The secrets of how Earth’s continents first formed over 2.5 billion years ago lie in relatively recent geologic events that took place 10 million years ago in what is now Panama and Costa Rica.
By studying recent volcanic activity in that region, the authors of a new Nature Geoscience paper set out to unravel the mysteries surrounding the extreme continent-building that occurred billions of years earlier, as well as how Earth’s life and climate have continued to be profoundly affected by those processes over the past 70 million years.
The findings, Virginia Tech assistant geology professor Esteban Gazel and his colleagues write, provide new insight into the formation of the planet’s continental crust, which are masses of rock that are buoyant and rich with silica (a compound which combines silicon and oxygen).
Earth sure is crusty
“Without continental crust, the whole planet would be covered with water,” explained Gazel, a geoscientist at the Blacksburg, Virginia-based university. “Most terrestrial planets in the solar system have basaltic crusts similar to Earth’s oceanic crust, but the continental masses – areas of buoyant, thick silicic crust – are a unique characteristic of Earth.”
The Earth’s continental mass formed approximately 2.5 billion years ago in the Archaean Eon. During that time, the planet was roughly three-times hotter than it is today and had significantly more volcanic activity. Life would have been very limited, and many scientists believe that most of the continental crust was generated during this period in the planet’s history.
Furthermore, the belief is that the material used to create that crust is continually recycled through collisions of tectonic plates on the planet’s outermost shell. However, the new study indicates that newer, younger continental crust has been produced throughout Earth’s history.
Newer crust is still formed
“Whether the Earth has been recycling all of its continental crust has always been the big mystery,” said Gazel. “We were able to use the formation of the Central America land bridge as a natural laboratory to understand how continents formed, and we discovered while the massive production of continental crust that took place during the Archaean is no longer the norm, there are exceptions that produce ‘juvenile’ continental crust.”
He and his co-authors reconstructed the evolution of modern-day Panama and Costa Rica using geochemical and geophysical data to show how those landmasses were generated when a pair of oceanic plates collided, melting iron- and magnesium-rich oceanic crust over the past 70 million years. That melting produced the modern-day Galapagos Islands by reproducing the conditions of the Achaean Era and providing the “missing ingredient” of continental crust creation.
By testing the material and observing seismic waves travelling through the crust at speeds near those observed in continental crust worldwide, they found that the geochemical signature of erupted lavas first reached continental crust-like composition some 10 million years ago. Also, they conducted a survey of volcanoes from areas where two oceanic plates interact and found additional examples of where juvenile continental crust that had recently formed.
The research also raises questions about the global impact that this newly-formed continental crust has had over the years, as well as its potential role in the evolution of life. For instance, the authors said, the formation of a land bridge in Central America resulted in the seaway’s closure, which altered the circulation of the ocean, separated marine life, and changed the climate.
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“We’ve revealed a major unknown in the evolution of our planet,” said Gazel, the senior and corresponding author of the study.