Giant Meteorite Impacts May Have Formed The Continents

Earth. 4.6 billion years ago, dust and gas coalesced to form our planet. In Earth’s early days, it was a ball of hot, molten rock. Shortly after Earth formed, a Mars-sized object named Theia smashed into our planet. This impact created the Moon.

Things didn’t settle down after Theia’s impact and the creation of the Moon. For the first billion years, Earth faced a constant barrage of meteorites. This culminated in the Late Heavy Bombardment which lasted from approximately 4.1 - 3.8 billion years ago. It’s safe to say that early Earth would have been hellish, which is why the first aeon is named the Hadean after the Greek god Hades.  

Today the Earth’s surface has solidified, and thankfully there are a lot fewer meteorite impacts that cause significant damage - though our home planet is bombarded thousands of times per day by smaller objects, mostly no bigger than a range of sizes, stretching between specs of dust and up to about the size of basketballs. We often don’t see or hear the effects of these atmospheric impacts, because 70% of our planet is covered in ocean, and away from populous locations, where people can witness such events.

But despite our planet having a fairly similar origin to the other rocky planets in our Solar System, we have a few key differences. Life is one of those differences (insofar as we know), but so are the continents that human beings and many other lifeforms live on. Continents are the giant plates of landmass that float around on Earth’s hot, viscous mantle. These giant structures move over the course of millions to billions of years, crashing into each other and sometimes forming mountain ranges, or subducting under each other forming trenches. The highest (Mt. Everest - 8,848 metres above sea level) and lowest points (Mariana Trench - 11,034 metres below sea level) on Earth are a result of these giant plates, undergoing tectonic processes. 

One key aspect of having continental drift, which is also unique to Earth, is the presence of liquid water, which tectonic activity relies upon to ensure continual movement and replenishment. Other planets (like Venus) also have plates, but in the absence of water, these have long since become stagnant. 

So why do we have continents?  

A theory that has been around for decades is that giant meteorite impacts from Earth’s first billion years of history may be responsible. Now, new research from Curtin University provides the strongest evidence to date that this theory is correct.

The team of researchers began by investigating zircon crystals and their oxygen isotope composition from the Pilbara Craton in Western Australia. Cratons are very stable parts of the continental crust, so are able to survive and preserve rocks for billions of years. The Pilbara is around 3.5 billion years old and is one of Earth’s best-preserved pieces of Archean crust. Zircon crystals are also very stable. During their formation, they trap Uranium which decays to Lead very slowly but at a steady rate, meaning these zircon crystals can be accurately dated. The oxygen isotopes also trapped in these zircons provide information about the mantle composition and geological processes at that time.

“By examining tiny crystals of the mineral zircon in rocks from the Pilbara Craton in Western Australia, which represents Earth’s best-preserved remnant of ancient crust, we found evidence of these giant meteorite impacts,” said Dr Tim Johnson, from Curtin University’s School of Earth and Planetary Sciences.

“Studying the composition of oxygen isotopes in these zircon crystals revealed a ‘top-down’ process starting with the melting of rocks near the surface and progressing deeper, consistent with the geological effect of giant meteorite impacts.”

“Our research provides the first solid evidence that the processes that ultimately formed the continents began with giant meteorite impacts, similar to those responsible for the extinction of the dinosaurs, but which occurred billions of years earlier.”  

The team also compared data from the Pilbara with the WA’s Yilgarn Craton (also home to the oldest terrestrial material on Earth - 4.4 billion-year-old zircon crystals), Canada’s Slave Craton and Superior Provence, and West Greenland. 

Studying the formation and evolution of Earth’s continents is incredibly important given that’s where the majority of Earth’s biomass is, not least of all human beings, and also the majority of Earth's important mineral deposits.

“Not least, the continents host critical metals such as lithium, tin and nickel, commodities that are essential to the emerging green technologies needed to fulfil our obligation to mitigate climate change,” said Dr Johnson.

“These mineral deposits are the end result of a process known as crustal differentiation, which began with the formation of the earliest landmasses, of which the Pilbara Craton is just one of many.”  

“Data related to other areas of ancient continental crust on Earth appears to show patterns similar to those recognised in Western Australia. We would like to test our findings on these ancient rocks to see if, as we suspect, our model is more widely applicable.”