Earth’s Moving Crust May Occasionally Stop





When an ocean plate collides with another ocean plate or with a plate carrying continents, one plate will bend and slide under the other. This process is called subduction. As the subducting plate plunges deep into the mantle, it gets so hot it melts the surrounding rock. The molten rock rises through the crust and erupts at the surface of the overriding plate. (Woods Hole Oceanographic Institution)
When an ocean plate collides with another ocean plate or with a plate carrying continents, one plate will bend and slide under the other. This process is called subduction. As the subducting plate plunges deep into the mantle, it gets so hot it melts the surrounding rock. The molten rock rises through the crust and erupts at the surface of the overriding plate. (Woods Hole Oceanographic Institution)

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.

Plate Tectonics May Grind To A Halt, Then Start Again





Pacific Ocean off California. Most of today's subduction zones are located in the Pacific Ocean basin. If the Pacific basin were to close then most of the planet's subduction zones would disappear with it. (Credit: Michele Hogan)
Pacific Ocean off California. Most of today’s subduction zones are located in the Pacific Ocean basin. If the Pacific basin were to close then most of the planet’s subduction zones would disappear with it. (Credit: Michele Hogan)

Plate tectonics, the geologic process responsible for creating the Earth’s continents, mountain ranges, and ocean basins, may be an on-again, off-again affair. Scientists have assumed that the shifting of crustal plates has been slow but continuous over most of the Earth’s history, but a new study from researchers at the Carnegie Institution suggests that plate tectonics may have ground to a halt at least once in our planet’s history–and may do so again.



A key aspect of plate tectonic theory is that on geologic time scales ocean basins are transient features, opening and closing as plates shift. Basins are consumed by a process called subduction, where oceanic plates descend into the Earth’s mantle. Subduction zones are the sites of oceanic trenches, high earthquake activity, and most of the world’s major volcanoes.



Writing in the January 4 issue of Science, Paul Silver of the Carnegie Institution’s Department of Terrestrial Magnetism and former postdoctoral fellow Mark Behn (now at Woods Hole Oceanographic Institution) point out that most of today’s subduction zones are located in the Pacific Ocean basin. If the Pacific basin were to close, as it is predicted to do about in 350 million years when the westward-moving Americas collide with Eurasia, then most of the planet’s subduction zones would disappear with it.


This would effectively stop plate tectonics unless new subduction zones start up, but subduction initiation is poorly understood. “The collision of India and Africa with Eurasia between 30 and 50 million years ago closed an ocean basin known as Tethys,” says Silver. “But no new subduction zones have initiated south of either India or Africa to compensate for the loss of subduction by this ocean closure.”



Silver and Behn also present geochemical evidence from ancient igneous rocks indicating that around one billion years ago there was a lull in the type of volcanic activity normally associated with subduction. This idea fits with other geologic evidence for the closure of a Pacific-type ocean basin at that time, welding the continents into a single “supercontinent” (known to geologists as Rodinia) and possibly snuffing out subduction for a while. Rodinia eventually split apart when subduction and plate tectonics resumed.



Plate tectonics is driven by heat flowing from the Earth’s interior, and a stoppage would slow the rate of the Earth’s cooling, just as clamping a lid on a soup pot would slow the soup’s cooling. By periodically clamping the lid on heat flow, intermittent plate tectonics may explain why the Earth has lost heat slower than current models predict. And the buildup of heat beneath stagnant plates may explain the occurrence of certain igneous rocks in the middle of continents away from their normal locations in subduction zones.



“If plate tectonics indeed starts and stops, then continental evolution must be viewed in an entirely new light, since it dramatically broadens the range of possible evolutionary scenarios,” says Silver.

Geologist Discovers Three New Minerals


On a geological expedition along the windswept slopes of the Larsemann Hills in Antarctica, UMaine geologist Edward Grew collected samples of the area’s unique rock formations that would later reveal three minerals previously unknown to science. The minerals, stornesite-(Y), chopinite and tassieite, are extremely rare and represented only by microscopic samples collected by Grew.



The unique mineralogy of the Larsemann Hills, located on the eastern shore of Prydz Bay in Princess Elizabeth Land, inspired Grew and his fellow researcher Chris Carson (now at Geoscience Australia) to make the four-month expedition in 2003 – 2004, which was funded by the National Science Foundation and made possible by the Australian Antarctic Division.


Grew and his colleagues identified and characterized the minerals using cutting edge technologies. Martin Yates used the powerful electron microprobe at UMaine to image the new minerals and measure their chemical compositions. Next, the minerals were sent to Olaf Medenbach at the Ruhr University (Bochum, Germany) and Thomas Armbruster at the University of Bern (Switzerland), who determined the new minerals’ optical properties and crystal structures, respectively. Then Grew submitted a complete dataset for each mineral to a special commission of the International Mineralogical Association, which formally approved them as valid new species. Grew has discovered a total of ten new minerals, and sees each as an opportunity to expand scientific understanding of the Earth and its complex geological processes.



“When new minerals are identified, some have little significance, and some end up being tremendously important,” said Grew. “They all tell us something about how rocks form. Ultimately, discoveries like these contribute to our understanding of the origin of rocks, plate tectonics and other processes, and give us valuable insights into temperature, pressure and other conditions within the Earth at different points of its history.”