The rock known as gabbro features in many trendy kitchen countertops, where its durability, heat resistance, and bold black-and-white veining make it an attractive addition. Its chemistry also made it ideal for creating the early building blocks of the continents, according to a new study published in Nature Communications.
Early Earth was covered in a hardened shell of dark basaltic crust that crystalized as the planet’s primordial ocean cooled. How the lighter and chemically distinct continental crust formed later is still under debate.
The new study proposes that gabbros melted within early Earth to form rocks known as TTGs—a mix of tonalite, trondhjemite, and granodiorite—which made up most early continental crust.
“Many people are preparing their dinner on the type of rock that was responsible for making our modern continents.”
“Many people are preparing their dinner on the type of rock that was responsible for making our modern continents,” said the study’s first author Matthijs Smit, a geochemist at the University of British Colombia in Canada. Smit was struck by the revelation while casually cutting an onion for a meal at a friend’s house.
The crusts making up the continents and the ocean floor are physically and chemically different. Oceanic crust is thin and dense and made primarily from gabbro and its surface equivalent, basalt. Continental crust, on the other hand, is thicker and less dense and is made from lighter rocks that contain mostly silicate minerals—those that have abundant silicon and oxygen.
According to some theories, continents were first formed mainly by subduction, as one tectonic plate slid under another, or by meteorite impacts that fractured and altered the early crust. But the new study suggests instead that the early continents formed from the melting of gabbro in crustal regions known as protocontinents roughly 3.5 billion years ago, during the Archean eon.
Archean TTGs are difficult to study, in part because they are so old: Plate tectonics has trundled along for the past 3 billion years or so, subducting, morphing, and eroding TTGs from early Earth and affecting their composition.
Creating Crust
To dig into the past, researchers looked at data from thousands of TTG samples from locations where the ancient crust is exposed around the world—including the Rocky Mountains in Canada—which are detailed in the GEOROC (Geochemistry of Rocks of the Oceans and Continents) data repository at the University of Göttingen in Germany.
The TTGs had a distinctive signature of trace elements—a high ratio of the rare earth element lanthanum compared to samarium and high levels of europium—that indicated that gabbros were the source rocks, Smit said.
This new understanding enabled the researchers to determine that Archean TTGs had formed at the bases of areas of thickened oceanic plateaus above hot regions of the mantle.
“This is a new idea and quite different from previous models.”
Earlier studies had suggested that Archean TTGs had formed from melting within oceanic plateaus, geochemist Balz Kamber from Queensland University of Technology in Australia wrote in an email. But the new study is the first to identify gabbros as the source. “The idea had not previously been tested with such a thorough assessment of published geochemical data for TTGs,” Kamber wrote. Kamber was not involved in the new study.
Geologist Hugh Rollinson, a professor emeritus at the University of Derby in the United Kingdom, added that he’d like to see a study of areas where such TTGs were exposed on the surface, rather than the latest study’s statistical approach to all TTG samples in the GEOROC database. But he stressed that the specific geochemical mechanism proposed in the study was a novel approach to the issue of the formation of Archean TTGs: “This is a new idea and quite different from previous models,” he said.
—Tom Metcalfe (@HHAspasia), Science Writer