Remnants of an Ancient Supercontinent Discovered on a Remote African Island

Geologists have found 'impossible rocks' at a remote African Island. Now they say the weird island contains an intact chunk of an ancient continent

Between the east coast of Africa and Madagascar is a remote island called Anjouan.

Born of ancient volcanic eruptions about four million years ago, due to its “fiery” origins, the entire island should be composed uniformly of tuffaceous rocks and volcanic basalt: a dark colored rock, derived from lava.

It should, but it is not. In fact, it’s anything but what scientists expected.

During the various expeditions to the island, scientists have found many things on the island that shouldn’t be there.

Anjouan is made almost exclusively of blackish volcanic basalt that rose from the seafloor. Geochemist Steven Goldstein searches a creek bed for anything unusual. Image Credit: Earth Institute.
Anjouan is made almost exclusively of blackish volcanic basalt that rose from the seafloor. Geochemist Steven Goldstein searches a creek bed for anything unusual. Image Credit: Earth Institute.

For more than a century have geologists been documenting evidence of a type of rock that can be found in Anjouan and should not be there.

These mysterious formations come from the continental crust, more specifically from the delta of a river or a beach.

During the 1890s, scientists from France discovered outcrops with quartzite. It is a rock rich in mineral quartz and is typically found on continents.

Nearly one hundred years later, a group of scientists from the University of Columbia (USA) and the University of Western Brittany (France) found the mysterious outcrops in the dense jungle that covers the island.

Following suggestions from local guides, they discovered that the nearby ridge of Habakari N’gani consists almost entirely of quartzite.

“This thing is big,” said Cornelia Class, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory of the quartzite formation. “Much bigger than we thought. This is a major discovery.”

That quartzite shouldn’t be there.

Currently, the origin of the quartzite on the remote island is not yet clear, but the geologists formulated a preliminary hypothesis: traces of quartzite could be the remains of an ancient supercontinent.

Gondwana 420 million years ago. View centered on the South Pole. Image Credit: Wikimedia Commons.

More than 200 million years ago, Madagascar and Africa were united in the supercontinent of Gondwana, along with South America, India, Australia, and Antarctica.

For millions of years, erosion and sedimentation formed a thick layer of sediments rich in quartz at different points.

According to geologists, Gondwana began to separate about 180 million years ago.

Somewhat later, between 165 and 130 million years ago, Madagascar split from East Africa and became an island.

It is believed that during this continental ‘goodbye’, pieces of continental rock were trapped in the oceanic crust that formed in the opening basin.

Around four million years ago, the volcanism that would later form the island of Anjouan took place.

The fragments of ancient sediments rich in quartz, later transformed into quartzite and still trapped in the seabed, were raised by the volcanic activity that created the island.

This is huge.

If the theories are correct, the island of Anjouan may be the only volcanic island in the world that features an intact part of the ancient supercontinent, according to the researchers.

Anjouan stands alone—the only island in the world formed by volcanism that also contains an intact chunk of a continent.

“This is contrary to plate tectonics,” said Class.

“Quartzite bodies do not belong on volcanic islands.”

Earth Institute
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