Deep beneath California's Sierra Nevada mountains, geologists have found evidence of Earth's rigid outer layer sinking into the mantle, like a sugar cube slowly descending and dissolving into a pool of hot syrup.
This is known as lithospheric foundering, a process where a portion of Earth's lithosphere (the rigid outer layer of the Earth) becomes too dense and sinks into the underlying, softer mantle.
Imagine the lithosphere as a solid crust floating on a more fluid, hotter mantle. Over an extremely long time, certain parts of this crust (often in the lower lithosphere) may become so dense that they sink into the mantle, similar to how a solid object might descend into a gloopy liquid.
It's important to remember that Earth isn't a ball of solid rock. Instead, it's a dynamic, layered planet with a solid outer shell that's effectively floating atop a hotter, more fluid mantle. At its center is a solid core of iron and nickel, surrounded by a liquid core of metals.
Some scientists have long suspected that lithospheric foundering might play an important role in the geological workings of Earth, but solid evidence has been lacking.
In a new study, two researchers from the University of Colorado Boulder and the University of California San Diego collected seismic data and earthquake records to show there have been small seismic tremors (ranging from magnitude 1.9 to 3.2) over 40 kilometers (25 miles) beneath the central Sierra Nevada mountain range.
The data suggests the mantle below the Sierra Nevada has a distinct layer, which becomes thinner as it extends farther north. This, they claim, fits in with the idea that a piece of the lithosphere beneath the southern Sierra sank millions of years ago through lithospheric foundering. This process seems to have been occurring for at least 3 million years and may be continuing to progress northward.
"We image the imprint on rocks at the current crust-mantle boundary that recorded a dense portion below it peeling off under the Sierra Nevada in California," the authors write in their summary.
"The removal happened several million years ago in the southern part of the mountains and is still in progress under the central part, causing very deep small earthquakes, while the northern part still retains its dense layer."
Despite how it might sound on the surface, the research does not suggest that California will be swallowed up into Earth's mantle (at least not anytime soon). Foundering involves the super-slow movement of deep subsurface layers and won't involve the collapse of external landmasses.
That said, these kinds of processes might provide some insight into the workings of Earth's surface, such as the continent-building activity that shifts and shapes the world's landmasses through plate tectonics.