Herdman Symposium - Liverpool 2006

The Herdman Symposium is an annual event organised by the student geological society of the University of Liverpool. The theme this year was ‘Pushing the Boundaries… exploring the boundaries of time and space on Planet Earth’. The day consisted of five excellent lectures, at once diverse yet linked by the overarching theme, interspersed by tea coffee and a rather nice buffet lunch.

The Core-mantle boundary region – seismic observations and possible processes by Dr Tine Thomas of Liverpool University took us 2800 km down to the last few 100 km of the mantle.

Fundamentally seismic studies are based on the principle that shock waves will travel at different velocities depending on the elastic properties of what they are travelling through. Using earthquake derived shock waves, models have been developed which initially identified the three part division on the earth into crust, mantle and core. Subsequently, as data quality and analytically techniques improved, more complicated models have been developed. Seismic data available for study is mainly derived from arrays of sensors which were put out by all sides during the Cold War to detect nuclear testing by the opposition. One consequence of this is that the arrays are not necessarily positioned ideally for studies of the deep mantle. The lecture outlined that difficulty and also pointed out the current limitations in interpreting seismic waves due to the inherent properties of these waves. One big problem faced is trying to understand in two and three dimensions what the seismic data is telling you in just one. She referred to ‘tomography’ which is the technique of using waves from different positions about a 2D surface to understand what that 2D surface looks like.

A view of the core mantle boundary from www.columbia.edu/itc/ldeo/mutter/index.html

The evidence now indicates that the base of the mantle (the D” region) has a complex structure, on a scale at the lower limit of resolution of existing analytical techniques (i.e. c. 50 km) to major structures many hundreds of kms in size. There are zones which scatter the waves which might be areas of partial melt and zones where the waves travel with extremely low velocities which may be the roots of hot-spots. At the top and bottom of the D” layer (c2800-2900 km down) the velocities of waves alter over a range, indicating a gradual alteration from the zone above, not a sharp discontinuity.

The top of the D” appears to have a complicated topography, i.e. it is far from a smooth spherical shape evenly distributed around the earth, but the base of the D” at the core-mantle boundary appears much smoother.

Dr Thomas presented four possible explanations for the data:
1. The D” region is composed of slabs derived from the upper mantle ‘slab grave yards’;
2. The region is the point where up-wellings from the core originate;
3. It is a zone of changing chemical composition;
4. It is the result of phase transitions of the mineral perovskite

Perovskite is essentially calcium titanium oxide mineral and laboratory and theoretical work on its behaviours at the temperatures and pressures which exist in the D” region does explain the observed seismic data. Although this is a plausible theory Dr Thomas did point out that is not the only valid model. The lecture did emphasise the message that we do not strictly know what the centre of our planet looks like, we only have a range of models, which is a rather humbling thought.

Niall Clarke