Continental collapse: New insights into North American crust sinking
Beneath the surface of the United States, the Earth's crust is gradually subsiding, creating a novel geological phenomenon. Scientists from the USA and China have discovered that this may result from a tectonic plate submerging into the Earth's mantle. Their findings have been published in the journal "Nature Geoscience".
The Earth's crust beneath the Midwest United States is steadily sinking. In the Earth's upper mantle, clusters of molten rock form, which, over time, gain mass and collapse further down. This process, revealed through seismic monitoring, highlights how thin the lithosphere is in this region. Researchers from the University of Texas at Austin discovered that the bottom of the North American continent is subsiding, possibly due to a tectonic plate submerging into the Earth's mantle. The phenomenon is detailed in the journal "Nature Geoscience".
The sinking of the lithosphere indicates that something is happening inside the planet
The lithosphere in this region is a craton – a large, stable part of the Earth's crust that remains unchanged for extended periods. Cratons are considered nuclei around which continents form. Approximately 35 such structures are known worldwide. This phenomenon, termed lithosphere dripping, also occurs in other parts of the world, yet its discovery presents new opportunities to understand Earth's dynamic geological processes. "We made the observation that there could be something beneath the craton," said Junlin Hua, the study's lead author.
Geophysicist Thorsten Becker from the University of Texas at Austin, co-author of the study, stresses in a university release that this is a significant discovery: "This sort of thing is important if we want to understand how a planet has evolved over a long time." "It helps us understand how do you make continents, how do you break them, and how do you recycle them," she added.
Lithosphere dripping occurs when the lower part of the Earth's crust heats to a specific temperature. As the rock melts, a drop forms, which eventually becomes heavy enough to detach and sink deeper into the planet. Scientists liken this phenomenon to an extreme version of a pitch drop experiment.
In some regions, such as the Andes or the Anatolian Plateau in Turkey, this process can cause rifts on the Earth's surface, revealing activity occurring below. In this case, the team led by seismologist Junlin Hua from the University of Science and Technology of China utilised seismic data to reconstruct activity at the bottom of the Earth's crust.
What are the consequences of this discovery?
The discovery of craton "dripping" is notable because it is occurring now, allowing scientists to observe the process in real-time. Scientists reassure us that there is no need to fear that the continent will hollow out or that the subsidence of the continent will significantly alter the landscape in the near future. The processes in the Earth's mantle driving this phenomenon are very slow.
This study is part of a larger project that developed a new seismic model for North America. This model, created by Stephen Grand and his team, has uncovered new details about the geological processes occurring in the crust and mantle beneath North America. "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," said Becker.
This model helped scientists infer that the Farallon Plate, an oceanic tectonic plate that subducted beneath North America over the past 200 million years, may be driving this process. Although this plate is separated from the craton by approximately 600 kilometres, it influences the continent by directing mantle material flow in such a way that it shears the base of the craton and releases volatile compounds that weaken its base.
