Deep mantle plumes feeding periodic alignments of asthenospheric fingers beneath the central and southern Atlantic Ocean

What is it about?

Hotspot volcanoes like Hawaii occur within, rather than at the boundary between, tectonic plates on Earth due to the motion of the plates above a hot mantle upwelling or ‘plume’. In this well-established paradigm of hotspot volcanism, seamounts form chains wherein the older seamounts are found further away from the location of active volcanism. Several lines of volcanoes located in the middle of tectonic plates on each side of the south-central mid-Atlantic ridge do not conform to this paradigm. These volcanic features are thought to be related to plumes of hot material rooted deep in the Earth's mantle, but because of the lack of a clear age progression in several of them, they cannot be explained by the classical plume model. Using detailed seismic imaging that allows us to map the whole mantle in this region using different types of seismic waves from distant earthquakes, we show that these volcanic lines are the manifestation of active mantle flow in periodically spaced channels beneath the lithosphere. The channels are aligned in the direction of motion of the tectonic plate above them, each associated with a separate group of mantle plumes rising from the core-mantle-boundary. These plumes fuel volcanic hotspots at the Earth’s surface, each with a unique chemical signature. As they rise, plumes in each group come together at certain depths before splitting off again.

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Why is it important?

This study clarifies some of the pathways of upwellings from the deep mantle, indicating that rising mantle plumes interact with more complex convective motions in the top 1000 km of the mantle, and possibly play an active role in shaping them and the motion of lithospheric plates above them. This study also informs our understanding of the nature of the Large Low Shear Velocity Provinces (LLSVPs), large-scale seismic structures at the bottom of Earth’s mantle beneath Africa and the Pacific Ocean. The origin and nature of the LLSVPs remains largely unknown, but understanding their structure and properties will allow an improved understanding of role in Earth’s evolution. This study adds to mounting evidence that the African LLSVP is not a uniform, unbroken pile of dense material extending high above the core-mantle boundary, but rather a collection of mantle plumes rooted in patches of distinct chemical signature.