The Ontong Java Plateau in the western Pacific is the largest oceanic plateau on Earth – a colossal pile of ancient lava covering roughly 1.5 to 2 million square kilometres, with crust up to 35 kilometres thick. But new research reveals that the volcanic event that built it did more than deposit magma on the seafloor. It physically fractured and chemically transformed the underlying tectonic plate itself.
The finding, published in Geophysical Research Letters, challenges the conventional view that large igneous provinces sit passively atop oceanic plates that remain largely unchanged beneath them.
What the seismic waves reveal
Led by Azusa Shito, a lecturer at Okayama University of Science, the research team used seismic waveform modelling to peer through the Ontong Java Plateau into the plate beneath. Data came from ocean-bottom seismometers deployed across the plateau and from island-based instruments around the Solomon Islands.
Two key findings emerged.
First, the plate beneath the OJP has a composite internal structure. Horizontal layers – the familiar fabric of oceanic lithosphere formed at mid-ocean ridges – are crisscrossed by vast swarms of vertical dikes, channels through which magma once rose from the mantle.
Second, seismic wave velocities through this region are unusually slow. Both Po waves (compressional) and So waves (shear) travel more slowly than expected through ordinary oceanic lithosphere, a signature of chemical alteration. Magma from a deep thermochemical plume did not simply fracture its way through the plate; it chemically reacted with the surrounding peridotite in a process called refertilisation, reintroducing melt components into rock that had previously been depleted.
“This shows that large-scale volcanic activity can significantly change both the physical structure and chemical composition of an oceanic plate,” Shito said.
A volcanic event on a planetary scale
The Ontong Java Plateau formed between 110 and 120 million years ago during the Early Cretaceous, when a massive mantle plume rose beneath the Pacific plate. Together with the related Manihiki and Hikurangi plateaus – now separated by seafloor spreading – the combined Ontong Java-Manihiki-Hikurangi system covered roughly one per cent of Earth’s surface, with an estimated total magmatic volume of 80 million cubic kilometres.
That makes it the largest volcanic outpouring in Earth’s known history, dwarfing even the Deccan Traps and the Siberian Traps in total volume.
The study’s co-authors include Akira Ishikawa from the Institute of Science Tokyo, Masako Yoshikawa from Hiroshima University, and a broader team from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and other Japanese institutions.
What this means for understanding plate tectonics
The finding has implications beyond the Ontong Java Plateau itself. Oceanic plates have traditionally been thought of as relatively simple structures – cold, dense slabs of lithosphere that form at spreading ridges and subduct at trenches, with their internal fabric largely set at birth. This study suggests they can be dramatically modified later, when a large igneous province erupts through them.
The scale of the modification matters. The dike swarms detected beneath the OJP are not minor features; they represent an extensive network of magma conduits that physically disrupted the plate’s existing layering. The chemical refertilisation, meanwhile, changed the plate’s density and seismic properties in ways that persist 100 million years later.
The eruption has also been linked – tentatively – to oceanic anoxic events in the Early Cretaceous, periods when the oceans became depleted in oxygen and marine life suffered widespread die-offs. The study itself does not directly address this link, but the sheer volume of lava and associated gases makes the connection plausible.
Caveats
The study relies on seismic waveform modelling, an indirect method that infers structure from how waves travel through rock. The resolution is limited by the density of seismometer coverage, and the interpretation of chemical alteration from wave speeds depends on laboratory measurements of how altered peridotite affects seismic velocities. No explicit limitations were published alongside the findings.
Traducido por Alessandra
Sources
- Shito, A., Suetsugu, D., Ishikawa, A., Yoshikawa, M., Isse, T., Shiobara, H., Sugioka, H., Ito, A., Ishihara, Y., Tanaka, S., Obayashi, M., Tonegawa, T., Yoshimitsu, J. (2025). “Seismic evidence for large-scale physicochemical modification of oceanic plate during the formation of the Ontong Java Plateau.” Geophysical Research Letters, Vol. 52, Issue 19. DOI: 10.1029/2025GL115219
- ScienceDaily: “Earth’s biggest volcanic event transformed an oceanic plate“