Roiling, incandescent magma and boiling gases covered the earth in the wake of its formation 4.6 billion years ago. Regions of this fiery sea eventually cooled enough to crust over, leaving the planet’s first hard rocks floating like slag on the white-hot liquid. But they were nothing more than a thin veneer. The thick roots of terra firma were much longer in the making.

Exactly how—and how quickly—continents arose and grew is a matter of ongoing debate. Scientific wisdom long held that the earth’s inner workings alone drove continent formation. But recent findings have turned the spotlight toward a once heretical idea: that large asteroid impacts played a constructive role as well.

A basic assumption was that asteroid bombardments—frequent during the earth’s infancy—had all but petered out by about 3.8 billion years ago. By then, the planet had cooled enough for nascent oceans to harbor microscopic life. Major impacts since that time were typically considered rare and utterly destructive. (Think demise of the dinosaurs.)

Recently, though, scientists have been forced to wrestle with the discovery of an unexpected series of massive blows between 3.8 billion and 2.5 billion years ago, a span of the earth’s youth known as the Archean eon. The crust-obliterating reputation of asteroids seems at odds with a hallmark of the Archean: it was the most productive period of continent formation in all of earth history. By some estimates, 65 percent of today’s continental crust came into being during that time.

Attempting to reconcile this apparent conundrum, geologists are scouring the ancient rock record for clues about how these colossal collisions shaped the planet. One of these geologists—Andrew Y. Glikson, a professor at the Australian National University in Canberra—has been convinced by 40 years of fieldwork that extraterrestrial impacts actually aided the growth of the planet’s first continents, including ones with remnants now preserved in the ancient cores of South Africa and Western Australia.

Many scientists cast Glikson’s assertion a tentative glance, arguing that direct evidence for what was happening on the ancient earth is exceedingly rare and controversial. Yet computer simulations of the potential effects of large impacts lend some intriguing support to his hypothesis. It may be too early to overhaul the classic view of the early evolution of continents, but even skeptics agree it is time to consider the earthly outcomes of these powerful forces from space.

Land, Ho!
Scientists spent decades deciphering the origin of continents before the potential influence of Archean asteroid impacts came into focus. These efforts have always been tricky because creating a continent is such a complicated process; it requires building up a slab of crust so thick and buoyant that it can no longer sink back into the earth’s hot interior. That quality is what makes today’s continents so different from the crust underlying the oceans. Relatively thin and dense, iron-rich ocean crust sinks easily, most of it within a mere 200 million years of its formation. Continental crust, on the other hand, is packed with lower density rocks such as granite, which have kept some ancient fragments afloat, like icebergs at sea, for nearly four billion years.

The story of the earth’s first continent varies from one textbook to another, but one common version unfolds something like this:

During brief reprieves from the heavy asteroid bombardment following the planet’s birth, the earth’s natural tendency to cool caused the surface to crust over repeatedly. This crust was not perfectly continuous; it consisted of several dozen pieces that skidded across the ever churning magma. Like hot wax rising in a lava lamp, plumes of hot mantle rock rose up, cooled slightly as they moved across the surface, and then sank—easily dragging those original, ultradense crustal fragments down with them. Meanwhile volcanoes spewing gases from the earth’s interior created a primitive atmosphere, and rain condensed out of the sky, forming shallow oceans atop the thin, crusted magma.

The embryo of a continent formed, so continues this storyline, when the heat from a rising plume partially melted a patch of the dense crust before it could sink—allowing lighter minerals, which have a lower melting point, to separate out. More buoyant than the surrounding rock, this newly separated magma tended to rise up; once hardened, this lighter rock was less likely to sink later on.

Repeated cycles of partial melting and separation of lighter magma led eventually to the production of granite. It is impossible to know the precise timing of this process, but at least one trace from the first 160 million years of the earth’s infancy remains: tiny, 4.4-billion-year-old zircon crystals eroded from a primordial granite and were later deposited within younger sedimentary rock formations in what is now Australia [see “A Cool Early Earth?” by John W. Valley; Scientific American, October 2005].

These traces of early granite were probably a minor component of the first masses of rock to grow thick enough to protrude above the early oceans. And they certainly would have been a far cry from today’s continents, which cover 30 percent of the planet’s surface and are on average 35 kilometers thick. Early protocontinents probably gained stature slowly, much as landmasses do today: collisions among them merged thickened crust into larger masses, and hot mantle plumes triggered surges of fresh magma from below.

By three billion years ago, most geologists agree, the earth had its first bona fide continent: a barren, volcano-strewn mound of rock almost certainly smaller than present-day Australia. It is even possible that ancient cores, or cratons, of present-day Australia and Africa were part of that original continent. Western Australia’s Pilbara craton and the Kaapvaal craton of South Africa’s scenic Barberton Mountain lands “are stunningly similar geologically,” notes geologist Bruce M. Simonson of Oberlin College, who has spent months combing the dry, brushy, hillside outcrops of both regions. “I’m a firm believer that Barberton and Pilbara are one continent that got split in two.”

Where on the globe the first continent sat is unknown, but as the earth’s hot interior continued churning, that landmass split apart as others sprung up. A well-documented series of breakups and mergers of continents ensued, leading eventually to the modern arrangement.

Knowing Where to Look
The dance of crustal plates clearly explains the transition of continents from youth into adulthood. But what transpired beforehand is rife with uncertainty. That is why geologists turn to those ancient landforms in South Africa and Australia for clues about continental nativity. Compared with the cratons of other modern continents, Kaapvaal and Pilbara have undergone less metamorphism and remain some of the most well-preserved traces of Archean-aged crust. Of particular interest within these cratons are greenstone belts—rock formations that took shape between 3.5 billion and 2.4 billion years ago, right as the first continents were coming to be.

Since the 1970s most geologists have interpreted greenstone belts as ancient analogs to the strings of volcanic islands that arise along the overlapping edges of colliding crustal plates—and later become part of a continental landmass. Crustal collision continues over millions of years, and the lower plate dives ever deeper into the earth’s hot interior, forming a deep trench known as a subduction zone. As the islands ride the sinking plate toward the trench, these thicker parts are thrust onto the side of a looming landmass; rather than being pulled down with their parent crust, they are scraped right off the top. The Sierra Nevada and other mountain ranges of the western U.S. glommed onto western North America in this way.

Yet this modern style of continental growth cannot explain all the geologic features seen in the greenstone belts, Glikson notes. When studying the South African and Australian belts in detail years ago, he found that the belts’ oldest segments—those between three billion and 3.5 billion years old—all appeared to have accumulated vertically, as eroded material was laid down in layers between dome-shaped bodies of granite-forming magma pushing up from below. These formations showed none of subduction’s telltale signs: sediments and volcanic material that accrued horizontally as two crustal fragments collided.

A dearth of evidence for subduction is not surprising. Most researchers agree that plate tectonics was probably less efficient in the early Archean, if it operated at all. The planet was hotter then, and so less vigorous was the lava lamp–like convection that drives plate motion. Still, something swift must have taken a hand in the formation of the oldest parts of the Archean belts, Glikson says. The specific ages of various rocks within them suggest that massive granite bodies were emplaced in a series of abrupt, well-defined episodes. But if subduction was not the driving force, what was?

These difficulties led Glikson to seek new explanations for what shaped the Archean earth. He knew that one factor most geologists had ignored was the potential effect of collisions by asteroids and comets. Asteroid bombardment peaked around 3.9 billion years ago, yet studies of moon craters indicate large impacts continued until about 3.2 billion years ago. Could those later bombardments have been involved? The first step in finding out would be identifying good evidence of such strikes on the earth. Had this evidence been destroyed, or were geologists looking at it without recognizing it?

Solid Blows
A pair of American geologists answered the latter question in 1986. During their annual re­­search excursions to the greenstone belt in the Barberton Mountains, Donald R. Lowe of Stanford University and Gary R. Byerly of Louisiana State University had stumbled across a thin layer of ancient ocean sediment containing hundreds of hollow, glasslike beads. On closer inspection, these sand-size spheres appeared nearly identical to the so-called impact spherules that be­­came some of the strongest evidence of an asteroid striking the planet 65 million years ago, ending the reign of the dinosaurs. These Barberton spherules, dated to 3.2 billion years ago—plus another spherule bed found in Australia’s Pilbara craton—became the first evidence that large extraterrestrial objects smashed into the earth during the Archean.

Additional discoveries followed. Knowing that the spherule layer from the dino-killing impact showed up around the globe, Lowe and Byerly soon correlated the Australian bed with a 3.5-billion-year-old impact they found in Barberton. They also discovered two more 3.2-billion-year-old spherule beds in South Africa. Simonson, too, ran across unexpected spherule beds during his explorations of iron formations in the Pilbara region in the early 1990s, extending the surprising series of asteroid strikes just beyond the end of the Archean eon, 2.5 billion years ago.

Inspecting the Archean greenstone belts with ancient impacts in mind gave these geologists additional insight into the asteroids and their aftermath. From the magnesium- and iron-rich composition of the spherules, for example, Lowe and Byerly deduced that the errant space rocks most likely struck the dense rock of an ocean basin—probably a fair distance away from the regions where the preserved spherules landed. Signs of globe-sweeping tsunamis that accompany each of the spherule beds they have uncovered in South Africa further corroborate that the asteroids smashed into an ocean rather than an exposed landmass, they say.

Glikson noted that the timing of some strikes coincides with the formation of “an abundant supply of angular boulders, including blocks up to 250 meters across,” in the Pilbara region. Such jumbled blocks are the shattered result of the rise and collapse of the earth’s surface along major earthquake faults in the area. Indeed, intense swarms of strong earthquakes would be one of the most immediate effects of a large asteroid impact.

Clearly, the early Archean impacts were not something the planet took lightly. Lowe and Byerly estimated their asteroids were big: between 20 and 50 kilometers in diameter, based on the distribution of spherules and other comparisons with the ejecta from younger impacts. (For comparison, best estimates suggest that the errant asteroid that killed the dinosaurs was no more than 15 kilometers wide.) Such indications of the asteroids’ size fueled Glikson’s notion that they could have played a role in continent formation. He soon began drawing attention to other abrupt changes in the rock record right around the time of what he sees as a particularly illuminating trio of impacts: those Lowe and Byerly found clustered in South African sediments deposited around 3.2 billion years ago.

In a recent technical paper, Glikson observes that the timing of these impacts coincides with major signs that these regions were rising above sea level for the first time—presumably forming a new continental landmass. Specifically, the rock record laid down before the impacts consists of thick layers of ocean crust and types of sediments that form on the seafloor. During the period encompassing the asteroid strikes, those basalt layers are deformed, uplifted and eroded—kinds of upheaval easily attributable to the shock of asteroid collisions, he explains. In contrast, all the rocks formed after the time of the impact trio represent the eroded remnants of rocks that could have formed only on land. This change suggests that not long after the asteroid strikes, great forces within the earth raised the crust above the surface of the ocean, granites and other continental-type rocks formed, and they eventually eroded.

Glikson further suggests that the asteroid strikes themselves were the source of this upheaval. Most critical to his argument are the great masses of granite-forming magmas that intruded into both the Pilbara and Kaapvaal regions from below about 3.2 billion years ago. The similar timing of the asteroid impacts and the formation of this new magma was more than a mere coincidence, Glikson argues; they were cause and effect. He asserts that their planet-altering forces “caused major uplift of earlier nascent continents and intrusion of granitic magmas, both testifying to the violent origin of at least some parts of continental crust.” The critical question is: What heating process generated the magma? Glikson’s answer: the disruptive force of the 3.2-billion-year-old asteroid impacts shifted mantle convection patterns, triggering new mantle plumes that rose up and heated the crust from below.

Constructive Criticism
The plausibility of Glikson’s assertion hinges in great part on the size of the errant asteroid. From the perspective of the earth’s inner workings, a rock the size of the dino-killing asteroid would be hardly more than a “bug on the windshield,” Simonson suggests. But if the early Archean impacts were truly double that size, they could have left a more lasting impression. In particular, impacts as large as 50 kilometers in diameter could indeed shift patterns of heat flow inside the earth, says geophysicist Jay Melosh of Purdue University. Based on computer simulations of hypothetical impacts that he and his colleagues have developed for other purposes, Melosh describes how a sufficiently large early Archean asteroid impact might actually help a continent bulk up.

In this hypothetical scenario, Melosh assumes that an asteroid 50 kilometers wide smacks into an ocean basin at about 20 kilometers per second. Such an impact does not excavate a crater; instead it generates an enormous sea of molten rock some 500 kilometers across and nearly as deep. If such an asteroid-induced magma lake forms atop a mantle plume, its intense heat stifles the rising plume and then deflects it to surrounding regions. A plume deflected underneath dense ocean crust might generate new islands that might much later find their way to a subduction zone and thicken a growing continent from the side. Or if the deflected plume happened to rise below a protocontinent already containing less dense rock, the new heat source might be sufficient to produce fresh upward surges of granitic magma such as those located in the greenstone belts of Pilbara and Kaapvaal, thereby thickening the continent from below.

But this scenario is rife with uncertainty, Melosh warns. Proving that a given asteroid deflected mantle plumes to create specific continental embryos found in the rock record is virtually impossible. The craters the asteroids generated have long since been subducted or eroded away. And even if a plume was indeed responsible for the production of the granite, who is to say it was not already rising beneath a protocontinent even before the asteroid hit?

In the end, Glikson has illuminated an amazing coincidence in timing between the early Archean asteroid strikes and insurgence of new magma in ancient fragments of today’s continents—and he has tied them together with a credible mechanism for how a cosmic strike could actually lead to the production of such magma. “It’s a very possible hypothesis of what might have happened,” Lowe says. “But it’s only one interpretation.” Undoubtedly, though, planet-altering impacts interrupted the earth’s internal dynamics—and their violence may not have been entirely destructive.