NOVA Online | Garden of Eden | Seychelles Through Time

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Seychelles Through Time
by Paul Howell

As you read this article, refer as often as you wish to the accompanying animation. The animation, which you control using your mouse, shows the breakup of the ancient supercontinent of Gondwana from 150 million years ago to the present, and on to a (speculative) 50 million years into the future.

The Seychelles—tiny little islands, remote location, beautiful coral beaches. Few destinations could hold more allure as prototypical tropical islands. Yet the allure of the Seychelles for geologists is not their nature as oceanic islands; on the contrary, geologists are fascinated by the Seychelles because they are so very un-oceanic in nature.

The Hawaiian and Polynesian islands started life as volcanoes in the middle of the Pacific; Iceland, Bermuda, and the Azores did the same in the Atlantic. But the Seychelles are not volcanic at heart; they are granitic, and granite is so un-oceanic a rock that its presence virtually defines the existence of a continent. Granite islands in the middle of an ocean? Hmmm....

Moreover, the Seychelles are tiny pinpoints of islands sticking up in the ocean, but not up from the ocean's great depths. Instead, they arise from a broad undersea "platform" that is much shallower than the surrounding ocean: the Mascarene Platform, which measures approximately 60,000 square miles and rises one to two miles above the surrounding seafloor.

Aha! you claim. This is the lost island of Atlantis, foundered into the Indian Ocean. The merest hint of Atlantis' former glory is now left as the gemlike Seychelles islands, the Atlanteans replaced by corals, sharks, and the archipelago's present, relatively small population of humans.

Alas, that's not the interpretation favored by the geology of these islands. The Mascarene Platform is granite at its core, with a mantle of basalt and limestones. The granite comes from the platform's association with the subcontinent of India. Before they split apart, the two had been intimately linked at least since the formation of these 700-million-year-old igneous rocks. Together they stayed, along with the rest of Gondwana, the supercontinent of the southern seas, until Gondwana began to break up about 200 Ma (million years before present).

See animation of Gondwana breakup
(requires QuickTime)

Non-QuickTime version

As you can see in this animation of former plate positions, India, along with Madagascar and the Seychelles/Mascarene Platform, broke away from Africa about 150 Ma. Plate boundaries then shuffled a bit, and around 84 Ma, India/Seychelles left Madagascar behind and continued northward toward Asia. This put a little distance between the escapees and Madagascar, but then a second reshuffling of the plates and some interaction with the Reunion Hotspot—a hot plume of upwelling mantle material much like the hotspot that generates Hawaiian volcanism today—created a new rift that separated the Mascarene Platform from India about 64 Ma.

This was a momentous occasion geologically. The separation of the Seychelles "microcontinent" (the Mascarene Platform) was accompanied by a tremendous outpouring of basaltic lava from this hotspot, giving birth to a mass of igneous rock called the Deccan Traps, which today covers much of western India. One of the most voluminous episodes of continental volcanism known, it coughed up an area of basalt one to two miles thick and blanketing 400,000 square miles. Because the Mascarene Platform was just beginning to separate from India at the time, much of this basalt covers the platform as well, though the eruptions did not smother all of the older granite.

The Deccan Traps eruptions slightly postdate the calamitous demise of the dinosaurs at the end of the Cretaceous Period about 65 Ma. In the early 1980s scientists suspected that the enormous release of noxious volcanic gases associated with the Deccan Traps may have caused the end-Cretaceous mass extinctions, though now geologists believe the two events were largely independent. Since then, evidence has mushroomed in support of a meteorite-impact explosion on the Yucatán peninsula of Mexico as the cause of this mass extinction.

But (the reasoning goes) couldn't that impact explosion—the largest explosion on Earth since an even larger impactor hit us about four billion years ago and spawned the moon—have possibly triggered the outpouring of these Deccan Traps basalts, located almost precisely on the opposite side of the planet? Such antipodal activity from major impacts is known on other planets, most spectacularly on Mercury. Although a few scientists still argue for such a relationship, it now appears not to be the case. The overwhelming evidence of radiometric dates on the basalts show that the Deccan Traps eruptions postdate the end-Cretaceous impact in Mexico, and thus the rifting of the Seychelles away from India in no way can be blamed for the deaths of our dinosaur friends.

See animation of Gondwana breakup
(requires QuickTime)

Non-QuickTime version

Following this 64 Ma rift, India moved off to the north and collided with the soft underbelly of Asia , eventually wreaking havoc in the form of the Himalayas and the Tibetan Plateau. By contrast, the Seychelles were happy to stay put in the middle of the Indian Ocean. And there the islands remain to this day, still sinking slowly—only a fraction of an inch every millennium—but remaining as a tiny and ever-so-appealing archipelago of coralline beaches and crystal seas even as the last granite peaks approach their watery final resting place. As with the coralline atolls of the Pacific, there is no geologic reason that continuing subsidence will necessarily result in the total disappearance of this Atlantis. The corals are fully capable of growing fast enough to keep up with this sinking, thereby perpetuating their existence for many eons to come (see Build an Island).

Paul Howell is an associate professor in the Department of Geological Sciences at the University of Kentucky. He created the Gondwana-breakup animation presented here.