Spatial distribution of the Pdiff postcursors in Hawai’i. Credit: Science Advances (2026). DOI: 10.1126/sciadv.adz1962

Mantle plumes beneath volcanic hotspots, like Hawaii, Iceland, and the Galapagos, seem to be anchored into a large structure within the core-mantle boundary (CMB). A new study, published in Science Advances, takes a deeper dive into the structure under Hawaii using P- and S-wave analysis and mineralogical modeling, revealing its composition and properties.

Mega-ultra low velocity zones

It is known that anomalous structures exist within Earth’s lower mantle, including large low-velocity provinces (LLVPs) and ultra low-velocity zones (ULVZs), which cause seismic waves to slow down dramatically. Larger ULVZs, typically referred to as mega-ultra low velocity zones, are found near the CMB and often beneath oceanic hotspots like Hawaii. Mega-ULVZs can be over several hundred kilometers in length. Previous studies have linked these megastructures to mantle plumes and some say they may preserve primordial geochemical signatures.

However, current tomographic methods have been unable to fully analyze mega-ULVZs, and their composition and origin remain unclear. Seismic waves, on the other hand, present a way to investigate ULVZs, largely due to the effect ULVZs have on wave velocity.

A deeper look with the help of S-waves

Prior research on mega-ULVZs mainly focused on shear wave, or S-wave, data, with limited use of pressure waves (P-waves). This is mostly due to weaker signals and technical challenges associated with P-waves. However, the team working on the new study saw an opportunity in the use of P-waves, along with S-waves, measuring changes in their velocities (V). In the study they used a method involving joint seismic analysis of core-diffracted P and S waves from earthquakes with magnitudes greater than 6.5, which occurred between 1990 and 2021.

"By analyzing waveform similarities, we trace Pdiff postcursor patterns geographically and identify their moveout, enabling measurements of amplitude decay analogous to those used in the recent Sdiff postcursor analysis. A joint analysis of P- and S-wave datasets, coupled with 3D wavefield simulations, enables estimates of relative reduction in Vs versus Vp (henceforth RS/P) for the Hawai’ian mega-ULVZ," the study authors explain.

The team found that the mega-ULVZ beneath Hawaii has a ratio of S- to P-wave speed reduction (RS/P) between 1 and 1.3. They note that other seismic studies have reported the presence of ULVZs with RS/P values closer to 3 or 4, and that these higher values are more consistent with the presence of partial melting. The mega-ULVZ under Hawaii, on the other hand, is better explained by a solid structure in their simulations.

Iron drives thermal conductivity

Using the data from their seismic wave analysis, the team also conducted mineralogical modeling to determine possible mantle compositions and to consider the potential for partial melting. The models showed that the structure likely consists of solid iron-rich material, identifying magnesiowüstite [(Mg,Fe)O] as a best match.

The study authors write, "A range of (Mg,Fe)O concentrations can reproduce the seismic models depending on the distribution of stress among the mineral phases, bounded by the Voigt and Reuss mixing cases. Lower levels of iron enrichment yield slightly poorer fits to the seismic model but remain within acceptable limits given mutual uncertainties.

"Small degrees of partial melting (e.g., 1%) in such iron-enriched rock are acceptable but do not provide a better fit to the seismic models. On the other hand, partial melting of the pyrolitic lowermost mantle or the subducted oceanic crust does not appear consistent with the inferred seismic wave speed reductions."

The researchers say that the high iron content points to high electrical conductivity, which in turn implies the existence of elevated thermal conductivity in the rock. This means that iron-rich mega-ULVZs, like the one under Hawaii, may be active participants in the localized long-term plume generation seen there. They say that this also influences convection in the mantle, instead of simply serving as an indicator of hot upwelling regions.

Written for you by our author Krystal Kasal, edited by Gaby Clark, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You’ll get an ad-free account as a thank-you.

Publication details

Doyeon Kim et al, Seismic and mineralogical evidence for an iron-rich mega–ultralow-velocity zone beneath Hawai’i, Science Advances (2026). DOI: 10.1126/sciadv.adz1962

Key concepts

mantle plume

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