What are we working on?
Even though we are slowly filling in the details, there is still so much to discover about our planet. As our team questions more about the composition, conditions, and movement of material within the mantle, we are slowly able to play a part in piecing together more of an understanding about our planet.
The mantle transition zone
Around 410 and 660 km depth, seismologists have detected small jumps in seismic wave speed (or velocity) everywhere around the globe. By collaborating with mineral physicists, seismologists discovered that these jumps are caused by a common mantle mineral, olivine. Olivine changes to a higher density state under immense pressure. We map in detail where these jumps in seismic wave speed occur in interesting places like mantle plumes and subducting slabs for two reasons:
- The change to a higher density phase of olivine has an effect on the dynamics of the mantle. The phase change at 410 km speeds up convection, but the phase change at 660 km slows down convection, trapping diving tectonic plates above it and mantle plume material below. This can cause a degree of layering in mantle convection that we are still trying to understand!
- The exact depth at which the phase change occurs depends on temperature. This means that we can use detailed maps of the topography of the jumps in seismic wave speed as a thermometer hundreds of kilometres inside Earth!
Deep mantle mountains a.k.a. LLVPs
Other scientists in the group look even deeper, at structures that sit thousands of kilometres deep on top of the core-mantle boundary. Deep Mantle Mountains are regions thousands of kilometres across and over 1000 kilometres high through which seismic waves travel slowly. So far we have found two, and they exist on either side of the Earth. One beneath Africa and the other beneath the Pacific. They occupy approximately 8% of the entire mantle, and an impressive 6% of the overall Earth. Scientists call these Large Low Shear Velocity Provinces or LLVPs. Deep mantle mountains are probably made of a different material to the rest of the mantle and could be the source of the strange material compositions of mantle plumes. What exactly they are is still unknown. We try to better image these deep mantle mountains by using high frequency waves. We can only do this in specific places where the locations of earthquakes and seismic stations allow us to do so.
Red blobs sitting on the core-mantle boundary indicate the potential shapes of the Deep Mantle Mountains or LLSVPs as imaged using seismic waves. Cottaar & Lekic (2016)
Mantle plume anchors a.k.a. ULVZs
Some researchers in the group zoom into even smaller structures at the bottom of mantle plumes and on the edge of LLSVPs. Here we see patches of 20 km thick material with extremely slow seismic wave speeds , known as Ultra-Low Velocity Zones (ULVZs). The material here is likely to be very dense and able to anchor mantle plumes, hence the nickname Mantle Plume Anchors. To see these patches we have to use high frequency waves that have interacted with the core-mantle boundary. We study these zones because we want to understand what they are, and how they formed.
Cartoon of cross-section of the Earth, with a zoom-in on the ULVZ at the base of the mantle on top of the boundary with the core.
In one approach, we push the boundaries of these methods to image at the scale of several kilometres within the Mantle Plume Anchors. We want to map where they occur at the core-mantle boundary. Currently they have been found beneath Hawaii, Iceland, the Galapagos, and Samoa, all places where we think there is a mantle plume and volcanic islands exist at the surface. In a sense we are trying to produce the first geological map of the core-mantle boundary!