How Carbon Causes Continents To Crack Up

The Natron Valley, Tanzania is part of the East African rift where part of the continent is breaking away. Roby1960/Shutterstock

We know atmospheric carbon dioxide can heat the planet, and drown coastlines, but that may just scratch the surface of its impacts on the planet. New research indicates it can even cause continents to break apart – in fact, it appears to be facilitating Africa's slow tectonic divorce right now.

Volcanoes can release large amounts of carbon dioxide along with all the other material they spit out, although contrary to a widespread myth, the quantities released in the modern era are small compared to human activities. The volcanoes of the African Rift Valley are particularly carbon intensive, a discovery that has led scientists on a long investigation to understand why.

Rifts involve the splitting of a craton, the oldest and most stable part of a continental plate. Professor Tobias Fischer of the University of New Mexico proposed the carbon dioxide released in the process comes from the craton's base, where it has been accumulating for billions of years. "The model suggests that this accumulated carbon originates from subducting oceanic plates and deep mantle plumes," Fischer said in a statement.

However, when Fischer published this idea he was still unable to explain how gasses buried beneath so much rock could make its way to the surface. Fischer now thinks he has answered this question with a new paper in Nature, and his solution could explain continental rift formation.

The East African rift is gradually pushing a coastal strip away from the rest of Africa. It is thought to release about as much CO2 as a small country. Fischer's co-authors sampled gasses from rift valley hot springs and found they divide into two very distinct groups. Some release very little carbon dioxide, instead being mostly nitrogen and helium from the crust, while others contain carbon dioxide laced with helium whose isotopic signature identifies it as coming from Earth's mantle.

The Natron Valley, Tanzania is part of the East African rift where part of the continent is breaking away. Carbonate Volcano Oldoinyo Lengai in the background has clued scientists in to the role of carbon dioxide in this process. UNM

The key to understanding the difference is Tanzania's volcano Oldoinyo Lengai, famous for its cool lava. "This volcano erupts lavas that are so liquid they move like motor oil. The reason for this is that they are devoid of the silica that makes up most igneous rocks but contain about 30 percent carbon,” Fischer said. The rocks formed from these lavas are even known as carbonatite. Widespread carbonatite through northern Tanzania indicates many such volcanoes exist but Oldoinyo Lengai is the only one active in recent times.

The shape of the underside of a craton focusses carbon until it reaches a section think enough for it to rise, eventually forcing a rift to open. Muirhead et al/Nature

The paper concludes the underside of cratons includes steep slopes that focus carbon into pockets. Just as carbon-rich lava flows more easily at the surface, the presence of carbon beneath a craton slopes reduces friction, pushing melted material sideways until it reaches thinner parts of the craton. There the carbon-rich magma rises, producing a volcanic province and breaking the craton apart, eventually leading to the division of the continental plate and a new seaway.

 

 

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