Earth’s plate tectonics traced back to a “tipping point” 3.2 billion years ago. The Earth’s surface is constantly changing today, with tectonic plates shifting and moving, forming mountain ranges, ripping apart the seafloor, and causing massive earthquakes. Geophysicist Dave Stegman of the Scripps Institution of Oceanography in La Jolla and colleagues used observational data and computer models to develop a new formula for how the speeds of Earth’s plates depend on the size and edges of subduction zones. About 200 million years ago, the continent of Pangea broke apart into two huge pieces, with the supercontinent Laurasia to the north and Gondwana to the south.
A new study describes how the Earth’s surface moves, and the findings could potentially upend conventional wisdom about plate tectonics.
This new thinking, if correct, explains a long-standing mystery about how the planet’s crust moves and collides over time, causing volcanoes and earthquakes and changing the Earth’s surface. The idea could also explain why some of the rocky plates that make up Earth’s outer surface move faster than others.
Scientists have long believed that a slow ooze of hot, molten rock that lies beneath the Earth’s surface, in a region called the mantle, set the huge rocky slabs that make up the crust in motion. As the viscous mantle rises, it drags the plates along, pushing them together in some places and tearing them apart in others.
However, scientists could not explain why some parts of the earth’s crust move much faster than others.
The subduction of the Farallon Plate beneath the North American continent over 30 million years created the geological structure known as the Basin and Range Province, an area of the western United States that covers much of Nevada, shown here in a topographic model. Mike Sandiford/University of Melbourne
A new study suggests that these tectonic plates control their own speed more than the mantle does. The results, detailed in the July 16, 2022, issue of the journal Science, provide a new explanation for why some plates are fast and others are slow, relatively speaking, which will help scientists piece together how Earth’s surface evolved into its current shape.
“The Earth’s surface is covered with tectonic plates that move relative to each other at speeds of centimeters per year,” said geologist and study co-author Wouter Schelaert of Monash University in Melbourne, Australia. “These plates meet at deep-sea trenches, plate boundaries, where one plate is subducted under another in so-called subduction zones. The speeds of these plates and the speeds of the boundaries between these plates vary greatly across the Earth.”
Geophysicist and co-author Dave Stegman of the Scripps Institution of Oceanography in La Jolla, California, Schellart and their colleagues used observational data and computer models to develop a new formula that describes how Earth’s plate velocities depend on the size and edges of subduction zones.
“Computer models show that the subducted portion of the tectonic plate pulls on the portion of the plate that remains on the Earth’s surface,” Stegman said. “This pull results in either plate motion or plate boundary motion, with the size of the subduction zone determining how much of each.”
The discovery explains why the Australian, Nazca and Pacific plates are moving four times faster than the smaller African, Eurasian and Juan de Fuca plates.
The study also helps explain why western North America looks the way it does today. Around the time the supercontinent Pangea began to break apart, a tectonic plate called the Farallon Plate sank into the mantle beneath the west coast of North and South America, slowly changing the landscape.
supercontinent Pangea
“This plate has slowed as it moves east from about 4 inches (10 centimeters) per year about 50 million years ago to just 0.8 inches (2 centimeters) per year today,” Schellart said.
The plate’s movement slowed as the subduction zone it was sinking into shrank from 8,700 miles (14,000 kilometers) wide to just 870 miles (1,400 kilometers).
“This had a dramatic impact on the topography and structure of the North American continent,” Schellart said. “Until 50 million years ago, the west coast of North America was characterized by a massive mountain range, similar to the modern Andes in South America, stretching from Canada in the north to southern Mexico in the south.”
As the subduction zone shrank, tension along the west coast of North America decreased, causing the giant mountain range to collapse. This created the Basin and Range Province, a 772,000-square-mile (2 million square-kilometer) region of elongated basins and ranges that characterizes the modern landscape of western North America.
Earth’s plate tectonics traced back to ‘tipping point’ 3.2 billion years ago. Researchers analysing ancient sediments in Australia have found evidence that Earth’s layers began to mix, a sign of plate tectonics, about 1.3 billion years after the planet formed.
Now, a new study adds to a growing body of evidence that this dynamic began 3.2 billion years ago. While there is disagreement in the geoscience community about exactly when Earth became more than just a blob of hot, undifferentiated rock, the new study suggests that the transition occurred about 1.3 billion years after the planet formed.
In 2020, Zheng Xiang Li, a geodynamicist at Curtin University in Australia, and colleagues reported a shift in the chemistry of rocks that formed in the mantle about 3.2 billion years ago, hinting that a “mixing” process had taken place. This process would have involved the movement of minerals from the crust into the mantle and the movement of mantle rocks to the surface — fingerprints of plate tectonics. Other researchers have also seen evidence of a shift during this same time period; for example, a 2020 study in the journal Science Advances found magnetic evidence of large-scale plate movement 3.2 billion years ago.
But there is still debate about when and how these processes began. In the new study, Lee and Luc Doucet, a geochemist at Curtin University, and their colleagues focused on large lead and zinc deposits in Australia. The scientists used the ratios of molecular variations in uranium, thorium, and lead as a clock to measure events that happened deep in Earth’s history.
Traces of Earth’s plate tectonics found in 3.2 billion-year-old sediments. visdia/Getty Images
Study co-author Denis Fougerouse of Curtin University said in a statement that the age of the sediments in Australia ranges from 3.4 billion to 285 million years.
The new analysis again pointed to 3.2 billion years as a turning point, Li said. Before then, Earth had differentiated into the “layer cake” structure of core, mantle and crust that we see today. That layering was driven by gravity, with heavier elements sinking into the core and lighter elements rising into the crust, Li said.
However, 3.2 billion years ago, these layers began to mix: plate tectonics pushed layers of the Earth’s crust back into the mantle, and forces such as volcanism brought elements of the mantle to the surface.
The researchers also found that the onset of this process was complex and not necessarily consistent across the planet. The new findings, published in the August issue of Earth-Science Reviews, suggest that researchers need to recalibrate the uranium-thorium-lead dating system to capture these nuances.
“If we’re not careful about this, we could have tens of millions or hundreds of millions of years of error in our age estimates,” Li said.
The researchers are now using computer modeling to understand how plate tectonics likely began 3.2 billion years ago. The planet’s cooling from a magma ocean to something more temperate and solid may have played a major role. “Our ultimate goal is to document how the entire Earth evolved from the early red ball through plate tectonics to the green marble we have today,” Li said.
Melting glaciers 10,000 years ago accelerated continental drift and volcanic activity, a process that could be repeated as modern ice sheets melt. About 10,000 years ago, as the last ice age ended, the movement of the North American continent and the widening of the Atlantic Ocean may have temporarily accelerated. Melting glaciers may have played a key role in this process, according to a new study by researchers at the University of Colorado Boulder. In the study, the geophysicists used computer models to simulate conditions on Earth up to 26,000 years ago. During this period, the massive ice sheet that covered much of North America as far south as modern-day Pennsylvania began to retreat. As the ice melted, vast volumes of water entered the oceans, causing sea levels to rise by about 1 cm per year.
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Scientists have found that this global melting may also have had unexpected consequences, including on plate tectonics, or the internal clockwork that has been tearing Earth’s continents apart and squeezing them back together for billions of years.
The team calculates that the movement of the North American continental plate may have accelerated by 25% as the ice melted. Between about 12,000 and 6,000 years ago, the area of the Mid-Atlantic Ridge, located between the North American and Eurasian plates, may have increased by as much as 40%.
Today, the ice over Greenland is not melting fast enough to have a significant impact on continental drift around the world. But it could still have a major impact on Iceland over the next few hundred years, especially if the glaciers begin to disappear at an accelerated rate.
Antarctica, the fifth largest continent, wasn’t always so isolated; it was once part of a larger supercontinent. So when did it become its own continent?
Today, Antarctica is the largest piece of ice on Earth, covering more than 5.4 million square miles (14 million square kilometers). Hidden beneath the ice, Antarctica’s rocks reveal the continent’s dynamic history.
Many of the rocks that outcrop in Antarctica are part of the Transantarctic Mountains, which reach heights of about 14,700 feet (4,500 meters), or higher than the Rocky Mountains in North America. The vast continent is divided into two halves: East and West. From what geologists can piece together, East Antarctica is a craton, an ancient continental block of the Earth’s crust and uppermost mantle made up of igneous, sedimentary, and metamorphic rocks, some more than 3 billion years old, Ives said. In contrast, West Antarctica is relatively young, made mostly of volcanic rock forged in the tectonically active Ring of Fire around the time the supercontinent Gondwana began to break apart during the Jurassic Period (201.3 million to 145 million years ago), according to the latest findings from Live Science.
Gondwana has ancient roots: It formed about 600 million years ago during the late Ediacaran period, before Pangea existed. Beginning about 200 million years ago, Pangea broke into two huge pieces, with the supercontinent Laurasia to the north and Gondwana to the south. About 180 million years ago, Gondwana, which included parts of modern-day Antarctica, Africa, Australia, India, and South America, began to break up into the continental fragments we’re more familiar with today. Basaltic rocks found on Antarctica’s eastern edge match those found in South Africa, representing early rifts in Gondwana, according to Discover Antarctica.
Discover Antarctica
Antarctica was warmer during the Mesozoic era (252 million to 66 million years ago) than it is today, a 2006 modeling study found, and in the Cretaceous period (145 million to 66 million years ago) it was a temperate rainforest teeming with dinosaurs and other ancient life forms. In fact, it served as a key southern passage; for tens of millions of years, South America, Antarctica, and Australia remained connected, allowing flora and fauna to move across their vast expanses. For example, fossil evidence shows that marsupials, which originated in North America at least 125 million years ago, moved south to South America and east across Antarctica before eventually arriving in Australia at least 55 million years ago.
Scientists aren’t sure when Antarctica officially became isolated, losing its land connections to Australia and South America. However, recent research suggests that the Drake Passage between Antarctica and South America and the Tasman Gate between Antarctica and Australia opened just as the Eocene epoch gave way to the Oligocene epoch, about 34 million years ago, “within a few million years,” Lamanna said.
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After the final break, Australia moved north while Antarctica began to drift south. When the Drake Passage and Tasmanian Gate opened between the continents, they allowed cold water to flow continuously around Antarctica, isolating the continent from warm ocean currents. At the bottom of the planet, Antarctica began to freeze.
The opening of the Drake Passage and the Tasmanian Gateway allowed the Antarctic Circumpolar Current to fully form. These were the final “events” in the breakup of Gondwana.
Today, Antarctica plays an integral role in the Earth’s climate system. The massive ice sheets that cover the continent reflect incoming sunlight, keeping the continent cool. As the planet warms due to climate change, the ice sheets will melt, exposing more rocks for study, but also reflecting less incoming sunlight and perpetuating planetary warming.
Ice sheets on land also extend into adjacent waters, such as the Southern Ocean, Weddell Sea, and Ross Sea, forming ice shelves that break up from time to time. As more ice is released, it melts, adding vast amounts of fresh water to the Earth’s oceans. Today, scientists are actively studying ice in Antarctica and sediment in adjacent seas and oceans to understand past climate fluctuations and explain how climate change may affect the entire planet.
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