Investigating what causes the morphology of Large low-shear Velocity Provinces (LLSVPs)

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When earthquakes happen, the energy propagates through the Earth's interior in the form of seismic waves. It is found that seismic waves propagate remarkably slower in two regions of the lowermost mantle beneath Africa and the Pacific Ocean than the surroundings. These regions are called the "large lower shear velocity provinces", or LLSVPs, which cover ~30% area of the Earth's core. The height of the LLSVPs ranges from 100s of km to ~2,000 km at different locations, with the Pacific LLSVP taller than the African LLSVP. The steepness of the LLSVP edges also changes significantly from place to place. The shape of the LLSVPs is controlled by, and provides important information on the density and viscosity structures of the deep mantle, which are linked to the origin of the LLSVPs, the dynamics of the deep mantle and the evolution of the Earth. It is thus critical to understand how density and viscosity structures of the deep mantle control the shape of the LLSVPs. Human beings are not able to go into the deep Earth to examine the LLSVPs in detail, but the interaction between the LLSVPs and the surrounding mantle should be governed by the fundamental physics laws of conservation of mass, momentum and energy. In this research, the team will follow these physical laws and perform numerical simulations to quantify the relationship between the shape of the LLSVPs and the density and viscosity structures of the deep mantle. The results should not only provide new understanding on the causes of the complex shapes of LLSVPs, but also help constrain the density and viscosity structures of the deep mantle. The project will also lead to understanding about the fundamental physics in the Earth's deep mantle. This project provides support to an early-career scientist and a 2nd year graduate student. It also provides teaching materials that will be used in undergraduate and graduate classes, and promotes educational outreach to the public at outreach events and via the internet.The structure and dynamics of the lowermost mantle play a critical role in the Earth's evolution. Seismic studies have revealed two large low-shear velocity provinces (LLSVPs) in the lowermost mantle. Complex features of the LLSVPs have been suggested by seismic observations, with variable topography, internal structure and steepness of the LLSVP edges. The morphology of the LLSVPs is mostly a result of the physical interaction between the LLSVPs and the surrounding mantle, which is in turn largely controlled by the viscosity and density of the lowermost mantle. However, the viscosity and density of the lowermost mantle remain not well constrained. It has been suggested that the LLSVPs are caused by compositionally distinct thermochemical piles. In this project, the team will use numerical modeling to quantify the relationship between the morphology of thermochemical piles and the deep mantle viscosity and density structures, and to explore conditions that lead to structures in the deep mantle with similar features to the seismically observed LLSVPs. The results will help understand the morphology of the LLSVPs, and provide constraints on the density and viscosity of the deep mantle. They will also lead to understanding about the fundamental physical processes and mechanisms controlling the interaction between the LLSVPs and the surrounding mantle.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.