Earth Has Lost and Gained Many Oceans

June 10, 2021

So, not man-made.

In a darkened room at the American Museum of Natural History in New York, a wall of unassuming stone stretches nearly to the ceiling. At first glance, it looks like a slab destined for a kitchen island or countertop, with black, white, and pink speckles mixing in bands of minerals that stretch far above my head. But then the display light flicks from white to black, and the 10-ton rock glows neon orange and green.

“You cannot help but drop your jaw,” says George Harlow, the curator of the museum’s newly renovated Mignone Halls of Gems and Minerals, where the rock stands.

The stunning vibrancy betrays the minerals’ uniqueness: They formed on the bottom of a now-vanished ocean some 1.2 billion years ago, at a time when spindles of algae smaller than rice were among the largest forms of life. In this ancient ocean, metal-rich particulates burbled up from hydrothermal vents and settled to the seafloor in layers, creating a particular mix of elements that now fluoresce when exposed to ultraviolet light.

The rocks are a vivid reminder of just how much our oceans have changed over billions of years of history—driven by the planet’s ever-shifting network of tectonic plates. These shifts ripple like falling dominoes through geologic, atmospheric, and biological systems, influencing everything from the diversity of Earth’s minerals to the paths of ocean currents and atmospheric flow. And all of this influences life as we know it today.

“The changes in the entire Earth system that take place as part of that changing geography are profound,” says Shanan Peters, a geoscientist at the University of Wisconsin-Madison, who specializes in the co-evolution of life and Earth’s systems.

Preserved seafloor slabs such as this one on display, along with a slew of other geologic clues, are helping scientists recreate the tangled history of oceans lost to time—the Iapetus, Rheic, Tethys, Panthalassic, Ural, and more. Just like these ancient bodies of water, our modern oceans will also eventually close, and others will form anew.

As Harlow simply puts it: “Things haven’t stopped.”

Clues etched in the seafloor

Our planet’s ever-shifting tectonic plates not only raise mountains and carve out valleys, but they also send the oceans opening and closing in cycles—”almost like an accordion,” says Andrew Merdith, a tectonic modeler at the University of Leeds.

The movement is partly driven by subduction zones, in which one plate plunges beneath another. This action recycles the seafloor into the bowels of our planet and tugs along the land behind, narrowing the gaps between continents.

Ghosts of oceans past

The magnetic record, however, is imperfect: “The further we go back in time, the less and less oceanic rocks we have to deal with,” says Grace Shephard, geophysicist, and expert in plate tectonic reconstructions at the University of Oslo. Except for a small swath of rock underlying the Mediterranean—which is a remarkable 340 million years old—much of the seafloor dates back a mere 100 million years ago, and the majority is younger than 200 million years old.

Scientists, however, have found a way to identify the floors of vanished seas that have sunk into Earth’s mantle and are now hiding in an oceanic graveyard.

The method involves looking at the speeds of seismic waves from earthquakes rippling through the planet. Lost bits of the ocean floor can remain relatively cool for some 250 million years or so, and seismic signals differ when passing through cold slabs versus Earth’s sizzling innards.

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