Most present-day oceanic crust is at most 200 million years old, and its formation is ongoing and well understood: When a gap opens between tectonic plates, magma bubbles up into the gap, cools, and spreads out from the initial ridge. Its thicker and less-dense cousin, continental crust, is much older; three-quarters of present-day landmass worldwide formed in the Archean eon, from 4 to 2.5 billion years ago. Because the continents have been around so long, figuring out their genesis is a challenge. Earth’s geological evolution gradually and often violently transforms and mixes continental crust until evidence of the distant past is erased.
Since the mid 1960s, one theory has argued that continents formed at the sites of collisions with asteroids tens or hundreds of kilometers wide. The resulting heating and removal of Earth’s primordial crust would have triggered a sequence of events in which the mantle swelled up and overfilled the crater to create a plateau. That seed would then have grown into a continent.
The idea makes chronological sense; the solar system suffered a barrage of asteroids around 3.9 billion years ago, in a period known as the Late Heavy Bombardment. The Moon’s craters are evidence of that bombardment (see the article by Brett Denevi, Physics Today, June 2017, page 38), but the evidence on Earth has been limited. And no concrete proof has connected the Late Heavy Bombardment to the formation of continents.
Now Tim Johnson of Curtin University in Australia and his colleagues have found support for the theory that continents formed at the sites of giant impacts. Their description of the emergence of the Pilbara craton—the best-preserved remnant of Earth’s ancient continental crust—may explain the formation of Earth’s continents.
Johnson and his colleagues pulled data from 26 craton rocks that were gathered from the Pilbara region of Western Australia and are 3.6 to 2.9 billion years old. They focused on grains of the magmatic mineral zircon, a common accessory mineral in igneous rocks. Such grains are useful because their compositional ratio of oxygen-18 to oxygen-16 isotopes provides a snapshot of their environments—materials, temperature, and so on—at the time they formed.
The team measured the 18O/16O isotope ratio of the Pilbara zircon samples. The data fell into three distinct time periods: stage 1, for samples older than 3.4 billion years; stage 2, for samples aged 3.4 to 3.0 billion years; and stage 3, for samples younger than 3.0 billion years. The median isotope ratio noticeably increased from stage 1 to stage 2 to stage 3.
Johnson and his colleagues argue that the data fit with what would be expected from a giant impact. In particular, the low 18O/16O isotope ratio in stage 1 indicates the minerals formed from a material source near Earth’s surface that was hydrothermally altered at high temperatures, as expected if a burning meteorite crashed into, cracked, and melted a primordial crust covered with ocean water. Other theories for how the continents formed—such as that hot matter bubbled up from the mantle—predict the oldest zircons would have origins from deep below the surface and thus a lower ratio.
The researchers’ next step is to investigate other well-preserved ancient rocks in northwest Canada, southern Greenland, and other territories to see if giant impacts could explain all continental masses, not just the Pilbara region. Their initial look at data from the Slave craton in Canada is promising. (T. E. Johnson et al., Nature 608, 330, 2022.)
