Time and tide may wait for no man, but continents occasionally do. That's the conclusion of a study published today in Science, which finds that the inexorable drift of Earth's tectonic plates isn't inexorable at all. In fact, the planet could be headed for another pause in continental drift, with uncertain and possibly ominous consequences.
Nearly a century ago, German geophysicist Alfred Wegener noticed that the continents resemble puzzle pieces that once could have fit together. He proposed that the continents actually had drifted apart, in tiny but measurable annual increments, over hundreds of millions of years. After some initial disagreement within the scientific community, most research since has supported Wegener's hypothesis, eventually establishing the primary driver of plate tectonics as rising heat from deep inside Earth's interior. That heat, called thermal upwelling, reaches the surface in the form of eruptions of magma that force apart the continents and form new oceans.
That's what continues to happen today along the Mid-Atlantic Ridge, a 15,000-kilometer-long undersea mountain range that began forming about 200 million years ago and eventually pushed the Americas away from Europe and Africa. On the other side of the globe, that process is causing the Pacific Ocean to shrink and gives rise to very active earthquake zones and volcanism as the seabed slips under eastern Asia and the western Americas.
Now two researchers from the Carnegie Institution of Washington in Washington, D.C., have found evidence that the continents weren't always in motion. During at least one period, the scientists have concluded, plate tectonics might have stopped entirely. In what began as "a thought experiment," says geophysicist and lead author Paul Silver, he and postdoctoral fellow Mark Behn of Woods Hole Oceanographic Institution in Massachusetts tried to imagine what would happen if the process of subduction--when one plate slips underneath another--stops. They found evidence of such an event by examining the changing ratios of the elements niobium and thorium and of two isotopes of helium in ancient rocks. The elements serve as reliable markers of how rapidly Earth's internal temperature has cooled over the eons, and the team's analyses suggest that a slowdown in cooling means a corresponding slowdown or stopping of plate tectonics. Silver and Behn conclude that subduction virtually ceased about 1.5 billion years ago.
What's more, the researchers predict that the planet could undergo a similar event in about 350 million years, when the Pacific Ocean Basin closes as the Americas link up with Asia and Australia. That could be of more than paleontological interest, Silver says, because the loss of subduction and its associated volcanic activity would stop the cooling of the atmosphere by the dust from periodic eruptions--the planet's natural antidote to global warming. Hence, Silver says, the development "could have a dramatic impact on climate and thus surface life."
The paper sheds new light on an important but little-discussed aspect of tectonics and "will have a permanent impact on our way of interpreting observations [of tectonic history]," says geophysicist Rainer Kind of GeoForschungsZentrum, an earth science institute in Potsdam, Germany. Geophysicist Derrick Hasterok of the University of Utah, Salt Lake City, agrees. The study presents "an elegant description of how variable plate tectonics has been throughout Earth's history," he says, and the findings could even help explain some mysterious sea-level variations over the last 4 billion years.
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