Shaking up Earth

When plate tectonics emerged in the 1960s it became a unifying theory, “the first global theory ever to be generally accepted in the entire history of earth science,” writes Harvard University science historian Naomi Oreskes, in the introduction to Plate Tectonics: An Insider’s History of the Modern Theory of the Earth. In 1969, geophysicist J. Tuzo Wilson compared the impact of this intellectual revolution in earth science to Einstein’s general theory of relativity, which had produced a similar upending of thought about the nature of the universe.  

Plate tectonics describes how Earth’s entire, 100-kilometer-thick outermost layer, called the lithosphere, is broken into a jigsaw puzzle of plates — slabs of rock bearing both continents and seafloor — that slide atop a hot, slowly swirling inner layer. Moving at rates between 2 and 10 centimeters each year, some plates collide, some diverge and some grind past one another. New seafloor is created at the center of the oceans and lost as plates sink back into the planet’s interior. This cycle gives rise to many of Earth’s geologic wonders, as well as its natural hazards.

Plate tectonics is “the first global theory ever to be generally accepted in the entire history of earth science.”

Naomi Oreskes

“It’s amazing how it tied the pieces together: seafloor spreading, magnetic stripes on the seafloor …  where earthquakes form, where mountain ranges form,” says Bradford Foley, a geodynamicist at Penn State. “Pretty much everything falls into place.”

With so many lines of evidence now known, the theory feels obvious, almost inevitable. But the conceptual journey from fixed landmasses to a churning, restless Earth was long and circuitous, punctuated by moments of pure insight and guided by decades of dogged data collection.

Continents adrift

In 1912, German meteorologist Alfred Wegener proposed at a meeting of Frankfurt’s Geological Association that Earth’s landmasses might be on the move. At the time, the prevailing idea held that mountains formed like wrinkles on the planet as it slowly lost the heat of formation and its surface contracted. Instead, Wegener suggested, mountains form when continents collide as they drift across the planet’s surface. Although now far-flung, the continents were once joined together as a supercontinent Wegener dubbed Pangaea, or “all-Earth.” This would explain why rocks of the same type and age, as well as identical fossils, are found on either side of the Atlantic Ocean, for example.

This idea of drifting continents intrigued some scientists. Many others, particularly geologists, were unimpressed, hostile, even horrified. Wegener’s idea, detractors thought, was too speculative, not grounded enough in prevailing geologic principles such as uniformitarianism, which holds that the same slow-moving geologic forces at work on Earth today must also have been at work in the past. The principle was thought to demand that the continents be fixed in place.

German geologist Max Semper disdainfully wrote in 1917 that Wegener’s idea “was established with a superficial use of scientific methods, ignoring the various fields of geology,” adding that he hoped Wegener would turn his attention to other fields of science and leave geology alone.“O holy Saint Florian, protect this house but burn down the others!” he wrote sardonically.

The debate between “mobilists” and “fixists” raged on through the 1920s, picking up steam as it percolated into English-speaking circles. In 1926, at a meeting in New York City of the American Association of Petroleum Geologists, geologist Rollin T. Chamberlin dismissed Wegener’s hypothesis as a mishmash of unrelated observations. The idea, Chamberlin said, “is of the foot-loose type, in that it takes considerable liberty with our globe, and is less bound by restrictions or tied down by awkward, ugly facts than most of its rival theories.”

One of the most persistent sticking points for Wegener’s idea, now called continental drift, was that it couldn’t explain how the continents moved. In 1928, English geologist Arthur Holmes came up with a potential explanation for that movement. He proposed that the continents might be floating like rafts atop a layer of viscous, partially molten rocks deep inside Earth. Heat from the decay of radioactive materials, he suggested, sets this layer to a slow boil, creating large circulating currents within the molten rock that in turn slowly shift the continents about.

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Holmes admitted he had no data to back up the idea, and the geology community remained largely unconvinced of continental drift. Geologists turned to other matters, such as developing a magnitude scale for earthquake strength and devising a method to precisely date organic materials using the radioactive form of carbon, carbon-14.

Data flood in

Rekindled interest in continental drift came in the 1950s from evidence from an unexpected source — the bottom of the oceans. World War II had brought the rapid development of submarines and sonar, and scientists soon put the new technologies to work studying the seafloor. Using sonar, which pings the seafloor with sound waves and listens for a return pulse, researchers mapped out the extent of a continuous and branching underwater mountain chain with a long crack running right down its center. This worldwide rift system snakes for over 72,000 kilometers around the globe, cutting through the centers of the world’s oceans.

Armed with magnetometers for measuring magnetic fields, researchers also mapped out the magnetic orientation of seafloor rocks — how their iron-bearing minerals are oriented relative to Earth’s field. Teams discovered that the seafloor rocks have a peculiar “zebra stripe” pattern: Bands of normal polarity, whose magnetic orientation corresponds to Earth’s current magnetic field, alternate with bands of reversed polarity. This finding suggests that each of the bands formed at different times.

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Meanwhile, growing support for the detection and banning of underground nuclear testing also created an opportunity for seismologists: the chance to create a global, standardized network of seismograph stations. By the end of the 1960s, about 120 different stations were installed in 60 different countries, from the mountains of Ethiopia’s Addis Ababa to the halls of Georgetown University in Washington, D.C., to the frozen South Pole. Thanks to the resulting flood of high-quality seismic data, scientists discovered and mapped rumbles along the mid-ocean rift system, now called mid-ocean ridges, and beneath the trenches. The quakes near very deep ocean trenches were particularly curious: They originated much deeper underground than scientists had thought possible. And the ridges were very hot compared with the surrounding seafloor, scientists learned by using thin steel probes inserted into cores drilled from shipboard into the seafloor.

In the early 1960s, two researchers working independently, geologist Harry Hess and geophysicist Robert S. Dietz, put the disparate clues together — and added in Holmes’ old idea of an underlying layer of circulating currents within the hot rock. The mid-ocean ridges, each asserted, might be where circulation pushes hot rock toward the surface. The powerful forces drive pieces of Earth’s lithosphere apart. Into the gap, lava burbles up — and new seafloor is born. As the pieces of lithosphere move apart, new seafloor continues to form between them, called “seafloor spreading.”  

The momentum culminated in a two-day gathering of perhaps just 100 earth scientists in 1966, held at the Goddard Institute for Space Studies in New York. “It was quite clear, at this conference in New York, that everything was going to change,” University of Cambridge geophysicist Dan McKenzie told the Geological Society of London in 2017 in a reflection on the meeting.

“It was quite clear, at this conference in New York, that everything was going to change.”

Dan McKenzie

But going in, “no one had any idea” that this meeting would become a pivotal moment for the earth sciences, says seismologist Lynn Sykes of Columbia University. Sykes, then a newly minted Ph.D., was one of the invitees; he had just discovered a distinct pattern in the earthquakes at mid-ocean ridges. This pattern showed that the seafloor on either side of the ridges was pulling apart, a pivotal piece of evidence for plate tectonics.

At the meeting, talk after talk piled data on top of data to support seafloor spreading, including Sykes’ earthquake data and those symmetrical patterns of zebra stripes. It soon became clear that these findings were building toward one unified narrative: Mid-ocean ridges were the birthplaces of new seafloor, and deep ocean trenches were graves where old lithosphere was reabsorbed into the interior. This cycle of birth and death had opened and closed the oceans over and over again, bringing the continents together and then splitting them apart.

The evidence was overwhelming, and it was during this conference “that the victory of mobilism was clearly established,” geophysicist Xavier Le Pichon, previously a skeptic of seafloor spreading, wrote in 2001 in his retrospective essay “My conversion to plate tectonics,” included in Oreskes’ book.

Plate tectonics emerges

The whole earth science community became aware of these findings the following spring, at the American Geophysical Union’s annual meeting. Wilson laid out the various lines of evidence for this new view of the world to a much larger audience in Washington, D.C. By then, there was remarkably little pushback from the community, Sykes says: “Right away, they accepted it, which was surprising.”

Scientists now knew that Earth’s seafloor and continents were in motion, and that ridges and trenches marked the edges of large blocks of lithosphere. But how were these blocks moving, all in concert, around the planet? To plot out the choreography of this complex dance, two separate groups seized upon a theorem devised by mathematician Leonhard Euler way back in the 18th century. The theorem showed that a rigid body moves around a sphere as though it is rotating around an axis. McKenzie and geophysicist Robert Parker used this theorem to calculate the dance of the lithospheric blocks — the plates. Unbeknownst to them, geophysicist W. Jason Morgan independently came up with a similar solution.

With this last piece, the unifying theory of plate tectonics was born. The hoary wrangling over continental drift now seemed not only antiquated, but also “a sobering antidote to human self-confidence,” physicist Egon Orowan told Science News in 1970.

People have benefited greatly from this clearer vision of Earth’s workings, including being able to better prepare for earthquakes, tsunamis and volcanoes. Plate tectonics has also shaped new research across the sciences, offering crucial information about how the climate changes and about the evolution of life on Earth.

And yet there’s still so much we don’t understand, such as when and how the restless shifting of Earth’s surface began — and when it might end. Equally puzzling is why plate tectonics doesn’t appear to happen elsewhere in the solar system, says Lindy Elkins-Tanton, a planetary scientist at Arizona State University in Tempe. “How can something be a complete intellectual revolution and also inexplicable at the same time?”
— Carolyn Gramling

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These tools have offered new views of Earth’s exterior and opened new windows into its interior. In the 1960s, for example, researchers had demonstrated that underwater mountain chains called mid-ocean ridges were places where two tectonic plates pulled away from each other, and where new seafloor was forming. But in the early 1970s, scientists for the first time saw the consequences close-up, with the first manned submersible explorations of a mid-ocean ridge in the Atlantic Ocean.

And mid-ocean ridges held more surprises: A few years later, oceanographers exploring another mid-ocean ridge, the Pacific Ocean’s Galápagos Rift, discovered the first known hydrothermal vents, fissures that spew superheated, mineral-rich water. To the amazement of the team members, the vents were teeming with giant tube worms and clams and other forms of life.

Abundant new seismic data and amped-up computing power, meanwhile, led to insights about the mysterious regions where Earth’s tectonic plates were sinking back into the planet’s interior, dubbed “subduction zones.” A heavy, sinking slab of lithosphere, researchers discovered, can exert an extremely powerful pull on the rest of the plate — the first suggestion that subduction might be one of the main engines keeping plates on the move. Scientists also found that those plates might descend much deeper into Earth’s interior than once thought, and could play a big role in stirring up circulation of dense, hot rock within the mantle, the 2,900-kilometer-thick layer between the planet’s rigid outer layer and its superheated, metallic core.

Other researchers began to probe the strangeness of “hot spot” volcanoes, such as Hawaii’s island chain, which are located puzzlingly far from the edges of tectonic plates. Once thought to originate from magma pooling just under the surface, seismic images of the mantle suggested that the volcanoes are instead fueled by giant, buoyant plumes of hot, molten material originating hundreds to thousands of kilometers deep inside the planet, some nearly to its core.

But for every new discovery or question answered, dozens more arise about the dynamic nature of the planet. Here are a few of the big ones:

Can we predict earthquakes?

“The big question is whether there is any hope of being able to predict earthquakes,” says seismologist Lynn Sykes of Columbia University.

“The big question is whether there is any hope of being able to predict earthquakes.”

Lynn Sykes

To try to answer that, scientists are looking to understand the physics of how faults move. It’s not an easy problem: As tectonic plates grind against one another, stress builds in the rocks, creating complex networks of fractures. These fault zones can include both microscopic cracks and vast fissures. Some faults may suddenly slip, causing an earthquake; others may inch along more slowly, possibly heralding a much larger quake in the near future. A quake can “jump” from one fault to another as the locus of stress shifts. The presence — or removal of — groundwater adds yet another wrinkle. Water circulating underground may help lubricate a fault, or it may add new stress.

Scientists are trying to find order in this seeming chaos. Seismic networks have dramatically expanded over the past few decades, with about 26,000 seismic stations currently installed around the globe. More stations mean more precise measurements of where a quake started and how quickly it travels, adding a few extra seconds to minutes of warning.

But scientists are hunting for more data. One strategy is to squeeze as much information as possible out of existing seismic records. By training computers to distinguish even the tiniest of quakes from other kinds of ground-shaking, like passing traffic, scientists increased the number of Southern California quakes logged in one decade by a factor of 10, for example. Another strategy is to dramatically ramp up how much data is collected in the first place. Some scientists are experimenting using underground fiber-optic cables to create dense seismic arrays, a technique called distributed acoustic sensing.

Earthquakes at subduction zones are particularly tricky to understand, Sykes says. Subduction zone quakes are responsible for some of the most destructive quakes on record, including the 2004 magnitude 9.1 rupture off Indonesia that spawned a deadly tsunami that killed over 250,000 people, and the 2011 magnitude 9.0 quake off Japan that launched a tsunami, killing more than 15,000 people. The 2011 quake also crippled the Fukushima Daiichi power plant, releasing radioactive particles into the atmosphere and groundwater.

And these killer quakes are notoriously hard to anticipate. Subduction zones create extremely deep underwater trenches, making it very difficult to install sensors on the subducting plate that could help identify where strain might be accumulating ahead of a future quake. “A lot of the activity happens offshore,” where the sinking plate lies beneath very deep waters, Sykes says. “So you can’t sit right on top of it.”

To get around this problem, the Japan Coast Guard has been testing a novel idea: Combining data from GPS systems installed on land with acoustic data collected from a ship. This combination enables scientists to keep an eye on subducting plates and look for changes to the shape of the seafloor that might presage a quake. Gravity-detecting satellites, sensitive to shifting landmasses, might help too; researchers suggest that satellites may have detected deformation in the Japan subduction zone months before the 2011 quake.

Why do volcanoes erupt?

As with earthquakes, anticipating volcanic eruptions remains tricky. Scientists can detect rumbling within a volcano caused by moving magma by using seismometers, and GPS stations can detect changes in land elevation, including those caused by the swelling of magma beneath a volcano’s flanks.

But what those movements mean is not obvious. “Each volcano has its own personality,” and it’s difficult to classify them into broad categories, says John Vidale, a seismologist at the University of Southern California in Los Angeles. Sometimes magma moving beneath a volcano just pools in large underground chambers. Sometimes it pushes right to the surface. Different volcanoes also have different magma “plumbing” systems: In some, like Washington’s Mount St. Helens, the magma rises up from a vast, deep underground reservoir to another large chamber just below the surface. In others, like Hawaii’s Kilauea, long conduits snake sideways, feeding lava into numerous rifts at the surface.

How different plumbing systems can affect the explosiveness of an eruption is still unclear. Even intensely studied volcanoes such as Kilauea continue to surprise — in 2018, for example, the usual slow ooze of lava from the volcano was punctuated by explosive bursts of a much more gas-rich lava from some of the fissures.

Sometimes a series of deep, slow earthquakes turns out to presage a powerful eruption, as in the case of the Philippines’ Mount Pinatubo eruption in 1991. Other times, such quakes are just quiet grumbles. Telling which is the case for a particular volcano can be more of an art than a science, Vidale adds.

How do “hot spots” form?

Most of Earth’s volcanoes form at the edges of tectonic plate boundaries. But some of Earth’s most famous volcanoes — such as those of the Hawaiian Islands — pop up in the middle of a plate, and are fueled by isolated “hot spots” of magma rising from deep within the mantle. Scientists are still trying to fathom why and how these hot spots form.

“Each volcano has its own personality.”

John Vidale

The Hawaiian Islands have been a geologic puzzle for decades. Even before plate tectonics theory, scientists wondered what forces could create a 2,400-kilometer-long string of volcanoes, all neatly lined up like ducklings in a row (including many that are underwater). In 1963, geophysicist J. Tuzo Wilson suggested that the new idea of seafloor spreading might have something to do with it. If the seafloor were sliding across a stationary region of magma just beneath Earth’s lithosphere, or a hot spot, the result could be a linear march of progressively older volcanoes.

By 1971, the plate tectonics revolution was under way, but Hawaii was still a puzzle. Geophysicist W. Jason Morgan took up the hot spot concept, but went deeper. He suggested that hot spots are fueled by plumes of magma rising up thousands of kilometers from the base of the mantle, where it meets Earth’s core.

Morgan’s mantle plume hypothesis remains the dominant idea even today. But proving it has been tricky, because scientists have almost no direct data from Earth’s interior, and must infer what exists there using indirect methods. One tool that scientists do have is seismic tomography, a visualization technique similar to a CT scan. Using multiple seismic waves from earthquakes, scientists can create 3-D images of the interior of the Earth, based on observations of where the waves slow down or speed up due to changes in temperature or mineral composition. With seismic tomography, scientists have spotted deep plumes, just as expected, beneath Hawaii and Samoa. But such deep plumes haven’t been found beneath other hot spots around the globe, such as beneath the Yellowstone hot spot. In some cases, this may be an imaging problem; some plumes may be too small to detect using this technique.

Why and how hot spot plumes form in the first place is still mysterious. Some scientists suspect that the plumes might be connected to another long-standing mantle mystery. In the 1980s, using seismic tomography, scientists discovered two massive “anomalous zones” near the bottom of the mantle, regions where seismic waves travel much more slowly than they do through the neighboring rocks. One lies beneath Africa (later dubbed “Tuzo” by geophysicists) and another beneath the Pacific Ocean (a.k.a. “Jason”). These regions could represent piles of long-ago subducted lithosphere, and their geochemical makeup looks a lot like the lava erupted from some — but not all — hot spot volcanoes.

Scientists are hard at work trying to determine whether and how exactly these anomalous zones might give rise to hot spot plumes. But there is still a lot of uncertainty about the ultimate link between hot spot volcanoes, subducted plates and Earth’s innards.
— Carolyn Gramling

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Then, suddenly, atmospheric oxygen levels surged, a phenomenon now known as the Great Oxidation Event. The cause of that abrupt chemical transition is a long-standing mystery.

But one possibility is that the Earth moved.

A massive surge of volcanic eruptions about 2.5 billion years ago may have spurred the event, says James Eguchi, a geochemist at the University of California, Riverside. Using clues provided by changing levels of carbon and oxygen isotopes in carbonate rocks, Eguchi and colleagues have suggested that the lava and bursts of carbon dioxide into the atmosphere from such eruptions would have warmed the planet and increased rainfall. And that would have kicked weathering into high gear, Eguchi says.

“[Plate tectonics is] responsible for mediating the climate on long geological timescales.”

Bradford Foley

Volcanic bursts of carbon dioxide in general are known to play a key role in keeping the planetary carbon cycle moving. As acidic, CO₂-laden rainwater reacts with the rocks, pulling the carbon out of the atmosphere, it forms new minerals that wash into the sea. Microscopic cyanobacteria, or blue-green algae, in the oceans flourish as they gobble up carbon-rich minerals, adding oxygen to the atmosphere. Some of the minerals form carbonate rocks, sequestering even more of the carbon. Eventually, those seafloor carbonates, riding atop a sinking tectonic plate, are carried into Earth’s hot interior. They melt and the new magma rises, to be erupted anew out of volcanoes.

Eguchi and colleagues suggest that the combination of continued weathering of volcanic rocks and increasingly efficient cyanobacteria pumping out more and more oxygen helped the gas accumulate. Oxygen levels grew from near zero to about 21 percent of the atmosphere — paving the way for life on Earth as we know it. 

“It’s a big cyclic process that ties Earth’s interior to its climate, as well as to life,” Eguchi says.

What caused the sudden uptick in volcanic activity 2.5 billion years ago is uncertain, he adds. But scientists suspect that plate tectonics may have had many false starts and stutters before really kicking off. The proliferation of volcanoes could signal a major tectonic transition, perhaps to much faster-moving plates or to more widespread, even global, plate movement, he says.

Fast forward to about 252 million years ago, when the Earth experienced another dramatic transition —this one conclusively pinned to volcanic villains. One of the most devastating eruptions in history caused a climate cataclysm that killed off about 90 percent of species, a mass extinction known as the “Great Dying.”

It started with a plume of hot magma that rose from deep inside Earth to pool just beneath the surface before violently erupting. Over 3 million cubic kilometers of molten rock blanketed much of what is now Siberia within just 1 million years. Far more devastating were the enormous pulses of carbon dioxide, methane and other climate-altering gases that that burst out near the end of the eruptions, possibly within just a few tens of thousands of years.

Those gases quickly spread around the globe, sending global temperatures soaring and turning once-temperate lands into deserts. Fluorine and chlorine gases ate away at the ozone layer, allowing ultraviolet rays from the sun to scorch Earth’s forests. The oceans also became deadly, with seawater temperatures rising by as much as 15 degrees Celsius while the waters also turned acidic and oxygen-poor, dissolving the shells of some ocean-dwellers while others gasped for breath.

Such calamitous, planet-overhauling volcanic events are rare in Earth’s history, and their link to Earth’s tectonic plates and the swirling of hot, molten rock is still a puzzle.

But Earth’s dynamism isn’t only destructive. By cycling carbon into and out of Earth’s interior, over and over again, plate tectonics has ended up keeping Earth’s temperatures remarkably stable. It has acted as a planetary thermostat for billions of years, says Bradford Foley, a geodynamicist at Penn State. “It’s responsible for mediating the climate on long geological timescales.”

Plate movements have also helped shape weather and climatic features we know today, from the Asian monsoons to the ice sheet covering Antarctica. Beginning around 55 million years ago, the northward push of the Indian subcontinent to collide with the Eurasian plate began to shove parts of the Tibetan Plateau skyward. The plateau effectively walled off central Asia from the Indian Ocean, preventing cold, dry air over Asia from venturing southward. Meanwhile the plateau absorbs enormous amounts of solar energy during the summer. All that heat warms the atmosphere above the plateau, and the rising hot air creates powerful atmospheric currents. Warm, moist air from above the Indian Ocean gets sucked in, producing the intense annual monsoon rains. Those rains in turn shape weather patterns from India to China to Japan.

Antarctica can attribute its current icy state to its separation from South America. Cores of sediment show that around 90 million years ago, the continent was covered with a swampy forest. That’s not because it was closer to the equator; the landmass has barely moved since that time. But about 35 million years ago, the pulling apart of the South American and Antarctic plates opened a deep seaway called the Drake Passage. It was just enough to allow the frigid Antarctic Circumpolar Current to encircle the continent, putting it in a deep freeze that continues today. Across the globe, the path of ocean currents is set by the distribution of landmasses and the shape of ocean basins; these currents ferry heat and so drive regional climates.

Over the past two centuries, humans have interfered with the slow, stabilizing influence of plate tectonics on Earth’s climate. We’ve cranked up the thermostat by adding large amounts of carbon dioxide to the atmosphere in a very short period of time. Those emissions are already leading to rapidly increasing global temperatures and changing precipitation patterns, raising sea levels and shifting ocean currents.

Scientists hope to better understand what Earth’s future climate might look like by studying past climatic states, including the influence of different levels of carbon dioxide in the atmosphere. But plate tectonics “won’t save us” from ourselves, says seismologist Lynn Sykes of Columbia University. “It doesn’t have much play in terms of changing things on a timescale of, say, 50 years,” Sykes says. “Plate tectonics kind of stands still on that timescale.”
— Carolyn Gramling

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Planetary scientists puzzle over whether and how these two facts might be related — and what it means for just how unusual Earth really is, says Lindy Elkins-Tanton, a planetary scientist at Arizona State University in Tempe. “Nobody knows how plate tectonics began on Earth, and why it didn’t begin elsewhere,” she adds. “It’s a mystery that connects to a lot of other mysteries, and one of those is habitability.”

We know plate tectonics plays a powerful role in keeping Earth habitable, primarily by moving carbon around. “It’s responsible for mediating the climate on long geological time scales, making sure the climate is more or less temperate for life,” says Roger Fu, a geophysicist at Harvard University.

“Nobody knows how plate tectonics began on Earth, and why it didn’t begin elsewhere. It’s a mystery that connects to a lot of other mysteries, and one of those is habitability.”

Lindy Elkins-Tanton

When two tectonic plates collide, one can slide beneath the other, carrying rocks bearing carbon deep into the planet’s interior. The subducting plate begins to melt, and volcanoes bloom on the overlying plate, belching carbon dioxide and other gases into the atmosphere. As carbon dioxide builds up, it warms the planet through the greenhouse effect.

This warmer atmosphere then speeds up weathering of rocks on Earth’s surface, by boosting the chemical reaction between carbon dioxide–rich rainwater and the rocks. Those reactions draw the gas out of the atmosphere to form new carbon minerals. The minerals wash into the ocean, where tiny ocean creatures use the carbon to build their calcium carbonate shells. Ultimately those creatures die, their shells sinking to the ocean floor and becoming carbonate rocks themselves. As more and more carbon dioxide gets sequestered away from the atmosphere in this way, the planet cools — until, eventually, the slow grind of plate tectonics carries the carbonate into the planet’s interior with a subducting plate.

This cycle, playing out over many millions of years, doesn’t just keep temperatures mild. The churning also keeps oxygen, nitrogen, phosphorus and other nutrients cycling through the atmosphere, oceans and rocks — and chemically transforms them into forms that living organisms can use.

“That’s not to say that life wouldn’t happen without plate tectonics,” Fu says. “But it would be very different.”

In fact, the first life on Earth may predate the onset of plate tectonics. The planet’s ancient rocks bear traces of life dating to at least 3.4 billion years ago, several hundred million years before the earliest known evidence for any plate motions, in the form of fossilized stromatolites, layered structures made of microbes and minerals. Similar microbial communities exist in modern times at hot springs, such as those of Yellowstone National Park. Some scientists to speculate that hot springs — which contain the biochemical recipe for life, including chemical elements, water and energy — may have set the stage for Earth’s earliest life.

It’s certainly theoretically possible for planets without plate tectonics — like the early Earth — to have livable atmospheres and liquid water, as well as abundant heat, says Bradford Foley, a geodynamicist at Penn State. Foley has simulated how much carbon dioxide could seep out from the interior of “stagnant lid” planets — planets like Mars and Mercury that have a single, continuous piece of lithosphere that sits like a cold, heavy lid over the hot interior. Even on these planets, Foley says, “we still have volcanism,” because there’s still hot rock circulating beneath that heavy lid. Those eruptions release carbon dioxide to the atmosphere and produce fresh new rock for weathering.

Volcanism on a climate-altering scale might not last as long as it does when plate tectonics keeps things churning along, but it theoretically could persist for 1 billion or 2 billion years, Foley says. That means that some stagnant lid planets could create an atmosphere and even have temperate climates with liquid water, at least for a time.

Then there’s Europa, Jupiter’s icy moon. The surface of the moon is broken into a mosaic of plates of ice that slide past and over and under one another, much like those on Earth. “Instead of subduction, it’s referred to as subsumption,” Fu says. But the result of this icy cycle may be similar to the hard-rock recycling on Earth, moving nutrients between surface ice and liquid ocean below, which in turn could help support life on the moon.

“What exactly plate tectonics is isn’t an answered question,” Fu says. The term, he says, has become a catchall that encompasses numerous physical features on Earth — mid-ocean ridges, subduction, moving continents — as well as geochemical processes like nutrient cycling. “But there’s no guarantee they always have to happen together.”

Scientists instinctively turn to Earth as a template for studying other worlds, and as an example of what to look for in the search for habitability, Elkins-Tanton says. “So many of the things we try to explain in the natural sciences relies on us being in the middle of the bell curve,” she says. “If it turns out we’re unusual, we’re a bit of an outlier, then explaining things is much harder.”

It may be that each world has its own eclectic history, she says. Earth’s happens to include the powerful cycle of plate tectonics. But life elsewhere might have found another way.
— Carolyn Gramling