Washington, Apr 1: Scientists have unveiled how continents were formed on Earth over 2.5 billion years ago - and how those processes have continued for the last 70 million years to profoundly affect the planet's life and climate.
A new study details how relatively recent geologic events - volcanic activity 10 million years ago in what is now Panama and Costa Rica - hold the secrets of the extreme continent-building that took place billions of years earlier.
"Without continental crust, the whole planet would be covered with water," said Esteban Gazel, an assistant professor of geology with Virginia Tech's College of Science.
"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," said Gazel.
The continental mass of the planet formed in the Archaean Eon, about 2.5 billion years ago. The Earth was three times hotter, volcanic activity was considerably higher, and life was probably very limited.
Many scientists think that all of the planet's continental crust was generated during this time in Earth's history, and the material continually recycles through collisions of tectonic plates on the outermost shell of the planet.
But the new research shows "juvenile" continental crust has been produced throughout Earth's history. "Whether the Earth has been recycling all of its continental crust has always been the big mystery," Gazel said.
"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," said Gazel.
The researchers used geochemical and geophysical data to reconstruct the evolution what is now Costa Rica and Panama, which was generated when two oceanic plates collided and melted iron- and magnesium-rich oceanic crust over the past 70 million years, Gazel said.
Melting of the oceanic crust originally produced what today are the Galapagos islands, reproducing Achaean-like conditions to provide the "missing ingredient" in the generation of continental crust.
The researchers discovered the geochemical signature of erupted lavas reached continental crust-like composition about 10 million years ago.
They tested the material and observed seismic waves travelling through the crust at velocities closer to the ones observed in continental crust worldwide.