America is Sinking - and Not Metaphorically
Did you feel that earthquake?
If you were in the city of Gaziantep, Turkey on February 6, 2023 you not only felt it, you were lucky if you lived through it.
The stress under your feet had been building for thousands of years. The massive Arabian Plate - a layer of rock 1.1 million square miles in size, underneath Saudi Arabia, Yemen, and most of the Arabian Peninsula - had been slowly pushing northward toward the even larger Eurasian Plate, at a rate of just a few centimeters a year. Even at this slow pace, intense horizontal pressure was building between these two massive tectonic plates. The area between the plates, known as the East Anatolian Fault, was absorbing this stress. It was only a matter of time before the pressure exceeded the strength of the rock, causing a sudden seismic rupture along the fault.
It finally happened on February 6, 2023, at 4:30AM. The plates slipped. Rather than the slow horizontal movement, the Eurasian Plate was suddenly pushed vertically. It's known in geology as a compression collision.
The result was the largest and most destructive earthquake to strike a populated area in the last decade. It registered 7.8 on the Richter scale, with an epicenter in the densely populated Southeastern Turkey and northwestern Syria. At least one after shock reached a 7.5 magnitude later that same day. No less than nine major Turkish cities suffered widespread destruction, along with four in Syria. Over 200,000 buildings were damaged or simply collapsed. Millions were left homeless.
Over 50,000 people died in the devastation.
Earthquakes are an unwelcome reminder the ground under us isn't stable. We stand on fractured crust in constant motion. Lucky for us, those massive plates sit on top of a stable core. The Earth has a solid layer of lithospheric rock, made from solid granite or peridotite or basalt, depending on where you're standing. And that layer has remained unchanged for billions of years.
It has a name, this ancient rock that is the ground we stand on. It's called the craton. The name comes from the Greek word kratos, meaning "strength". It's the immobile foundation under every continent on Earth. It's made up of two key parts: an upper crust, and an extension underneath that goes deep into the Earth's mantle and keeps our continents stable, like the keel on a sailboat stabilizing the craft against the ocean currents.
Except a new study published this year in the journal Nature Geoscience revealed even the craton isn't so immobile. In fact, there are disturbing changes happening under us right now. The North American craton - which sits beneath Canada and the central United States - is getting thinner. And parts have "dripped" into the Earth's mantle, disappearing completely.
Scientists at the University of Texas at Austin used detailed seismic imaging to look under North America, essentially performing a continent-sized CT scan. The team created a full-waveform seismic tomographic model of the continent. In other words, they used waves generated from earthquakes and explosions to create a 3D map of everything under us. And they got a look at the geological processes raging under the craton in real time. They found that the hot mantle of the Earth is sending plumes of material up to the underlying craton, melting the rock into stalagmite-like structures, or drips, the pieces eventually breaking off and disappearing into the deeper mantle.
Keep in mind, the Earth's mantle ranges from 932 degrees Fahrenheit to over 7,000 degrees at the center. So when they talk about "hot plumes", they are plenty hot enough to melt granite and slowly drain the rock into the mantle's center. The process has a name: "lithospheric delamination". Put simply, the bottom of the tectonic plates are detaching and sinking into the Earth's core.
The thinning of the North American craton is concentrated in the Midwest part of the United States. Will we see dangers as a result? Well, a thinner shell makes for a more flexible or unstable plate. New faults might form more easily, and old ones may be reactivated by the internal changes, potentially increasing the likelihood of tectonic stress releasing as earthquakes. As if tornadoes weren't bad enough.
The concept of cratons thinning or changing started in the 1980s and 90s, when geologists realized the bottom of the craton could potentially detach and sink. Studies in Colorado and parts of Southern Africa revealed areas where these shifts in the cratonic root seemed to have already happened over time. In the 2000s, when seismic measurement advanced, scientists were able to actually observe these deep structures. They could see, for example, that the North China Craton - which includes Beijing and parts of Inner Mongolia - was once stable but had gone through extensive change.
But this remarkable University of Texas study is the first time "cratonic thinning" has been captured in action. On the positive side, it gives scientists a new understanding how a planet evolves over time. Unfortunately, this may ultimately mean the very continent we live on will be recycled into the Earth.
Suddenly that mission to Mars is looking real smart.
