The Earth might feel solid as a rock under our feet. However, it's actually an active and energetic planet. Earthquakes shake the ground, enormous craters suddenly open up, and volcanos erupt to create new islands as if from nowhere. All thanks to the movement of tectonic plates. 

On such a dynamic planet, it may bring you comfort to know that there are stable regions. These are called cratons. These are the oldest, most stable and most solid features of the planet. In short, "as solid as a rock." But it turns out they may not be as stable as we once thought.

Specifically, the craton below North America. "We combined and analyzed several data sets from Earth's gravity field, topography, seismology, and crustal structure and constructed a three-dimensional density model of the composition of the lithosphere below North America," explained the GFZ German Research Centre for Geosciences' Mikhail Kaban, in a statement. "It became apparent that the lower part of the cratonic root was shifted by about 850 kilometers [530 miles]." The research, led by Kaban, is published in Nature Geoscience. It was described in their press release as "extremely deformed".

Cratons are regions at the cores of the continents that have always been thought to be stable thanks to their strong and thick lithospheric roots, which are buoyant when compared to the surrounding mantle due to their relatively lower temperature. The ongoing assumption has been that cratons have barely changed since their formation, 2.5 to 3.8 billion years ago.

But what has shaken up of the foundations of the North American craton? The scientists developed a model that mimicked how the Earth's mantle flows below North America. The model indicates that 200 kilometers (124 miles) below the surface, the craton's root has been flowing west-southwest, the same direction as the mantle around it, at a velocity of about 4 millimeters (0.16 inches) a year. This might seem slow, but over hundreds of millions of years it has led to the 850 kilometer (530 mile) shift relative to the center of the craton cited by Dr Kaban. 

There's no need to worry of course, it's our understanding that has changed, not the North American craton itself. It hasn't suddenly become unstable. "This indicates that the craton is not as solid and as insensitive to the mantle flow as was previously assumed," said Kaban in the statement. Instead, it seems that cratons can flow can along with the natural movement of the mantle. There might be much more interaction between cratons and the surrounding mantle than we ever thought.