Sigmundsson - Vol 442|20 July 2006 NEWS & VIEWS E....

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Vol 442 | 20 July 2006 251 PLATE TECTONICS Magma does the splits Freysteinn Sigmundsson A minor volcanic eruption in Ethiopia was the main visible clue to a massive injection of magma along the Afar rift last year. Such inconspicuous processes could have been crucial in early continental break-up. Earth’s surface is a mosaic of cool, strong rock plates that are moving continuously at rates of up to 12 centimetres a year. On page 291 of this issue 1 , Wright et al. report studies of tectonic events in Ethiopia that provide an unusual insight into what happens in Earth’s crust when two of these plates diverge. Divergent plate movements cause a build- up of tensional stress that is released in a process known as rifting. This consists of periods of large-scale faulting (the slippage of adjacent rock masses at fractures in Earth’s crust) and/or volcanic and magmatic activity. The relative contributions of fault movements and magmatic activity are unknown, as are how the deformation processes vary with depth and when exactly rifting episodes occur. Opportunities to observe rifting are rare: sep- aration occurs mostly on mid-ocean ridges and in only a few places above sea level. At any one location, a rifting episode might occur only once every few centuries. One place where rifting can be observed on dry land is along the East African and Afar rifts in Ethiopia, where the Nubian plate diverges from the Somalian and Arabian plates 2 . The quiet flow and accumulation of magma over a long period — from at least April 2004 to May 2005 — 5 kilometres below the surface of the Afar rift preceded a series of more than a hun- dred earthquakes in September 2005, of maxi- mum magnitude 5.6, that were registered across the globe. Continuous earthquake activ- ity, including a volcanic tremor, was observed on 24–26 September at the nearest seismic station in Addis Ababa, around 400 km away. A minor eruption occurred on 26 September along a 500-metre-long fissure on the flank of the Dabbahu volcano in central Ethiopia (Fig. 1), and ground fracturing occurred along a rift segment that was around 60 km long. Wright and colleagues 1 have revealed the exact course of these events, by interfero- metric analysis of images acquired by the European Space Agency’s Envisat satellite using the synthetic-aperture radar method. They compared images taken from the same
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Sigmundsson - Vol 442|20 July 2006 NEWS & VIEWS E....

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