The Earth's Deepest Scars

No matter how thoroughly researched and well-understood a scientific theory is, there’s always room for improvement, always something we missed. Plate tectonics is one of the most powerful and comprehensive theories in modern science, but according to a new study out today, we may have missed an important piece of the geologic puzzle. According to this research, tectonic activity may be controlled by forces deeper below the surface than we’ve realized.

Plate tectonics theory explains how the Earth’s crust moves and changes. The crust isn’t a solid shell over the Earth, it’s broken into fragments: plates. Each plate butts right up against all the surrounding plates, so there’s no space in between, but they can rotate in place, or shift like conveyor belts, sliding and grinding past each other, into each other, or underneath each other. Boundaries between plates are areas of massive geologic activity. Where plates move together, they form massive mountains or deep trenches; where plates pull apart, volcanic activity creates new crust. The surface of the Earth is constantly shaking, shifting, and deforming, mostly at these boundaries.

This is a map of the major tectonic plates of the Earth.
Notice the places where the plates meet - those are the very active plate boundaries.
Image from Wikipedia.

But in some cases, tectonic activity is common in places that aren’t at plate boundaries, or even all that close to them. Sometimes it’s easy to see why this happens. In some places geologists can easily detect fault lines or areas of weakness within the crust, likely leftover from ancient tectonic activity. When pressure is put on the plate at the boundaries, these old “scars” in the middle of the plate may act up. But sometimes this non-boundary activity happens where we don’t see obvious scars, and in other cases we do see “scars” but don’t see activity when we would expect it. So … what’s going on?

The layers of the Earth. Tectonic plates
are composed of lithosphere.
Image from Wikipedia.

The researchers of this new study suggest we haven’t been looking deep enough. Tectonic plates aren’t just composed of crust, their bases are actually made up of the upper-most layer of the mantle (together, the crust and upper-most mantle are called the lithosphere). Previous research has shown that “scars” like we see in crust are also present in the deep mantle portion of plates. Their research aimed to find out how much these mantle scars affect surface activity.

What they found was quite surprising. Based on sophisticated computer models of plate dynamics, not only do these deep mantle scars affect surface activity, they have a much greater impact than the scars in the shallower crust. This may explain why we see non-boundary activity where there’s no crust damage. Their models even showed that “scars” in the crust often won’t act up if the mantle below is un-damaged.

This makes me think of another example of tectonic activity happening mysteriously far from plate boundaries. The famous Hawaiian volcano Kilauea and the infamous Yellowstone super-volcano are both cases of volcanism far from the edges of their plates. Similar to the findings of this study, these eruptions are thought to be driven by regions of intense activity in the mantle, called mantle plumes.

Most of the places where we see these “mantle scars” are areas that used to be plate boundaries, places where continents fused together and subduction zones were sutured-over. With this in mind, the researchers suggest in order to understand modern tectonic activity, it is important to be aware of modern plate boundaries and ancient ones. With that in mind, they put together this “updated” Plate Tectonics map.

This is a map showing modern plate boundaries AND ancient, hidden plate boundaries.
The scars left in the deep lithosphere may be linked to lingering activity in the crust today.
Image from Heron et al. 2016.

For a real-world example of this phenomenon, the researchers point to China. Today it is one solid landmass, but it is the result of many landmasses fusing together over millions of years, and as a result has many “scars” deep below the surface. As India continues to push up against Asia, raising the Himalayas in the process, the pressure puts stress on those old wounds, causing deformation in places above the scars, such as at the Altyn Tagh Fault.

Just like strenuous activity may cause renewed pain in an old healed injury, it seems that the many healed-over scars of tectonic activity millions of years in the past still lurk below the surface, subtly informing the way our modern landscape shifts and shakes.  

References:

Heron et al. 2016. Lasting mantle scars lead to perennial plate tectonics. Nature Communications. [Link]