Geophysicists in Switzerland have found the first evidence for water deep inside the Earth’s upper mantle. Mark van der Meijde and colleagues at the Swiss Federal Institute of Technology in Zurich analyzed seismic waves from below the Mediterranean sea and believe that parts of the upper mantle could contain as much as 700 parts per million of water by weight. The results could help researchers better understand fault mechanisms and predict earthquakes (M van der Meijde et al. 2003 Science 300 1556).
The Earth is made up of a core, the mantle and the crust. Between the upper and lower mantle – a region that is 400 to 700 km deep – there are discontinuities at depths of 410 km and 660 km (figure 1). The mineralogy of the mantle changes at these discontinuities.
As much as 1500 parts per million of water can exist in the top part of the upper mantle. This water has been released from moving oceanic plates at depths shallower than 150 km. However, it is not known how much water is transported down to lower regions.
Van der Meijde and co-workers analyzed seismic waves from the plate boundary between Eurasia and Africa (figure 2). They looked specifically for waves that were created by structures in the discontinuous regions. “We found that the region at 410 km is much wider than expected,” team leader Suzan van der Lee told PhysicsWeb. “We believe that the widening – which is 20 to 30 km thick – is caused by the interaction of water molecules with the crystal structures in this region. Water has been mentioned as being related to causing deep earthquakes, so this data helps us to improve their location and understand faulting mechanisms”.
Van der Lee added that the team plans to look at other tectonic plate boundaries in the world, to see if it can find more evidence for water at such depths.
