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Earth is the only planet we know of with continents, the giant land masses that provide homes for humanity and most of Earth’s biomass. However, we still don’t have firm answers to some basic questions about continents: How did they form and why did they form where they did? One theory is that they were formed by giant asteroids that crashed into the Earth’s crust long ago. This idea has been proposed several times, but until now there was little evidence to support it.

In new research published in Nature, we studied ancient minerals from Western Australia and found tantalizing clues that suggest the giant impact hypothesis might be correct.

How is a continent made?

Continents are part of the lithosphere, the rigid rocky outer layer of the Earth formed by the ocean floors and the continents, of which the uppermost layer is the crust.

The crust under the oceans is thin and made of dark, dense basaltic rock that contains only a little silica. In contrast, the continental crust is thick and consists mostly of granite, a less dense, pale-colored, silica-rich rock that “floats” the continents.

Beneath the lithosphere is a thick, slowly flowing mass of nearly molten rock that lies near the top of the mantle, the layer of Earth between the crust and the core.

If part of the lithosphere is removed, the mantle beneath it melts as the pressure above is released. And giant meteorite impacts (space rocks tens or hundreds of kilometers in diameter) are an extremely efficient way to do just that!

What are the consequences of a giant impact?

Giant impacts explode large volumes of material almost instantaneously. Rocks near the surface will melt for hundreds of kilometers or more around the impact site. The impact also releases pressure on the mantle below, causing it to melt and produce a “droplet-like” mass of thick basaltic crust.

This mass is called an oceanic plateau, similar to the one under present-day Hawaii or Iceland. The process is a bit like what happens if a golf ball or pebble hits you hard on the head: the resulting bump or “egg” is like the ocean plateau.

Our research shows that these ocean plateaus could have evolved to form the continents through a process known as crustal differentiation. The thick oceanic plateau formed from the impact can heat up enough at its base to melt as well, producing the kind of granitic rock that makes up floating continental crust.

Are there other ways to make ocean plateaus?

There are other ways that oceanic plateaus can form. The thick crusts beneath Hawaii and Iceland were not formed through giant impacts, but by “mantle plumes,” streams of hot material rising from the edge of Earth’s metallic core, somewhat like in a lava lamp. As this rising plume reaches the lithosphere, it causes massive melting of the mantle to form an oceanic plateau.

So could plumes have created the continents? Based on our studies and the balance of different oxygen isotopes in small grains of the mineral zircon, which is commonly found in small amounts in rocks of the continental crust, we don’t think so.

Zircon is the oldest known crustal material and can survive intact for billions of years. We can also determine quite precisely when it was formed, based on the decay of the radioactive uranium it contains.

In addition, we can learn about the environment in which zircon was formed by measuring the relative ratio of oxygen isotopes it contains.

We observed zircon grains from one of the oldest pieces of continental crust in the world, the Pilbara Craton in Western Australia, which began to form more than 3 billion years ago. Many of the older zircon grains contained more light oxygen isotopes, indicating shallow melting, but the younger grains contain more mantle-like equilibrium isotopes, indicating much deeper melting.

This “top-down” pattern of oxygen isotopes is what you might expect after a giant meteorite impact. In contrast, in mantle plumes, melting is a “bottom-up” process.

It seems reasonable, but is there any other proof?

Yes here it is! Pilbara craton zircons appear to have formed in a handful of distinct periods, rather than continuously over time.

Except for the first grains, the other grains with isotopically light zircon are of the same age as the spherule beds in the Pilbara Craton and elsewhere.

Spherule beds are deposits of drops of material “splattered” by meteorite impacts. The fact that the zircons are of the same age suggests that they may have been formed by the same events.

In addition, the “top-down” pattern of isotopes can be recognized in other areas of the ancient continental crust, such as Canada and Greenland. However, data from other locations has not yet been filtered as carefully as the Pilbara data, so more work will be needed to confirm this pattern.

The next step in our research is to reanalyze these ancient rocks from other places to confirm what we suspect: that the continents grew at the sites of giant meteorite impacts. boom