Linking Surface Volcanism To Deep Mantle Processes

A geodynamic model showing mantle plumes (red) rise from the lowermost mantle to near the surface. Plume preferentially form at the edges of LLSVPs (green). Image by: Mingming Li, Arizona State University

Although most surface volcanism on the Earth is found at plate boundaries and is caused by shallow mantle processes, some surface volcanoes (e.g, Hawaii) occur far away from plate boundaries. It has been suggested that the formation of the intraplate volcanism is more related to processes in the deepest part of the Earth’s mantle.

The Earth’s mantle is composed of solid rocks. However, the rocks deform and move, over geological time-scale. The Earth’s core is hot enough to heat up the rocks near the core-mantle boundary, and the density of the heated rocks is reduced because of thermal expansion. Consequently, the hot and less dense rocks can move up from the deepest part of the mantle to near the surface. These hot and less dense rocks that rise from the deep mantle to near the surface are often called “mantle plumes”.

When mantle plumes reach near the surface, they sometimes produce extensive melting and cause surface volcanoes even within the plate. The large igneous provinces, or LIPs, which are regions with extremely large accumulations of volcanic rocks, are thought to be caused by large plumes. The LIP events are of particular interests to scientists since they are often accompanied by the releasing of a lot of gas into the atmosphere, which could significantly change the climate and may be responsible for mass-extinctions.

However, the question is what controls the location of LIPs. Seismic observations have shown that there are two large blobs in the lowermost mantle beneath Pacific and Africa, which have significantly low seismic velocities. These two large blobs are called “large low shear velocity provinces” or LLSVPs. The LLSVPs are thought to be hotter than their surroundings and may be the source location of mantle plumes that cause LIPs. Interestingly, previous studies have found that the initial eruption sites of LIPs preferentially occur above the edges of the LLSVPs. In other words, mantle plumes preferentially occur near the edges of LLSVPs. Then, the question is: why this is the case?

In a recent study published in Earth and Planetary Science Letters, we performed numerical simulations and theoretical analysis to answer what controls the source location of mantle plumes. We follow the basic physics laws of conservation of mass, conservation of momentum and conservation of energy and solve differential equations using supercomputers. We model the dynamics of the entire Earth’s mantle. Our numerical experiments show that the source location of mantle plumes is controlled by physics properties of the lowermost mantle, such as viscosity, thermal expansivity, and thermal diffusivity. When using Earth-like values for these physics parameters, we found strong mantle plumes do preferentially occur at/near the edges of the LLSVPs. Our results match the predictions of the classical theory of thermal boundary layer instability.

Thermal boundary layer theory predicts that thermal instabilities (e.g., mantle plumes) form when the thermal boundary layer reaches a critical thickness. Our results show that materials outside of LLSVPs are advected by mantle flow towards the edges of LLSVPs and the thickness of thermal boundary layer increases towards the LLSVP edges. Under the current Earth-like condition, the thermal boundary layer is difficult to become thick enough to develop plumes before reaching the LLSVP edges. As a consequence, strong mantle plumes preferentially occur at/near LLSVP edges.

This study, The source location of mantle plumes from 3D spherical models of mantle convection, was recently published in the Earth and Planetary Science Letters.

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