Continental shift: Uncovering the slow sink of North America
Beneath the surface of the United States, the Earth's crust is slowly sinking, leading to the emergence of a new geological phenomenon. Scientists from the U.S. and China have discovered that this could be due to a tectonic plate submerging into the Earth's mantle. Their findings were published in the journal "Nature Geoscience".
The Earth's crust under the Midwest United States is gradually sinking. In the Earth's upper mantle, clusters of molten rock form, which gain mass over time and collapse deeper. This process, uncovered through seismic monitoring, highlights how thin the lithosphere is in this region. Researchers from the University of Texas at Austin found that the sinking of the North American continent might be caused by a tectonic plate submerging into the Earth's mantle. This phenomenon is discussed in the journal "Nature Geoscience".
The subsidence of the lithosphere indicates that something is happening within the planet
The lithosphere in this region is a craton—a large, stable part of the Earth's crust that remains unchanged for a long time. Cratons are regarded as nuclei around which continents form. About 35 such structures are known globally. This phenomenon, termed lithosphere dripping, also takes place elsewhere in the world, but its discovery here offers new insights into understanding the 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."
Lithosphere dripping happens when the lower part of the Earth's crust heats to a certain 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 planet's surface, revealing activity occurring below. In this study, the team led by seismologist Junlin Hua from the University of Science and Technology of China employed seismic data to reconstruct activity at the base of the Earth's crust.
What are the consequences of this discovery?
The discovery of craton "dripping" is noteworthy because it is occurring now, providing scientists the opportunity to observe the process in real-time. Scientists assure there is no need to worry that the continent will become hollow or that the sinking will alter the landscape in the immediate future. The processes in the Earth's mantle driving this phenomenon are very gradual.
This study is part of a larger project that created a new seismic model for North America. This model, developed by Stephen Grand and his team, has unveiled new details about the geological processes 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 led scientists to infer that the Farallon Plate, an oceanic tectonic plate that subducted beneath North America for the past 200 million years, could be influencing this process. Though the plate is about 600 km (approximately 373 miles) away from the craton, it affects the continent by directing mantle material flow in a manner that shears the base of the craton and releases volatile compounds, weakening its base.
