A new study led by researchers at Cardiff University, the University of Oxford, the University of Bristol, and the University of Michigan has revealed that two continent-size regions in Earth's deep mantle have distinctive histories and resulting chemical composition, in contrast to the common assumption they are the same. The findings are available to read in the journal Scientific Reports .
Seismologists have long known that seismic waves – generated by earthquakes – do not travel through all parts of Earth's interior at the same speed. This principle has allowed them to visualise the inside of our planet, even at depths inaccessible by humans, using techniques similar to employed in CT scans for medical imaging.
Deep inside the mantle (the layer between Earth's iron core and its silica-dominated crust), there are vast areas beneath the Pacific Ocean and the African continent where seismic waves travel much slower than average. These "Large-Low-Velocity-Provinces" (LLVPs) are bigger than continents, up to 900 kilometres in height and thousands of kilometres wide.
One common hypothesis is that the LLVPs are made up of oceanic crust that was pushed into the mantle at subduction zones. This crustal material was then stirred through the mantle over millions of years and accumulated to form the LLVPs.
Researchers have typically assumed that both LLVPs are similar to each other in nature, e.g. chemical composition and age, because seismic waves travel through them in similar ways. But a new study, co-authored by Dr Paula Koelemeijer (Department of Earth Sciences, University of Oxford), has challenged this view by modelling the formation of the LLVPs through time.
By combining a model of mantle convection, including a reconstruction of how tectonic plates have moved over the Earth's surface over the last billion years, the study has been able to show that the African LLVP consists of older and better mixed material than the Pacific LLVP, which contains 50% more and younger subducted oceanic crust (and therefore is more different to the surrounding mantle). The resulting differences in density could also explain why the African LLVP is more diffuse and taller than its Pacific counterpart.
"As numerical simulations are not perfect, we have run multiple models for a range of parameters. Each time, we find the Pacific LLVP to be enriched in subducted oceanic crust, implying that Earth's recent subduction history is driving this difference." - Dr James Panton (Cardiff University), Lead Author
The models of this study also show that the Pacific LLVP is consistently replenished by fresh oceanic crustal material since 300 million years ago, because it is surrounded at the surface by a circle of subduction zones, known as the Pacific Ring of Fire. By contrast, the African LLVP does not receive new material at the same rate, and the material has mixed more with the surrounding mantle, lowering its density.
Until now, these differences have been overlooked because temperature is the dominant control on how fast seismic waves move through a material. The models presented in this study demonstrate that both LLVPs actually have the same temperature, which explains why they look seismically similar. This highlights the importance of combining different scientific disciplines to closely examine the inner workings of our planet.
"The fact that these two LLVPs differ in composition, but not in temperature is key to the story and explains why they appear to be the same seismically. It is also fascinating to see the links between the movements of plates on the Earth's surface and structures 3000 km deep in our planet." - Dr Paula Koelemeijer (University of Oxford), Co-author on the study
The high temperature of the LLVPs, and their positioning in the deep mantle on each side of the planet, means that they affect how heat is extracted from the Earth's core. This impacts convection in the outer core – a process that drives the magnetic field and protects us at the surface from harmful cosmic rays. If the African and Pacific LLVP are different, heat may no longer be extracted symmetrically, which could lead to magnetic field instability. This makes it important to understand the structure of the LLVPs and how they influence heat extraction from the core. Scientists now need to account for this asymmetry in mantle density within their models of the deep Earth. This poses a challenge for observations, as the data that are used often only provide information on symmetric structures in the Earth.
Dr Koelemeijer adds: "We now need to look for data that can constrain the proposed asymmetry in density, for example using observations of Earth's gravitational field."
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