This article is part of a package in collaboration with Forbes on time capsules, preserving information and communicating with the future. Read more from the report.
Stuff is old where I live, in greater Boston. Clapboard houses that list with age bear plaques touting the former residence of the town cordwainer or victualler. The gravestones, worn rough by New England winters, lay crooked, too, bearing similarly outmoded biblical names—a Lemuel here, an Ephraim there. Old, too, are the local churches that commended many of these souls to the great hereafter.
As for the building material that makes up these churches, well, that’s a little bit older still. Roxbury puddingstone, the mottled rock quarried nearby and used for much of the old church masonry in Boston, formed 600 million years ago in violent submarine landslides off the coast of a barren volcanic microcontinent that rifted off Africa. This is so long ago that—in the course of the perpetual wander of continents—the whole thing happened somewhere near the south pole. These sediments hardened to rock, then hitched a ride across a bygone ocean as part of a traveling tectonic plate before being sutured onto the rest of equatorial North America some 140 million years before the first dinosaur evolved.
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This is the rock that pokes out from underneath the fallen leaves and at the edges of Dunkin’ parking lots in the Boston area. Very little else has survived the intervening half-billion-year eon here, save a superficial veneer from the extremely recent last ice age—one that is surely doomed in the next few dozen millennia or so. Had one hoped to leave a time capsule for today’s Bostonians in the Permian period 250 million years ago, much less the Pliocene epoch four million years ago , they would have been completely, utterly screwed. This is also the case for anyone aspiring to send such an envoy into the geological deep future as well. Ephraim and Lemuel’s mortal remains, much less the local Dunkin’, will not survive into geological time. “Can any mountains, any continent, withstand such waste?” Charles Darwin once asked, referring to the defacing forces of erosion.
Mindful of my eon-old local rock, and given a charge from Scientific American to figure out how far into the deep future one could possibly even hope to send a time capsule here on Earth, I stumbled upon the humbling work of Steve Holland of the University of Georgia. I reached him at his office, and he gamely decided to play along with my thought experiment.
“Something like 10 to 15 kilometers of rock is gone above me right now,” Holland says, marveling at the vanished local mountains which should entomb his office deep within Earth. Their disappearance has much to tell us about the ravages of deep time. As Pangaea assembled from once disparate continents around 300 million years ago, the African Maghreb headbutted the eastern seaboard, pushing the Appalachian Mountains high into the sky—an American Himalaya that would have buried the Peach State. The collision, meanwhile, injected giant blobs of magma deep into the crust—perhaps some 10 miles or so belowground. But today that old magma offers its granite face to the sunlight here, everything else on top having been completely eroded away in the meantime. “That just blows my mind,” he says.
If we want to leave a time capsule, say, for inhabitants of the next supercontinent to find 250 million years from now, just like we find fossils from Pangaea 250 million years ago, then the ocean floor is a terrible repository.
If we aspire to send a time capsule deep into the future, then Holland’s work is sobering. In one of his papers, a map of North America shows where sediments, and therefore fossils, have been preserved from over the entire 20 million year-long Neogene period (23 million to 2.6 million years ago). Except for two tiny islands of preservation marooned in the middle of the continent and a fringing of old sediments along the coasts, it’s almost completely blank. “We have remnants of that sediment across the U.S.,” Holland says of the surviving islands of Neogene-aged stuff in the middle of the country. “But even all those areas are uplifting”—or being pushed up by tectonic forces, where the unyielding work of erosion will most certainly plane them down. “So [the sediment is] a few tens of millions of years old, but it’s not going to last a whole lot longer.”
Making it into the very long-term fossil record requires getting buried by sediment, which, given enough time, becomes sedimentary rock. There are extraordinary quasi-exceptions to this: an oddly rhino-shaped cave is etched into the basalts of the Pacific Northwest where an actual rhino was covered in lava 15 million years ago and left behind a cartoonish cavity of itself in the rock. But otherwise things don’t get preserved in lava. They get buried in things like muds, silts and clays, or they skip this step and make the rock themselves, like coral reefs do.
But this isn’t nearly enough. For safe passage to the far future, you need to make sure you get buried in what’s called a “sedimentary basin”—that is, a region that is sinking for larger tectonic reasons, making space (“accommodation” in geology) that sediments can fill. This is because a mastodon that gets buried in a swamp might last a few millennia in the dirt, but if that old sediment is part of a vast region that’s subtly uplifting, then everything, and that means everything, will be lathed down to nothing by the forces of erosion.
Examples of this relentless demolition abound. The late, great Ancestral Rocky Mountains once stood where the current ones do—and with equal grandeur—but were long ago worn as flat as a billiard table. If solid mountain ranges in the wrong place have no chance of making into the deep future, what chance would the hollow glass and steel façades of a human city, much less our time capsule, have?
In those rare places where the crust is actively sinking—in the sagging flanks adjacent to new mountain chains or in the drooping, stretched, taffylike crust where a continent is trying to tear itself in half—sediments will fill the space above the slumping crust until it’s filled. This is where the fossil record begins. Unfortunately, today only 16 percent of Earth’s land surface is constituted of such sedimentary basins.
“The other place people might think to put a time capsule is at the bottom of the ocean, on the abyssal plains, right?” Holland says. They would be fools. While continental crust floats above the mantle essentially forever, deep-ocean crust is far denser, so it gets continually fed to subduction zones at the edges of those ocean plates and destroyed. As a result, half the ocean floor is younger than 85 million years old. This sounds old, and it certainly is, but it’s still young enough to miss out on the first 80 percent or so of the age of animal life (and more than 98 percent of the larger history of Earth). If we want to leave a time capsule, say, for inhabitants of the next supercontinent to find 250 million years from now, just like we find fossils from Pangaea 250 million years ago, then the ocean floor is a terrible repository. “The oldest oceanic lithosphere we have is 180 million years, and the fate of most oceanic lithosphere is to get subducted,” Holland says. “So if you put it down there, you’re only going to get it for 200 million years. And we are in it for the long haul here.”
Yet we do have a vast and vastly older fossil record of the oceans than any of the existing ocean crust on Earth today. Some of that rock owes to pieces of deep ocean crust that occasionally get smudged onto the sides of the continents during collisions and survive the fate of the rest of their plates. Far more commonly, though, the seas themselves were draped high above the continental crust in the deep past, leaving a fossil record of life in the ocean in surprising places, like Kansas, which is haunted by the remains of giant aquatic reptiles called mosasaurs. And in fact, we still have vast shallow seas sitting atop continental crust today. These waterlogged swaths of the continents are what’s known as the continental shelves—those gently sloping extensions of the land that slink beneath the waves at shore’s edge and then far out to sea before finally diving into the abyss. If it’s stupid to put our time capsule on the vast deep ocean floor that gets continuously destroyed, what about these somewhat narrower perches just offshore?
“You do have a couple of things to contend with if you’re putting stuff on the continental shelf,” says Hannah Sophia Davies, a postdoctoral researcher of tectonics and sedimentary systems at the Free University of Berlin, who was similarly intrigued by my bizarre assignment and agreed to play along. The climate is always changing, you might have heard. What this has meant in practice over the past few million years, as the planet has plummeted in and out of extraordinary ice ages, is that there are equally extraordinary changes in sea level—from more than 400 feet lower than today at the depths of the glacial periods to perhaps more than 20 feet higher during temporary millennia-long breaks from the cold, like the one ongoing today. While the brief memory of recorded human history lulls us into an expectation of stable shorelines, the seas have in fact oscillated wildly over the millennia. And wherever they pause, they begin to chew away at the landscape.
“As the sea level changes, it progressively cuts into the land, so that might kind of erode the material away where you’re trying to preserve the time capsule,” Davies says. This is a problem because the sea level is definitely going to change—first, perhaps, by dozens of feet upward in the geological short term from human-made warming. But eventually our CO2 will be washed out of the system, and perhaps in 400,000 years we’ll drop back into a deep ice age. If so, the sea level will drop hundreds of feet, the shelves will once again be exposed to the bracing air, and erosion will reign.
What if we put our time capsule a little deeper, near the edges of the shelves, which always stay below sea level but still remain precariously perched above the ocean crust? “I would think that that’s not a particularly good idea,” Davies says, “because every now and again you have these massive submarine landslides called ‘turbidity currents,’ and those transport all the material offshore into the deep ocean. So they will probably just destroy anything that you put there.”
Even worse, the Atlantic continental shelf and other so-called passive margins, which just sit there, placidly collecting sediment unmolested by tectonics, don’t stay passive forever. In 1755 a preposterously giant earthquake leveled Lisbon, killing tens of thousands of pious churchgoers—on All Saints’ Day, no less. The magnitude 8.7 tremor was awful enough that, in the minds of some Enlightenment-era philosophers, it destroyed the idea of an all-powerful, kind and loving God. It might have also kick-started the destruction of the entire Atlantic Ocean. These may have been the initial grumblings of a new subduction zone, a tectonic maw that will someday invade the Atlantic Ocean through the Strait of Gibraltar and beyond, chewing up ocean crust as it spreads. If so, it would only mirror its more mature counterparts across the Atlantic today: two crescents of deep ocean trench where the seafloor is similarly being fed to the mantle. For their part, these American subduction zones may infect the rest of the western Atlantic, effectively throwing into reverse a tectonic spreading system that has been successfully pushing the ocean apart for 180 million years. Ultimately this may swallow the entire Atlantic as the planet inaugurates its next supercontinent. Needless to say, this would likely be bad for the fragile sediments of today’s Atlantic continental shelf.
Every message needs a receiver, even if it’s just to puzzle over some baffling zircons hundreds of millions of years from now.
Elsewhere the vast submerged swath of shelf from Australia to Vietnam, which hosted countless stegodons and later humans in the ice ages—and now hosts their fossils deep underwater—is similarly slated for destruction. “Australia is going to collide with Southeast Asia, which will generate a huge mountain chain,” Davies says. “And that happens super quick, in, like, the next 30 million years.”
Returning to land, what about that 16 percent of continental crust that is home to sedimentary basins? Well, most of it is desert, which brings us to the next hurdle: taphonomy, or the process of fossilization itself. If one is extraordinarily lucky, they can occasionally find the permineralized bones of a hapless prosauropod in the cliff walls of Navajo sandstone, killed by a sand dune collapse in the Jurassic, but never in much detail. “Sand is really porous, so sandstones don’t preserve fine detail,” Holland says. “So yeah, that would not be my favorite place to put something.”
By this point, having eliminated most of the world, I was stumped. You want to put your capsule in a sedimentary basin, hermetically sealed off from the oxidative ravages of the surface world, but probably not in a desert and not in—or perhaps even near—the ocean. Looking at Holland’s map, I had a breakthrough: bury it at the bottom of the Black Sea! After all, it’s in a sedimentary basin in the middle of a landmass, and it’s famously anoxic—even pickling the shipwrecks of Roman galleys in breathtaking detail. Nope. “That whole area—basically, as you go from the Himalayas over through the Middle East, up through Türkiye into the Alps—is just a fright zone,” Holland says about the impossibly complex and ongoing collision of Eurasia with Africa. “There’s so much collision that I think that whole area has a really poor preservation potential. Like, the Mediterranean is going to be gone.”
Okay, fine. Where are we going to put this thing?
“I like the East African rift,” he says. “I would probably put it there.”
Some 200 million years ago, when the planet had it in mind to break up Pangaea, the first attempts at tearing North America from Africa failed, leaving behind a necklace of deep, narrow rift valley lakes from Massachusetts to South Carolina that are not unlike Lake Malawi or Lake Tanganyika in East Africa today. These ancient lake beds still give up scaled fish fossils and lakeside crocodilian footprints as they erode from outcrops at the edges of parking lots in Newark or quarries just outside Washington Dulles International Airport. With this in mind, then, perhaps we should charter a pirogue out to the middle of Lake Malawi and drop our time capsule into the deepest, most anoxic part of the lake, cross our fingers and hope for the best. Or maybe there’s something we can do to help this process along.
We’ve avoided discussing so far what this thing should actually be made out of. And while a metal canister might do for a couple decades, we need to be more selective as we jump deeper into the geological future. Metal corrodes; glass devitrifies. Even our much-touted plastic legacy won’t last long in the geological record: it will degrade into a strange residue of long-chain organic biomarkers. “Chemical weathering is the real killer,” Holland says. And chiseling something into granite would be downright idiotic because the weathering and erosion of silicate rock such as granite is just about the most reliable thing that happens on our planet. “Minerals can be ranked in terms of their susceptibility to chemical weathering,” he says. “Something made of quartz is extremely resistant. And actually—I’m not sure how you get as much of it—but the most resistant thing I can think of is zircon.”
We still have near indestructible grains of zircon from the very dawn of Earth history almost 4.4 billion years ago, even though nothing else has survived from that primeval world, the early Hadean eon. “We have zircons that are basically as old as the Earth, right?” Holland says. “So if you could, and you wanted to make something was going to basically last forever, I’d make it out of zircon.”
It’s no small irony that the very reason this exercise is near impossible is the reason why we’re here in the first place.
While Davies is wary of Holland’s East African rift idea (fearing the capsule might meet an early grave at the bottom of a new East African Ocean), the wheels began turning when I mentioned Holland’s zircon plan. “Oh yeah, that’s good. You could, like, laser etch in a zircon…. It would even stand a chance of surviving orogeny,” she says, referring to the titanic mountain-building collisions that mangle and cook lesser minerals. “So actually, that’s an interesting discussion then because then you don’t really need to find it in outcrop. You could find it detritally.” In other words, you wouldn’t have to find the time capsule in the rocks where it was originally placed, which may erode away, but instead you could find it wherever it ended up.
“If it eroded down a mountain and you dug it up at the coast before it got to the continental shelf, or ended up buried in the ocean, maybe that would work,” Davies says, adding that it could be possible to build a zircon with a strange, unnatural isotope concentration that would signal its humanmade origin. “If you’re just kind of screaming into the void, ‘We were here,’ then it would maybe make sense to distribute a lot of these weird zircons, just to mess with future civilization. But then, I guess, it depends on what the point in the time capsule is: Are you making a Voyager disc? Are you saying, ‘Here’s humanity. Here’s what we were’?”
This leads to the final and perhaps most speculative part of an exercise that has long since veered into irresponsible speculation: someone has to find the damn thing. Every message needs a receiver, even if it’s just to puzzle over some baffling zircons hundreds of millions of years from now. This likely takes out the most obvious solution to all of the problems so far outlined above: simply find the most stable, interior part of a continent, far from any tectonic drama, drill a mile-deep hole, put your time capsule in there, and seal it up with whatever—cement, maybe. And indeed, this would almost certainly work. There’s just one problem. “You can put the time capsule in a deep borehole in the middle of the Earth and seal it up, but nobody’s ever going to find it,” Holland says.
To find our laser-etched, isotopically deranged block of zircon in the future, it’s not enough for it to be committed for safekeeping in a subsiding sedimentary basin or even dropped into some fathomless shaft in the bedrock. After all, there are miles-thick stacks of strata positively loaded with fossils beneath our feet that no one will ever study because they’ll never see the light of day. To actually transmit our message, then, our rocks have to be subsequently uplifted at some point hundreds of millions of years from now just enough to be eroded and revealed at the surface. But then you’d have to be there at just the right time to catch them before they’re inevitably eroded out of existence. And the prospect of being at the right place at the right time—in the window of a few decades or so to look for this thing when it’s exposed on the surface somewhere in our several hundred-million-year journey—well, this is all getting a little silly.
Our knowledge of the far future of plate tectonics peters out somewhere around 250 million years from now, and even then it’s an understatement to call our grasp of this future geography sketchy. Nevertheless, every 400 million to 600 million years, it seems, all of the continents tend to assemble into one hemisphere-spanning union called a supercontinent, with Pangaea providing the most recent example. By applying what they know about plate tectonics and subduction zones and running a model forward as far as is reasonable (and then quite a bit further), several groups of geoscientists have tried their hand at projecting the next supercontinent’s configuration some 200 million to 250 million years in the future. Three of the groups predict that a behemoth will be huddled around the tropics (although the fact that one group has it forming over the North Pole gives some indication as to the level of guesswork involved). The canonical version, called Pangaea Ultima, was imagined by Northwestern University geologist Christopher Scotese.
Pangaea Ultima is virtually a reprise of the previous Pangaea: the Atlantic Ocean ultimately closes in much the manner described above, with the Americas and Africa reversing course and lazily drifting back toward each other before slowly, if violently, reuniting 250 million years from now. If this happens, then Davies has her eye on Namibia.
Namibia is a sedimentary standout today. And it’s unlikely to be disturbed by any major tectonic disruptions in the very long haul—until that happy day when it crashes into the Americas and gets uplifted as part of a vast east-west trending mountain chain at the very heart of the supercontinent, not unlike the Central Pangaean Mountains hundreds of millions of years before them.
Discouragingly, even if paleontologists exist on the world of Pangaea Ultima 250 million years from now, and even if we luck out on everything outlined above so far, the rocks to which we entrust our capsule would have to end up on a part of the planet that these future paleontologists would be likely to study. This might seem like a strange quibble, but today our understanding of the history of life on Earth is hugely biased toward the fossil record of the Northern Hemisphere for very human reasons, up to and including the history of global economic development. And while speculating on the political economy of the next supercontinent might be even more ridiculous than musing about its tectonics, there are still reasons to worry about the prospects of anyone—no matter where they come from on the tree of life—ever carrying out fieldwork across vast swaths of Pangaea Ultima. That’s because except for its polar fringes, it will be an absolute hellhole.
Supercontinents are miserable places to begin with. The last Pangaea, for instance, featured a vast, arid equatorial interior that was virtually devoid of life, brutally hot and streaked in toxic, superacidic salt playas. The interior of the next supercontinent will likely be even worse. This is because our star will grow about 2.5 percent brighter by the age of Pangaea Ultima. Paleoclimatologist Alexander Farnsworth and his colleagues have produced a menacing picture of the climate of this world. Daily temperatures could exceed an unthinkable 50 to 60 degrees Celsius (122 to 140 degrees Fahrenheit) for months on end across the entire supercontinent. Mammals can’t survive sustained temperatures above 40 degrees C (104 degrees F)—a seemingly hard limit over our entire quarter-billion-year evolutionary history—and the components of photosynthesis break down at 40 to 60 degrees C. Unless future paleontologists restrict themselves to the polar fringes of Pangaea Ultima, they will die. “If the time capsule survives the continental collision, then maybe it would be exposed in your central Pangaea Ultima mountains,” Davies says. “But then, yeah, there’s the problem of getting at it when it’s 60 degrees [C] out.”
Where does that leave us? If nothing else, this ridiculous thought experiment should drive home what a churning, restless planet we live on. This exercise would be trivially easy on Mars or the moon because those are dead, hopeless worlds. It’s not difficult on Mars to find river and lake sediments from four billion years ago exposed on the surface today. The moon still bears the fresh wounds of an asteroid impact 4.3 billion years ago. On Earth there aren’t even chunks of rock that old, and the Chicxulub crater, the biggest impact crater known to have formed in the past billion years, is hardly visible on the planet’s surface at all, buried under of tens of millions of years of limestone and covered in jungle. If there were bigger impacts over that vast span of time than the one that wiped out the dinosaurs, then they’ve been all but erased.
This is because our planet is alive. Plate tectonics ceaselessly reworks Earth’s surface: it pushes up mountains and creates and destroys oceans. Weather wears those same mountains down, and rivers carve canyons, seeding the oceans with nutrients that slough off the land and fuel life. This patient demolition helpfully draws CO2 out of the air as well, maintaining a habitable temperature for complex life through the chemical alchemy of rock weathering and erosion, which transforms carbon in the air to limestone at the bottom of the ocean over hundreds of millennia. This sequestration of CO2 is almost perfectly in balance with its contribution to the atmosphere elsewhere as it vents from volcanoes—volcanoes fired by subduction, rifting and all the other processes that ceaselessly remake our surface world. It’s a good deal for life on Earth. And it’s no small irony that the very reason this exercise is near impossible is the reason why we’re here in the first place.
“I think it’s becoming more and more obvious to a lot of geologists that plate tectonics is necessary for the long-term habitability of a planet,” Davies says, considering the strange thought experiment I had recruited her into. “It’s almost an interesting kind of catch-22: you need plate tectonics to develop civilizations, but plate tectonics can quite easily just destroy any remnants of civilization on a planet.”
