A fascinating new study has shaken up our understanding of Earth’s oceans. It reveals that around 15 million years ago, something remarkable happened beneath the surface of the planet: the Earth “swallowed” billions of tonnes of water, causing sea levels to drop by a staggering 30 meters.
This discovery, published in February 2025 on the AGU portal by researchers looking into the geological forces that shape our oceans, challenges the conventional wisdom that sea levels are only affected by climate change. Instead, it highlights the powerful role of tectonic shifts and ocean floor dynamics. Let’s break down this extraordinary event and what it tells us about the planet’s natural processes.
The Geological Event That Changed the Oceans
When we think about sea levels, most of us tend to associate them with climate change, melting glaciers, and global warming. But this new research suggests that tectonic activity also plays a significant role in shaping the oceans. Between 15 and 6 million years ago, the Earth’s oceanic crust experienced a dramatic collapse. The ocean floor, which is constantly being created and destroyed by plate tectonics, began to sink deeper into the Earth’s mantle.
This shift, which slowed the production of oceanic crust by about 35%, led to the formation of deeper ocean basins. As a result, the amount of water those basins could hold decreased, and sea levels dropped by between 26 and 32 meters. The event was so significant that it left a lasting imprint on the planet’s hydrological balance. What’s truly remarkable is that this geological event occurred long before the human-induced climate change we’re now grappling with, showing that Earth’s oceans have always been in flux.
The Role of Plate Tectonics in Sea Level Changes
The process that triggered this drop in sea levels isn’t as simple as just “the ocean sank.” The Earth’s oceanic crust is constantly formed at mid-ocean ridges, where tectonic plates pull apart and magma rises to create new crust. Over time, older crust is pushed down into subduction zones where it melts back into the mantle. Normally, this cycle of creation and destruction keeps the oceanic crust at a relatively even level. But when this process slowed dramatically, it had a direct effect on the depth of the ocean basins.
The study highlighted how the change in the rate of seafloor expansion directly affected the shape and volume of the ocean. As the production of new oceanic crust decreased, the seafloor became less elevated, and the oceans had less room to hold water. This shift didn’t just affect the ocean floor—it also led to a marked decrease in the overall volume of seawater in the oceans, contributing to a significant drop in global sea levels. It’s a fascinating example of how tectonic forces, which operate on a scale much longer than human history, can have a major impact on Earth’s oceans.
How Tectonics Influenced the Climate
The collapse of the oceanic crust also had an unexpected consequence on the global climate. As the expansion of the seafloor slowed, the Earth’s mantle released less heat into the ocean, resulting in a noticeable reduction in volcanic activity. Volcanic eruptions, especially underwater, release large amounts of carbon dioxide (CO2) into the atmosphere, contributing to the planet’s greenhouse effect.
With fewer eruptions, less CO2 was released into the atmosphere, which helped to cool the Earth. This cooling, in turn, contributed to the expansion of ice sheets and glaciers, trapping more water on land and further lowering sea levels. In essence, this slow-down in tectonic activity acted as a natural counterbalance to the warming effect of volcanic emissions, giving the planet a brief cooling period. It’s another example of how interconnected Earth’s geological and climatic systems are, and how shifts in one can reverberate across the planet.
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