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.
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.
The slab of rock at the American Museum of Natural History, for example, hails from Ogdensburg, New Jersey, and was preserved during an ancient collision between the predecessor of North America and another ancient continent. The smashup obliterated the ocean between the landmasses, baking the layered seafloor sediments at high temperatures and pressures into the rock that stands today.
Yet the few bits of ancient seafloor preserved on dry land, such as the New Jersey rocks or a chunk of the mantle exposed in Maryland, can only give small hints about shifting oceans through time. To understand these movements better, some scientists turn to a record etched into the seafloor: magnetic minerals.
The birth of oceanic plates takes place along the longest mountain range in the world: an underwater chain known as the mid-ocean ridge. Snaking some 40,390 miles around our planet, the ridge marks where tectonic plates pull apart and hot rock from the mantle wells upward to fill the void. As this molten rock cools, some of its minerals align with the planet's magnetic field, creating a geologic barcode along the seafloor that adds new lines every time the field flips. Scientists can use these barcodes to track our shape-shifting oceans through time.
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.
"It’s always been a black box below our feet," explains Douwe van Hinsbergen, a plate tectonics specialist at Utrecht University in the Netherlands. But now seismic analyses allow scientists to study these ancient slabs and turn back the geologic clock, unwinding the subterranean forces that drive our world. These ghostly remains of seafloor lurk under nearly every continent, and van Hinsbergen and his colleagues cataloged almost a hundred in their so-called Atlas of the Underworld.
Among the oldest bits are remnants of oceanic plates up to about 250 million years old, which now sit at the boundary between mantle and core. That includes the Paleo-Tethys Ocean that once washed onto the shores of Gondwana, a supercontinent mostly made up of what is now South America, Africa, India, Arabia, Australia, and Antarctica.
Putting together these lost bits of seafloor, magnetic barcodes, and a slew of other geologic clues allowed a team of scientists to craft a stunning reconstruction of a billion years of our planet's past.
Merdith, one of the model's architects, notes that it's not the final word on Earth's early form, which may continue to shift as more data emerges. But playing the video of this dance of continents and oceans underscores the mesmerizing nature of our planet’s shape-shifting surface.
"It's all part of the global puzzle," Shephard says.
Ripples through the habitats of Earth
As oceans open and close and continents drift across the planet, the transforming environments set the stage for life’s transformations. The formation of a new ocean, for example, can be a boon for biodiversity, as seen during a spike that occurred when Pangea broke apart, according to work from Peters and his colleagues.
Pangea contained the ancestral groups of all major terrestrial creatures today, Peters explains. After the supercontinent fractured into bits, land animals evolved into a diversity of colors, sizes, and lifestyles on their isolated patches. New oceanic circulation paths also carried moisture to continental interiors, wettening previously desiccated belts. Meanwhile, fresh swaths of shallow sunlit waters opened along new continental shelves, where marine life thrives.
"Those shelf edges are prime real estate if you're a clam or fish or something like that," Peters says. When Pangea broke apart, life on Earth boomed.
Even small tectonic shifts can have drastic impacts on the surface world. One particularly startling example is the formation of the Isthmus of Panama, a sliver of land that bridges North and South America, Peters explains. Water streamed from the Atlantic to the Pacific through this oceanic artery before about 20 million years ago. But as the Pacific plate plunged under the Caribbean plate, it raised the seafloor and sent underwater volcanoes charging to the surface.
The watery connection between oceans began to narrow and eventually was cut off entirely. The change sent warm waters careening northward in a current now known as the Gulf Stream, which drove up temperatures in northwest Europe, imparting a relatively balmy climate in the region, despite it being a similar distance north of the Equator as chilly sections of Canada.
The change also set the stage for the modern conveyor belt of ocean currents, which controls storm patterns, nutrient flow, and more. "The closure of the Isthmus of Panama had a huge effect," Peters says.
Oceans yet to come
Many more world-changing tectonic shifts lie in our planet's future. Some 250 million years from now, Earth’s landmasses might all converge once more into a supercontinent: Pangea Ultima. In this potential scenario, crafted by Christopher Scotese, director of the PALEOMAP Project, the Atlantic Ocean nearly closes and is reduced to a modest inland sea.
But the geologic future remains uncertain. Perhaps just the opposite could happen and the Pacific Ocean closes, forming a supercontinent on the opposite side of the world dubbed Novopangea. Still other models suggest some combination of changes could cause the Atlantic and the Pacific to close as new oceans are born in Asia.
Whatever scenario lies in our distant future, tectonic changes are already afoot. Scientists believe Earth’s next ocean could form in the East African Rift Zone, where a rising plume of searing hot rocks is slowly forcing apart a swath of land along the continent’s eastern coast, explains Cynthia Ebinger, a geophysicist at Tulane University who has conducted extensive research in the region.
This splitting has very real consequences today, as revealed by the abundance of volcanism in this part of the world—including a devastating eruption of Mount Nyiragongo in the Democratic Republic of the Congo that recently displaced up to 400,000 people and killed at least 32. Another volcano, on the coast of Eritrea, is having a different impact: It's keeping the Red Sea at bay, protecting patches of northeastern Ethiopia that lie below sea level from floods, Ebinger says. A small ocean once formed in this region, and while the water has long since dried up, Earth's shifting plates may eventually unleash fresh floods.
While tectonics has been a key driver of our planet's geologic past and future, a different powerful force is mucking with Earth's processes today: us. Humans pump planet-warming gasses into the skies at unprecedented rates, altering oceanic and atmospheric circulation with deadly consequences. Humans are also mixing up ecosystems through importation and travel like never before.
"That’s a process the Earth has never seen before, ever. Not once," Peters says.
The age of humans is only a blip in geologic time, but our actions promise to leave indelible marks on the world, particularly the mixing of the biosphere, Peters says.
"It’ll be present in every organism effectively that exists in the future,” he says, “in the same way that Pangea exists in every organism present, essentially, on the Earth today."