At three moments in the past, Earth鈥檚 geological activity picked up the pace. Its tectonic plates moved 30 to 50 per cent faster than normal, and there were bursts of volcanic activity and mountain building that helped create supercontinents. These three geological big bangs may have played an important role in the evolution of life on Earth.
Kent Condie at the New Mexico Institute of Mining and Technology and his colleagues want to understand how Earth鈥檚 gigantic tectonic plates behaved in the past. In 2014, they suggested that the plates are moving faster now than they were a billion or so years ago. That conclusion came from a preliminary analysis of several types of data, including magnetic signals locked in ancient rock that give a sense of where on Earth鈥檚 surface the plates were at particular time point in the past 鈥� information that can help establish how fast the plates must have been moving at the time.
Now, Condie and his colleagues have analysed more data. They say there is no longer a signal suggesting the plates have accelerated through time. But in its place is another, even more intriguing pattern. At three points in Earth鈥檚 distant past 鈥� 600, 1100 and 1850 million years ago 鈥� the tectonic plates sped up for a few tens of millions of years, so the global average speed was between 30 and 50 per cent faster than normal.
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At the same time, there were peaks in volcanic activity and mountain building, heralding the formation of supercontinents 鈥� Nuna at about 1850 million years, Rodinia some 1100 million years ago and the coalescence of landmasses some 600 million years ago that would lead to the formation of Pangaea.
鈥淪omething dramatic was happening at those periods of time that was affecting all of these systems,鈥� says Condie. 鈥淪omething in the mantle.鈥�
The researchers think they are seeing evidence of a vast geological cycle that begins with the death of a supercontinent. As it breaks up, some slabs of Earth鈥檚 crust sink down into the mantle 鈥� the thick layer of hot rock between the crust and the core. Condie says the slabs take some 100 to 200 million years to drift down to the bottom reaches of the mantle. When they arrive there, the temperatures and pressures turn the former crust into a plume of abnormally hot rock, which gradually rises back to the surface.
鈥淲hen that hot plume hits the bottom of the tectonic plates, it increases plate speed, which in turn increases orogenic [mountain building] activity,鈥� says Condie, who presented the work at an .
鈥淭here is an ongoing debate about whether the Earth鈥檚 crust or the underlying mantle controls the speed of tectonic plates through time,鈥� says Hugo Olierook at Curtin University, Australia. 鈥淭he notion that the deep mantle is controlling the speed of Earth鈥檚 tectonic plates just before supercontinent assembly is fascinating.鈥�
Earlier this year, Olierook and his colleagues presented evidence that, 110 million years ago during the Cretaceous period, , which they also linked to the return of crust to the mantle.
鈥淪ometimes, the reconfiguration of tectonic plates can lead them to jam, which slows them down for several million to tens of millions of years until they can adapt to the new regime,鈥� says Olierook.
鈥淚 think the Cretaceous change is something different and not related to these three big bangs,鈥� says Condie, although it may take years before we fully understand why the plates suddenly speed up or slow down.
Whatever their cause, the three big bangs may have affected life. Condie says it might be no coincidence that two of the three roughly coincide with eventful episodes in life鈥檚 history 鈥� the and the appearance of animals about 600 million years ago.
鈥淧lume events pump carbon dioxide into the atmosphere,鈥� says Condie. 鈥淚 think there would be an effect on life.鈥�
