Scientists working at the University of Alberta have discovered evidence that may finally provide some answers to a question that has long puzzled geologists: how did the first continents form?
Before we get into the details of the study, let’s get a bit of the backstory.
When the earth first solidified, the surface rocks consisted primarily of basalt – the same type of rock that underlies the world’s ocean basins today. Since basalt has a consistently high density, it would have formed a relatively flat surface, unbroken by continents.
In contrast, the crust of modern Earth is broken into regions of high density basalt (the ocean basins) and relatively low density continental crust rocks, which tend to be enriched in silica. These lower density rocks rise above the oceans, forming the continents we inhabit today.
In recent times, there have been two competing theories about the origins of the continents. One theory posits that the first continental crust formed the same way it tends to form today – as one continental or oceanic plate subducts (sinks) beneath another, a complex series of geochemical reactions causes some of the melting plate to decrease in density and rise to the surface, erupting through volcanoes and adding to the continents along their margins (here’s a cool animation of the process).
The second theory holds that the first continents formed over a zone of mantle upwelling. Processes over upwelling zones can lead to separation of mantle rocks into high and low density components. Low density components rise to the surface, creating a landmass that rises above the seafloor. A modern landmass that formed by this process is Iceland (Iceland is a very unique place geologically – if you want to learn more, take a look at this documentary).
The University of Alberta team, led by Jesse Reimink, aimed to determine which theory was more likely. Many studies have attempted to find evidence for one theory or the other in the past, but this team had a new source of evidence – a rock unit known as “Idiwhaa” (a local Tlicho word for “ancient”).
This four billion year old rock unit, sourced from the Acasta Gneiss Complex of Canada’s Northwest Territories, has somehow escaped the deformation and metamorphosis that has affected most rocks of its age. This means that the team was able to perform geochemical analyses that haven’t been possible in the past.
The rock contains a variety of geochemical signatures characteristic of the rocks forming in Iceland today. For the first time, scientists have some direct geochemical evidence to support the mantle upwelling theory of continent formation. Once the incipient continents became large enough, the subduction related processes that dominate continent formation today would have become dominant, leading to to the formation of the modern continental land masses.