Much of what happens in Earth’s mantle is hidden from us. The combination of extreme heat, slow movement, and extreme pressure makes it impossible to know what is happening beneath our feet across much of the planet. At plate boundaries, we can have some idea because the mantle is being dragged along with the moving plates, but what about far from plate boundaries?
This sea cliff is found on the Atlantic coastline of Angola. This is an interesting area; it is far from any plate boundary but cliffs like these show that there has been uplift of rocks recently, geologically speaking. These cliffs are made of sediments that formed beneath the ocean waters and some force has pushed them upwards.
To understand these processes, researchers led by a team from Oxford University traveled to the top of one of these sea cliffs in Angola and found shells that must have formed in the ocean. They sampled them, took them to the lab, and found that they formed about 40-45,000 years ago, based on two different measurements that both agreed.
Those samples came from 25 meters above sea level today. Geologists generally know what sea level has done worldwide over the past few thousand years, so combining estimates of sea level change with the height above the sea level at that time means the coast of Angola is being uplifted, as something of a central dome, at a rate of ~2 millimeters per year. Since these shells were deposited, there has been nearly 80 meters of uplift. These rates are slower than plate tectonic rates that tend to be centimeters per year, but they’re still extremely high for something happening away from a plate boundary.
The uplift on the coast of Angola is hundreds of kilometers in diameter; it extends into a larger dome in the central part of the country, but ends before reaching Namibia. There is also an anomaly in the gravity over this country – extra gravity below the plateau suggests some force is pushing upwards and supporting the weight of the plateau.
The scientists believe the most likely cause for this uplift is flow in the mantle. Mantle flow is expected to be on the scale observed in this case and to move at about the scale seen here, plus there is no other obvious explanation for why these rocks are being uplifted.
The planet’s surface hides much of what is going on in the mantle, but recognizing geologically interesting settings such as this coastline can fill in part of that story.
Image credit: https://flic.kr/p/4KzG9Y
Original paper: http://go.nature.com/2eRRIdZ
