The oldest crust on Earth, known to be unchanging, is actually being altered in real time. The North American continent is “dripping” rock into the lower layers of the Earth, new research says, and in the process providing new information about how continents and landmasses evolve.
Between a rock and a hard place
The Earth’s continents contain cratons, which are ancient and stable pieces of the crust that very rarely change and deform. Any changes to cratons happen over a long geologic timescale. But a paper published in the journal Nature Geoscience found that the underside of the North American continent is “dripping” as we speak. Underneath the U.S. Midwest, the continent is losing blobs of rock into the Earth’s mantle.
Scientists had posited that there was something beneath the North American craton causing the drips, and indeed that something turned out to be a “chunk of oceanic crust that broke off from an ancient tectonic plate called the Farallon plate,” said Live Science. The slab of rock is applying a downward dragging force on the continent and is “both redirecting the flow of material in the mantle such that it is wearing away the bottom of the North American craton” and also “releasing volatile compounds that are weakening the base of the crust,” said Newsweek.
The Farallon plate is interacting with the whole North American craton, but the dripping is concentrated in the Midwest. The researchers used a full-waveform tomographic model of the North American plate that showed “extensive drip-like features,” said the paper. “Because of the use of this full-waveform method, we have a better representation of that important zone between the deep mantle and the shallower lithosphere where we would expect to get clues on what’s happening with the lithosphere,” Thorsten Becker, a professor at the University of Texas at Austin’s Department of Earth and Planetary Sciences and Institute for Geophysics and co-author of the paper, said in a press release.
Rocky rearrangement
This is not the first time scientists have found evidence of cratons changing. A study published at the end of 2024 found that the North China Craton had undergone deformation too. However, the interesting part of the North American craton discovery is that the changes are happening now, which “gives scientists an unprecedented opportunity to learn more about the geology of these lithospheric cores,” said Popular Mechanics.
There is much to be found in timely evidence. Geologists can “observe developments in close-to-real-time by tracking the rate at which seismic waves from earthquakes move through the craton,” said IFL Science. “That gives us a much better opportunity to make sense of the details than an example from the deep past.” The Earth’s geologic history has changed dramatically through the years. “This sort of thing is important if we want to understand how a planet has evolved over a long time,” Becker said. “It helps us understand: how do you make continents, how do you break them and how do you recycle them.”
The good news is that we will not feel any noticeable difference in our lifetime. “The mantle processes driving the dripping can influence how tectonic plates evolve over time, but they are very slow going,” said the press release. Also, the dripping is “expected to eventually stop as the remnants of the tectonic plate sinks deeper into the mantle and its influence over the craton fades” — a sort of tectonic law of diminishing returns.