Plate tectonics may not be a permanent feature of Earth. The process that forms mountains, sparks earthquakes and drives the planet’s continents to oh-so-slowly rearrange themselves could end billions of years in the future, new simulations suggest.
“We’ve known for a while that plate tectonics is only one of a spectrum of tectonic states that a planet could be in,” says Craig O'Neill, a planetary scientist at Macquarie University in Australia.
Planets like Mars and Mercury are in what’s called a stagnant lid state. The outermost shell, called the lithosphere, of these planets is too thick for the planet’s interior to break up and produce tectonic activity. Scientists had assumed that eventually Earth would reach a similar state, but evidence has been lacking, O’Neill says. “We just don’t have enough planets to be able to draw any real conclusions from.”
So O’Neill and his colleagues set out to model the evolution of the Earth and see what the future might hold for our planet. But even with modern supercomputers, there isn’t enough computing power to simulate the entire three-dimensional Earth over its whole history. Instead, the group built a simplified, two-dimensional simulation of Earth that models the planet’s evolution from its formation 4.5 billion years ago to more than 5 billion years into the future. Even then, a single run took 3 weeks, O’Neill notes.
The simplified model let the team try out different starting points for early Earth’s temperature, a variable that is currently unknown because we don't have any rocks from the first 500 million years of the planet’s history. “One of the great weaknesses in [our] understanding of Earth’s evolution at this point is we don’t know how it actually began,” O’Neill says.
Scientists used to assume that the process of accretion—when little bits of the early solar system glommed together to form a planet—was a fairly cool process, and that planets only heated up later as radioactive elements in the interior decayed.
“These days, we think there was quite a lot of energy brought in during the accretion process,” he says. “You’ve got a lot of big bodies smashing into each other. They generate a lot of heat through impacting.” And short-lived radioactive elements, such as aluminum-26 and iron-60, both of which can no longer be found in the solar system, may have heated things up further.
The team found that the starting state for the planet may dramatically affect its life cycle. When the planet in the model started off cooler, it quickly developed plate tectonics, losing the feature after only 10 to 15 billion years.
But a hotter Earth, which O’Neill thinks is more likely, results in a planet that is slow to develop plate tectonics. It starts off in a state similar to Jupiter’s moon Io, which is covered in active volcanoes but has no tectonic plates. The model then shows a planet on which plate tectonics switches on and off for 1 to 3 billion years. (This is a time period for our planet for which the geological record is spotty, and some geologists, including O’Neill, have concluded that there is a strong case for interspersed tectonics during this time. “It’s worth noting that that’s not entirely agreed upon,” he says.)
The simulations show an Earth that then eventually settles down into billions of years of plate tectonics before finally cooling off enough for that to end—in another 5 billion years or so. “At some point,” O’Neill says, “the Earth is going to slow down and that lithosphere is going to get thicker and thicker to the point where it’s too strong and too thick for the interior to be able to break it anymore.”
The researchers report their findings in the June issue of Physics of the Earth and Planetary Interiors.
Rocks “are the best things we have to rely on to tell us about the past,” says Bradford Foley, a geodynamicist at the Carnegie Institution of Washington. And without them, scientists have to rely on theoretical models. But there are a lot of uncertainties that get incorporated into them, Foley notes. For instance, O’Neill’s team could have gotten different results if they had used different formulas that describe the ways rocks form. None of the models being developed today to describe the evolution of the planet are anywhere close to definitive, Foley says.
But such models can help to explore what might have happened on Earth, as well as on other planets in the universe. Plate tectonics are important for Earth’s carbon cycle and help to regulate the amount of carbon dioxide in the atmosphere. “This cycle helps to keep Earth’s climate stabilized in a nice temperate range,” notes Foley. This is one of the reasons that scientists once assumed that a planet without plate tectonics couldn’t host life, or at least complex life.
Other factors, such as liquid water and the composition of an exoplanet’s atmosphere, may also play into a planet’s habitability, O’Neill notes. So it may be possible to find life somewhere in the universe on a planet that isn’t moving and shaking like Earth.