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Geological Background to Afar Rift Project
The African continent is slowly splitting apart along the East African rift valley, a 3000 km-long series of deep basins and flanking mountain ranges. In the remote Afar depression in northern Ethiopia, the Earth's outermost shell, usually a relatively rigid, 150 km-thick plate, has been stretched, thinned and heated to the point of breaking. Hot, partially melted rocks are rising up from the Earth's mantle and are either erupting at the Earth's surface or cooling just beneath it.
Satellite observations of the earth show that tectonic plates move apart, on average, very slowly: usually at a few centimetres per year, or about the rate of fingernail growth. Very occasionally, however, sudden large movements occur, often with devastating consequences. In September 2005, a series of fissures opened along a 60 km section of the Afar depression, as the plate responded catastrophically to forces pulling it apart. The rapidity and immense length of rupture are not unexpected, but have never before been measured directly.
The Afar depression is so hot and dry that almost no vegetation covers the rocks at the surface. This means we can use satellites images to measure the way the Earth's surface changes as faults move, and as molten rock moves up and along the fissures within the rift valley. In the two years following the initial activity, more dramatic surface changes have continued to take place, and earthquakes continue to stir the earth.
Field team walking towards fresh fault rupture. The small
cliff at the base of the hill moved by about 1.5 metres in September,
2005. Photograph by Tim Wright, January 2006.