The largest lava flow on earth
One hundred and twenty million years ago, deep beneath the Pacific Ocean, the Earth's mantle unleashed the single largest volcanic event in the last 200 million years. Over a period of three to seven million years, a massive mantle plume poured out an estimated 80 million cubic kilometers of basaltic magma. This colossal outpouring formed the Ontong Java Plateau, a submarine volcanic structure covering 1.5 million square kilometers, an area roughly the size of Alaska.
The plateau's crustal thickness is immense, reaching up to 36 kilometers, compared to the typical 5-7 kilometers of normal oceanic crust. The main phase of this eruption was geologically swift and violent, with magma eruption rates estimated as high as 22 cubic kilometers per year. This event was so large that it covered 1% of the entire planet's surface in lava. The source is believed to be the Louisville hotspot, the same mantle plume that later formed the Louisville Ridge seamount chain.
A global climate catastrophe
The Ontong Java event was not just a regional phenomenon; it triggered a global environmental crisis. The immense volume of lava released equally immense quantities of carbon dioxide and other volcanic gases into the atmosphere and oceans. This sudden influx of CO₂ caused a severe greenhouse effect, warming the planet and dramatically altering ocean chemistry.
The warming led to an important event known as Oceanic Anoxic Event 1a (OAE1a). As the oceans warmed, their ability to hold dissolved oxygen decreased. This, combined with changes in ocean circulation, led to widespread anoxia, or the near-total depletion of oxygen in the water. This event suffocated marine life, leading to a minor but significant mass extinction, particularly affecting species like nannoplankton. Evidence for this global suffocation is found in layers of black shale from this period, which are full of organic matter that failed to decompose in the oxygen-starved waters. Isotope studies of these ancient sediments provide a direct chemical link between the volcanic eruption to the deadly anoxic event.