Wrinkled Time

The Persistence of Past Worlds on Earth

Excerpts from a recent essay in Emergence Magazine

Marcia Bjornerud is a professor of environmental studies and geosciences at Lawrence University. She is a contributing writer to The New Yorker, Wired, The Wall Street Journal, and The Los Angeles Times and the author of Timefulness: How Thinking Like a Geologist Can Help Save the World. A Geological Society of America fellow and Fulbright alumna, Marcia has been named Outstanding Educator by the Association of Women Geoscientists. Her latest book is Turning to Stone: Discovering the Subtle Wisdom of Rocks.

Chronicling the way Earth archives Her geological history in the wrinkled strata just beneath our feet, Marcia Bjornerud orients us to the deep time-fulness of Earth—the four billion years of dynamism held in the ancient and ongoing story of rock.

Anyone born on Earth knows that our home is remarkable. If you were prompted to think about what makes this planet so special, you might mention the vast oceans and abundant oxygen, perhaps the protective magnetic field. But an equally exceptional yet less obvious attribute is the way that Earth preserves accessible records of countless earlier versions of itself, condensing events of billions of years into the volume of the present-day continents. In achieving this feat of compression, Earth effectively wrinkles time—accordioning eons and juxtaposing moments in its long history—through the medium of rocks.

In Earth’s crust, past worlds coexist with the present in the form of rocks from all chapters in the planet’s biography, but few of us give much thought to this extraordinary archive. If we would remember that we are first and foremost Earthlings, a term describing our shared heritage that should never have been ceded to science fiction, we might start paying more attention to what the planet itself has to say. We don’t realize how unusual it is to have easy access to nearly the entire history of our planet. None of our neighbors in space have maintained such a long and continuous rock record.

To grasp the peculiarity of Earth’s crustal archives requires an appreciation for the semiotics of rocks. First, rocks are best understood not as nouns but verbs, signifying events and processes. If nothing is happening on a planet, there will be no new rocks. Thinking of rocks as verbs is like seeing a painting not merely as an object that is, but as the manifestation of the motions that led to its creation. Like artists who paint only in oils, some planets work only in one medium: magma is the most common. Mercury, Venus, and the Moon fall into this igneous-only group. But generating magma requires a continuing heat source, and if a body cools too much, its output will end. This has been the fate of Mercury and the Moon, which are now silent museum worlds whose rocks are monuments to a dimly remembered volcanic past, vandalized by eons of meteorite impacts.

A wider range of creative possibilities is available to planets with liquid water, because erosion, sedimentation, dissolution, and reprecipitation make it possible to create new rocks by reconstituting existing ones. Among the inner planets, Mars and Earth have both practiced the watery art of sedimentation, but Mars lost its atmosphere early on and stopped recording its experiences while still in its childhood. Earth, in contrast, has continued to experiment with sedimentary techniques and has invented countless new ways to lay down rock, many done in collaboration with life-forms such as algae and coral.

So the first criterion for a long-duration planetary record is that rocks must be continuously generated over time—i.e., a planet must be active and dynamic, always creating new rock through one or more processes. But this creates a paradox, because on such a planet, the same processes that generate new rocks—magmatism, tectonics, sedimentation, erosion—are also likely to melt, alter, bury, and destroy older ones. One can easily imagine a world on which tectonic or hydrologic processes are so vigorous—like a dissatisfied artist who obsessively repaints the same canvas or destroys works as soon as they are finished—that no rocks of one period survive into the next.

Our sister planet Venus seems to behave in this impulsive way, at least episodically. Based on the relatively small number of craters on its surface, it appears to have had a global volcanic paroxysm that lacquered the entire planet in lavas a few hundred million years ago,¹ making any earlier records of its history permanently inaccessible. Given Venus’ subdued topography and lack of erosive processes, any older rocks that may survive beneath its surface cannot be “read”—there, the past is static and sequestered beyond our knowing.

And now we begin to see why Earth’s long rock record is exceptional. Even though Earth is exuberantly active, with countless parts in constant motion, from a churning mantle and shifting plates, to roaring rivers and crashing surf, this planet has consistently managed to set aside some of its rocky work over the past four billion years—and make those archives open to those of us who live on the surface. This is strange and remarkable.

How can a planet crenulate so much time into the finite dimensions of its outer crust? Two requirements need to be met. First, rates of rock formation must, on average, outpace rates of destruction, and second, the surviving rocks must be continuously reconfigured for access from the surface. Earth has rather elaborate mechanisms for accomplishing both.

The first requirement is in fact not true for rocks of the deep-sea floor. The oceanic crust, which covers more than two-thirds of Earth’s solid surface, lives fast and dies young. Made of the volcanic rock called basalt, the ocean floor is recycled at the same rate it’s created, with no net accumulation. Each year, about 20 km³ (4.8 mi³) of new ocean crust is extruded as lava from the mid-ocean ridges²—but an equal amount is consumed by subduction. If we had to rely on ocean crust to tell the Earth’s story, the narrative would take us back only to Jurassic time—just the last three percent of the planet’s history. (Also, it would not have much to say about what was happening on land during that time, like the rise and fall of certain terrible lizards and the subsequent proliferation of mammals.)

Thinking of rocks as verbs is like seeing a painting not merely as an object that is, but as the manifestation of the motions that led to its creation.

Fortunately, from an archival standpoint, Earth’s continents are underlain by a second, distinct type of crust with a much longer life expectancy. The heterogeneous but broadly granitic continental crust has too low a density to be taken back into the mantle by subduction, giving it the potential to survive billions of years. In fact, much of the extant continental crust formed prior to about three billion years ago. Continental crust is vulnerable to erosion—from which the ocean crust is exempt—but erosion is slow relative to the total volume of the continents, so even its incessant gnawing does not consume them entirely. And counterintuitively, erosion is actually essential to creating Earth’s archives—as well as making them accessible to land-based readers. Herein lies Earth’s secret to wrinkling time.

Most sediment that is eroded from the continents does not reach the deep ocean floor but instead gets deposited by rivers, which lose their steam when they reach the sea, in the shallow offshore regions called the continental shelves. The shelves are a liminal zone, untouched by the forces that destroy fully continental or fully oceanic crust. They are continental in the sense of being granitic, and therefore non-subductable, but marine in that they lie below sea level and are thus non-erodable. At the same time, they are sensitive to events on both land and sea, receiving detritus from continents while in communication with the global ocean. As a consequence, the continental shelves are the ideal site for long-term planetary recordkeeping.

ONE REASON the current moment feels so perilous is that we are hurtling forward with no shared vision—no common text or set of principles to consult for guidance. We humans have always craved a sense that we are part of something larger than ourselves, that our quotidian lives play out against the backdrop of some greater, grander presence. Traditionally, religions have met this yearning. But these days existential meaning is often sought in ways that, perversely, act to isolate us further: conspicuous consumption, political tribalism, conspiracy mongering, addictions of various sorts.

The spiritual solace we crave may lie in the records of deep time that are our common heritage as Earthlings. The rocky archives have been patiently awaiting our notice. In them, we may find reassurance in the persistence of earlier worlds all around us; a sense of wonder at how extraordinary their preservation is; gratitude for the way they permeate the present with mystery, gravitas, and the promise of continuity. A spirit of evolutionary camaraderie may come from the knowledge that we have shared the arduous journey to the present with so many other long-lived lineages and have kin everywhere in nature. Accepting that we too live in geologic time can free us from narcissism. Letting go of the illusion that only the present is real, allowing the undulations of time to wash over us, may carry us with less fear into the future.

We might begin to notice that in contrast to our technologies, which become outmoded in a matter of years, billion-year-old rocks still function perfectly well. We may then realize that the text of the Earth itself is full of guidance on durable design and start laying the groundwork for a post-Anthropocene world in which our species will have learned at last to be law-abiding Earth citizens. But this new relationship with time will require social change, which takes … time.

Still, we can make a start. We can write songs of gratitude for the biogeochemical cycles that keep the planet on an even keel. We can celebrate ecosystems—peat bogs and cloud forests, savannas, and kelp groves—and all their timeful idiosyncrasies. We can develop new geocentric curricula that teach children from an early age about how their magnificent planet works, from its mid-ocean ridges to subduction zones, continental shelves to crumpled mountain belts. And we can all strive to become more time-literate Earthlings, at peace with our temporal, and temporary, nature. Then we may learn to savor our moment on this wrinkled old planet, enfolded cozily within the creases of deep time.

Read the full essay at Emergence Magazine.