What’s pushing North and South America further apart from Europe and Africa? Study identifies geological event
Tectonic plates form deep beneath the oceans, making it very difficult for scientists to study their formation and evolution. In 2016, a research team went on a mission to the Mid-Atlantic Ridge to explore the mysteries of plate tectonics. The mission, nicknamed PI-LAB, was to scan deep beneath the ridge using seismic imaging techniques. The analysis reveals that a geological phenomenon is widening the Atlantic Ocean. The seismologists found that an upsurge of matter from deep beneath the Earth’s crust could be pushing the continents of North and South America further apart from Europe and Africa.
The plates attached to the Americas are moving apart from those attached to Europe and Africa by four centimeters per year. The findings, published in the journal Nature, provide a greater understanding of plate tectonics which causes many natural disasters around the world, including earthquakes, tsunamis, and volcanic eruptions.
“This was a memorable mission that took us a total of 10 weeks at sea in the middle of the Atlantic Ocean. The incredible results shed new light in our understanding of how the Earth interior is connected with plate tectonics, with observations not seen before,” says lead author, Matthew Agius, a former post-doctoral fellow at the University of Southampton and currently at Università degli studi Roma Tre.
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In between the continents lies the Mid-Atlantic Ridge, a site where new plates are formed and a dividing line between plates moving to the west (the North American and South American plates) and those moving to the east (the Eurasian and African plates). Material rises from beneath this ridge to replace the space left by the plates as they move apart, explain scientists.
This process is normally driven by distant gravity forces as denser parts of the plates sink back into the Earth. However, the driving force behind the separation of the Atlantic plates has remained a mystery because the Atlantic ocean is not surrounded by dense, sinking plates.
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The team, led by the University of Southampton, now suggest an upwelling in the mantle – the material between the Earth’s crust and its core – from depths of more than 600 kilometers beneath the Mid-Atlantic Ridge, which they say could be pushing the plates from below, causing the continents to move further apart. Upwellings beneath ridges are typically thought to originate from much shallower depths of around 60 km.
“Upwelling is a process in which deep, cold water rises toward the surface. Winds blowing across the ocean surface push water away. Water then rises up from beneath the surface to replace the water that was pushed away. This process is known as upwelling,” describes the NOAA.
According to Professor Mike Kendall from the University of Oxford, the work refutes long-held assumptions that mid-ocean ridges might play a passive role in plate tectonics. “It suggests that in places such as the Mid-Atlantic, forces at the ridge play an important role in driving newly-formed plates apart,” adds Kendall. He along with Dr Kate Rychert and Dr Nick Harmon from the University of Southampton led the experiment.
The mission
For the study, 39 seismometers were deployed at the bottom of the Atlantic over two research cruises on the RV Langseth and RRV Discovery, as part of the PI-LAB (Passive Imaging of the Lithosphere-Asthenosphere Boundary) experiment and EURO-LAB (Experiment to Unearth the Rheological Oceanic Lithosphere-Asthenosphere Boundary).
Spanning across 1,000 km, the seismometers were left operating for a year to record earthquakes from around the world. In total, the authors stayed 10 weeks at sea, deploying and recovering the stations.
Strong earthquakes from around the world were recorded on the PI-LAB seismic network. Seismic waves traveled through the deep Earth before reaching the stations. The data thus provided large-scale and high-resolution imaging of the mantle beneath the Mid-Atlantic Ridge. This high-quality seismic data were analyzed to reveal Earth’s hidden mysteries.
The seismic waves were used to image the different layers inside the Earth, particularly, the discontinuities at 410 km and 660 km depth between the upper and lower mantle known as the “mantle transition zone.”
“This is one of only a few experiments of this scale ever conducted in the oceans and allowed the team to image variations in the structure of the Earth’s mantle near depths of 410 km and 660 km – depths that are associated with abrupt changes in mineral phases,” according to the authors.
The observed signal was “indicative of a deep, sluggish and unexpected upwelling from the deeper mantle.” The observations imply material transfer between the lower and upper mantle that is linked to the ridge above. This indicates that “whole mantle convection” may be more prevalent than previously thought and could play a role in driving plate tectonics, explain researchers.
“The thinner than average mantle transition zone suggests anomalous high temperatures that facilitate material transfer from the lower to the upper mantle that may play a role in driving plate tectonics,” the findings state.
Dr Harmon notes that there is a growing distance between North America and Europe, and “it is not driven by political or philosophical differences - it is caused by mantle convection.”
Besides helping scientists to develop better models and warning systems for natural disasters, plate tectonics also has an impact on sea levels, and therefore, affects climate change estimates over geologic times scales. “This was completely unexpected. It has broad implications for our understanding of Earth’s evolution and habitability. It also demonstrates how crucial it is to gather new data from the oceans. There is so much more to explore,” emphasizes Dr Rychert.