Earth's Moving Crust May Occasionally StopJanuary 10, 2008
The motion, formation, and recycling of Earth's crust-commonly known as plate tectonics-have long been thought to be continuous processes. But new research by geophysicists suggests that plate tectonic motions have occasionally stopped in Earth's geologic history, and may do so again. The findings could reshape our understanding of the history and evolution of the Earth's crust and continents.
Synthesizing a wide range of observations and constructing a new theoretical model, researchers Paul Silver of the Carnegie Institution of Washington and Mark Behn of the Woods Hole Oceanographic Institution (WHOI) have found evidence that the process of subduction has effectively stopped at least once in Earth's past. Subduction occurs where two pieces of Earth's crust (tectonic plates) collide, and one dives beneath the other back into the interior of the planet.
Most of the major geologic processes on Earth-the formation of continents, the birth of volcanic island arcs, the opening and closing of ocean basins-are driven by tectonic plate motions and intimately linked to subduction and to seafloor spreading. If those processes were shut down, there would likely be a global decrease in earthquakes and volcanism.
Today, the vast majority of subduction occurs around the edges of the Pacific Ocean, which is slowly closing as the Atlantic Ocean opens. In roughly 350 million years, researchers estimate that the Pacific basin will be effectively closed and a new supercontinent will be formed.
Closure of the Pacific basin could shut down most of the Earth's capacity for subduction, unless the process begins somewhere else on the planet. However, there is no evidence that subduction is currently expanding or initiating anywhere else on the planet.
Though such a shutdown defies the prevailing wisdom about plate tectonics, Silver and Behn read the geologic evidence to suggest that just such a dramatic decrease in subduction happened about one billion years ago, after the formation of the supercontinent Rodinia.
Their findings-captured in a paper entitled "Intermittent Plate Tectonics?"-were published in the January 4 issue of the journal Science.
"The scientific community has typically assumed that plate tectonics is an active and continuous process, that new crust is constantly being formed while old crust is recycled," said Behn, an assistant scientist in the WHOI Department of Geology and Geophysics. "But the evidence suggests that plate tectonics may not be continuous. Plates may move actively at times, then stop or slow down, and then start up again."
Behn and Silver started their investigation by considering how the Earth releases heat from its interior over time, also known as "thermal evolution." If you take the rate at which the Earth is releasing heat from its interior today and project that rate backwards in time, you arrive at impossibly high and unsustainable numbers for the heat and energy contained in the early Earth. Specifically, if the planet has been releasing heat at the modern rate for all of its history, then it would have been covered with a magma ocean as recently as one billion years ago.
But we know this is not true, Behn said, because there is geological evidence for past continents and supercontinents, not to mention rocks (ophiolites) on the edges of old plate boundaries that are more than one billion years old.
The Earth cools more quickly during periods of rapid plate motions, as warm material is pulled upward from deep in the Earth's interior and cools beneath spreading ridges.
"If you stir a cup of coffee, it cools faster," said Behn. "That's why people blow on their coffee to get the surface moving."
"It is a similar process within the Earth," Behn added. "If the tectonic plates are moving, the Earth releases more heat and cools down faster. If you don't have those cracked and moving plates, then heat has to get out by diffusing through the solid rock, which is much slower."
Periods of slow or no subduction would help explain how the Earth still has so much heat to release today, since some of it would have been capped beneath the crust.
Silver and Behn conclude their paper by suggesting that there is a cycle to plate tectonics, with periods when the shifting and sliding of the crust is more active and times when it is less so. Rather than being continuous, plate tectonics may work intermittently through Earth history, turning on and off as the planet remakes itself.
The Woods Hole Oceanographic Institution is a private, independent organization in Falmouth, Mass., dedicated to marine research, engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the oceans and their interaction with the Earth as a whole, and to communicate a basic understanding of the ocean's role in the changing global environment.
The Carnegie Institution of Washington has been a pioneering force in basic scientific research since 1902. It is a private, nonprofit organization with six research departments throughout the U.S. Carnegie scientists are leaders in plant biology, developmental biology, astronomy, materials science, global ecology, and Earth and planetary science.
Woods Hole Oceanographic Institution
Related Subduction Current Events and Subduction News Articles
Diamonds used to 'probe' ancient Earth
Diamonds dug up from ancient rock formations in the Johannesburg area, between 1890 and 1930 - before the industrialisation of gold mining - have revealed secrets of how the Earth worked more than 3.5 billion years ago.
Reptile fossils offer clues about elevation history of Andes Mountains
On an arid plateau in the Andes Mountains of southern Bolivia, a Case Western Reserve University researcher flagged what turned out to be the fossil remains of a tortoise nearly five feet long -- a find indicating this highland was likely less than a kilometer above sea level 13 million years ago.
Age of blueschist is not an indicator of the date of emergence of plate tectonics
One of the big mysteries in the history of the Earth is the emergence of plate tectonics. When exactly did the processes of plate tectonics begin that today involve the subduction of oceanic plates? Scientific opinion varies widely as to this.
Minerals from Papua New Guinea hold secret for recycling of noble gases
With every breath we take, we inhale not only oxygen, but also a mix of gases. This mixture includes carbon dioxide and nitrogen, but also a gas called argon. Neon, the gas that illuminates the signs of all-night diners, is also in the mix.
How the Earth's Pacific plates collapsed
Scientists drilling into the ocean floor have for the first time found out what happens when one tectonic plate first gets pushed under another.
When did the Andes mountains form?
The Andes have been a mountain chain for much longer than previously thought, new research from the University of Bristol, UK suggests.
Discovery of hidden earthquake presents challenge to earthquake early-warning systems
Seismologists at the University of Liverpool studying the 2011 Chile earthquake have discovered a previously undetected earthquake which took place seconds after the initial rupture.
How did plate tectonics start on Earth?
Our planet Earth is the only planet in the Solar System that possesses Plate Tectonics. The Earth's surface is in a constant state of change; the tectonic plates together with the oceans and continents continuously slide along one another, collide or sink into the Earth's mantle.
Plate tectonics thanks to plumes?
"Knowing what a chicken looks like and what all the chickens before it looked like doesn't help us to understand the egg," says Taras Gerya.
Scientists map source of Northwest's next big quake
A large team of scientists has nearly completed the first map of the mantle under the tectonic plate that is colliding with the Pacific Northwest and putting Seattle, Portland and Vancouver at risk of the largest earthquakes and tsunamis in the world.
More Subduction Current Events and Subduction News Articles