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| Esteban Gazel, a geoscientist at Virginia Tech, collects samples of lava in a variety of locations, in this case Etna, Italy, to probe the chemical evolution of the planet. |
An international research team,
led by a Virginia Tech geoscientist, has revealed information about how
continents were generated on Earth more than 2.5 billion years ago -- and how
those processes have continued within the last 70 million years to profoundly
affect the planet's life and climate.
Published
online today in Nature Geoscience, the 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.
The
discovery provides new understanding about the formation of the Earth's
continental crust -- masses of buoyant rock rich with silica, a compound that
combines silicon and oxygen.
"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."
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."
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 traveling through the crust at velocities
closer to the ones observed in continental crust worldwide.
Additionally,
the researchers provided a global survey of volcanoes from oceanic arcs, where
two oceanic plates interact. The western Aleutian Islands and the Iwo-Jima
segment of the Izu-Bonin islands of are some other examples of juvenile
continental crust that has formed recently, the researchers said.
"This
is an interesting paper that makes the case that andesitic melts inferred to
derive ultimately by melting of subducted slabs in some modern arcs are a good
match for the composition of the average continental crust," said Roberta
L. Rudnick, a Distinguished University Professor and chair of the Department of
Geology at the University of Maryland, who was not involved in conducting the
research. "The authors focus primarily on Central America, but incorporate
global data to strengthen their case that slab melting is important in unusual
conditions of modern continent generation -- and probably in the past."
The study
raises questions about the global impact newly generated continental crust has
had over the ages, and the role it has played in the evolution of not just
continents, but life itself.
For
example, the formation of the Central American land bridge resulted in the
closure of the seaway, which changed how the ocean circulated, separated marine
species, and had a powerful impact on the climate on the planet.
"We've
revealed a major unknown in the evolution of our planet," said Gazel, who
was the senior and corresponding author of the study.
Story
Source:
The above
story is based on materials provided by Virginia
Tech. Note: Materials may be edited for content and length.
Journal
Reference:
- Esteban Gazel, Jorden L.
Hayes, Kaj Hoernle, Peter Kelemen, Erik Everson, W. Steven Holbrook,
Folkmar Hauff, Paul van den Bogaard, Eric A. Vance, Shuyu Chu, Andrew J.
Calvert, Michael J. Carr, Gene M. Yogodzinski. Continental crust
generated in oceanic arcs. Nature Geoscience, 2015; 8 (4): 321
DOI: 10.1038/ngeo2392
Note: This journal is an extract from Science Daily. Click Here to visit link
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