Science & technology
Earth may have been shaped by forces from outer space
The earth is a poor archivist.
The rigid tectonic plates of its outer layers are continuously in motion, sliding over one another to swallow almost all records of the past, melting them into the mantle and then casting them anew.
This makes it difficult to reconstruct the 4.5bn-year-old planet’s formative years.
A particular problem is the crucial biographical detail of how the first continents came to be.
Studies of ancient rocks suggest that fragments of solid crust existed as far back as 4bn years ago, when the Earth was mostly covered in water, but their genesis remains disputed.
While plate tectonics are the dominant mechanism for generating new continental crust today, in the distant past Earth’s subsurface temperatures are widely thought to have been too hot to allow plates to form.
A paper published this week in Nature attempts to resolve this conundrum.
Tim Johnson of Curtin University, Australia, and his colleagues argue that the planet’s transformation was not triggered by forces from within, but from outer space—in the form of collisions from meteorites that disrupted the surface, causing fragments of the crust to melt in the presence of water, cool and resolve themselves into buoyant patches of land that would later act as nuclei for new continents.
Dr Johnson’s thesis is not new, but the lack of direct evidence means it has never taken root.
The more popular explanation for the first continent formation points the finger at plumes of hot matter rising from the Earth’s mantle to the surface, much like bubbles in a lava lamp.
Then, if sufficiently hot, their underside would melt to produce buoyant granite, leading to the first continental plateaus.
The new study offers evidence that sits uncomfortably with this theory.
Dr Johnson and his colleagues looked at the Pilbara craton, an approximately 3.5bn-year-old slab of north-western Australia, roughly the size of Germany, which represents one of the few remaining scraps of the early Earth’s continental crust.