The planet's largest volcanic event
Beneath the southwestern Pacific Ocean lies the Ontong Java Plateau, the largest volcanic structure on Earth. Covering an area of 1.5 million square kilometers, roughly the size of Alaska, this large igneous province (LIP) was formed by the most voluminous volcanic event of the last 200 million years. Its creation began approximately 120 million years ago, during the Cretaceous period. Over a period of a few million years, an eruption released an estimated 80 million cubic kilometers of basaltic magma. This event built a section of oceanic crust that reaches a thickness of 36 kilometers, far greater than the typical 7 kilometers of oceanic crust.
The sheer scale of the eruption is difficult to comprehend. The magma eruption rate is estimated to have been as high as 22 cubic kilometers per year. Unlike volcanoes that build mountains reaching above sea level, most of the Ontong Java Plateau formed and remains deep underwater. Geological evidence shows it has a seismically slow mantle root that extends approximately 300 kilometers deep into the Earth.
A chemical message from the core-mantle boundary
The basalt rocks of the Ontong Java Plateau are solidified lava; they are geologic messengers from the planet's deep interior. Their chemical composition is distinct from the basalts that form at mid-ocean spreading ridges. Isotopic analyses of elements like strontium, neodymium, and lead show a composition that points to a much deeper origin. Scientists have traced this unique signature to a source within the Pacific Large Low Shear-wave Velocity Province (LLSVP), one of two massive, chemically distinct structures in the lower mantle, located near the boundary with Earth's core.
This deep source material, which contains primitive gases like helium-3 trapped since the Earth's formation, rose as a massive thermochemical mantle plume. According to recent models, this plume was hotter than the surrounding mantle and had a different chemical makeup. This chemical difference helps explain how such a vast amount of magma could erupt without producing the significant surface uplift that a purely thermal plume would cause. The rocks on the Sanriku Coast, part of a complex subduction zone, show the immense tectonic forces that shape our planet, similar to the forces that drove the Ontong Java eruption and continue to move continents and oceanic plateaus today.