Earth’s oldest rocks were formed by meteorites
A four-billion-year geological mystery resolves if cosmic intervention enters the picture. Ben Lewis reports.
The oldest rocks on Earth may have formed in an apocalyptically fiery barrage of meteorites that melted the planet’s crust more than four billion years ago, geologists report.
Analysis of four-billion-year-old silica-rich rocks from the Acasta River in Canada by researchers from Western Australia’s Curtin University, published in the journal Nature Geoscience, found that they probably formed at high temperatures and at a surprisingly shallow depth.
Such a violent beginning explains why the rocks, called the Idiwhaa gneiss, appear noticeably different to others typical of Earth’s ancient continental crust, say the scientists, led by Tim Johnson. (“Idiwhaa” means “ancient” in the local First Nations language.)
And that confirms that the early development of the Earth was shaped by a mixture of very different processes than are found today.
The ancient rocks found around the Acasta River are a type referred to as felsic, because of their high feldspar and silicon content. However, the early Earth sported a crust made principally of dark-coloured mafic rocks, which are high in magnesium and iron but low in silica.
In addition, scientists have long known that the Acasta rocks are different to the majority of felsic rocks seen today, such as granite.
This created a mystery. What caused rocks to form four billion years ago that were not only very different to the geological conditions of the time, but also very different to rocks seen today?
Examining the chemical and trace mineral content of the samples, and using modelling to investigate the processes leading to their formation, the researchers found that it was possible to derive these special felsic rocks by melting and re-solidifying the ancient iron-rich mafic crust.
However, very specific conditions would have been needed for the formation to have led to the formation of the rocks, involving low pressure in the surface layer of the crust combined with a high temperature. This led the researchers to consider a cosmic intervention.
“Our modelling shows that the Acasta River rocks derived from the melting of pre-existing iron-rich basaltic rock, which formed the uppermost layers of crust on the primitive Earth,” says Johnson.
“The uppermost three kilometres of mafic crust would have been melted in producing the rocks we see today.
“It would have needed something special to produce the 900 degree Celsius temperatures needed to generate these early felsic rocks at such low pressures, and that probably means a drastic event, most likely the intense heating caused by meteorite bombardment.”
It’s entirely likely that following the barrage this type of felsic rock would have been very common. However, after billions of years and drastic changes due to plate tectonics, almost none remain. Today, only this small area in northern Canada survives as a window into the violent early years of a young Earth.