcanoes in the world, and one of the oldest, having continuously operated for 65 million years (10-100 times longer than most volcanoes). The site of volcanism has remained stationary during all this time, and has burned a 5500-km long "hotspot track" of extinct undersea volcanoes into the tectonic plates that moved above it. This record of activity leads to the Deccan Trap flood basalt province in India, one of the most enormous outpourings of lava known on earth, which erupted when the hotspot was born 65 million years ago and has been invoked as a or the cause for the mass extinction that wiped out the dinosaurs and ended the Mesozoic era. La Réunion stands exemplarily for an entire class of hotspot-type volcanism that occurs far away from plate boundaries, and mostly in the world's oceans. Such volcanism does not fit the otherwise highly successful explanatory framework of volcanism being caused by plate tectonic processes, i.e., originating from no deeper than ~100 km. Instead, hotspot-type volcanism is hypothesized to originate from the deepest possible source, the earth's core at 2900 km depth. The core consists of molten iron, which is too dense to rise to the surface, but it should heat up the lowermost layer of the mantle, solid rock that should then become buoyant and viscously deforming, and rise to the surface in vertical, mushroom-like "mantle plumes". The initial arrival of a massive plume head would cause huge but short-lived flood basalt outpourings, whereas the thin, long-lived plume tails would sustain hotspot volcanism for tens of millions of years. Thus mantle plumes are the hypothesized mechanism by which the earth's core gets rid of its heat, largely decoupled from plate tectonics, and hugely disruptive on surface life from time to time. Although there are many surface observations supporting this hypothesis proposed in 1971, it has remained controversial, largely because due to the technical challenge of imaging mantle plumes at depth. They tend to operate under oceans (even to create their own oceans overhead), environments that are very difficult to instrument. RHUM-RUM successfully deployed the relatively novel technology of broadband ocean-bottom seismometers, with the aim of generating a seismological data set of earthquake recordings abundant enough to image the mantle column under La Réunion sufficiently well to settle the question of presence or absence of a plume. We also want to test hypotheses as to what happens to the hot plume material when it approaches the surface, since only a tiny fraction could get erupted on La Réunion. 48 German and 9 French ocean-bottom seismometers were operating autonomously on the seafloor for 13 months, over an area of 2000x2000 km2 surrounding the island. Two monthlong research cruises that deployed and recovered those instruments in 2012 and 2013 also collected bathymetric, magnetic and marine gravity data along ~15,000-km long cruise tracks. We also installed an additional 37 land seismometers between 2011 and 2015 on the islands of La Réunion, Mauritius and the Seychelles, Madagascar and the Iles Eparses. Designed as a close collaboration between German and French geophysicists, RHUM-RUM pooled the scientific expertise, marine resources, funding, and local infrastructure of the two countries. This meant we could mount the so far largest international effort to image an oceanic mantle plume. We recovered 100% of our instruments; the amount of recorded data was very good, and its quality was good to very good. We accomplished a rigorous assessment of data quality and issues, and prepared the recorded seismograms for all subsequent scientific analyses (which are beyond the scope of this grant but have been underway for a while now, funded by other DFG and French grants). Due to the novelty of the technology, technical problems and data glitches were expected and did occur. We diagnosed these and either remedied them via digital signal processing, or paved the way for subsequent studies to remedy them or take them into account. Project website, which includes a 51-min documentary about RHUM-RUM, two cruise blogs and several short videos about ocean-bottom deployments: www.rhum-rum.net/en/
Publications
- (2013). Investigating La Réunion Hot Spot From Crust to Core. EOS, Transactions American Geophysical Union, 94(23), 205-207
Barruol, G., & Sigloch, K.
- (2014). Cruise Report R/V METEOR, Cruise No. 101: METEOR-Berichte. RHUM-RUM - Seismological Imaging of a Mantle Plume under La Réunion, Western Indian Ocean - October 23 - December 4, 2013 - Port Louis (Mauritius) - Le Port (La Réunion, France)). Published by: DFG-Senatskommission für Ozeanographie, Bremen
Sigloch, K.
(See online at https://doi.org/10.2312/cr_m101) - (2016), The crustal structure beneath Mauritius from teleseismic P-receiver functions - oceanic or continental?, Geophys. Res. Lett., 43, 96369643
Singh, M., A. Kaviani, and G. Rümpker
(See online at https://doi.org/10.1002/2016GL070529) - 2016. Preliminary performance report of the RHUM-RUM ocean bottom seismometer network around La Réunion, western Indian Ocean. Advances in Geosciences, 41, pp.43-63
Stähler, S.C., Sigloch, K., Hosseini, K., Crawford, W.C., Barruol, G., Schmidt-Aursch, M., Tsekhmistrenko, M., Scholz, J.R., Mazzullo, A. and Deen, M.
(See online at https://doi.org/10.5194/adgeo-41-43-2016) - (2017), Seismic Anisotropy, Crustal Thickness Variations and Intraplate Seismicity in the South-West Indian Ocean, Dissertation, Goethe-University Frankfurt
Singh, Manvendra
