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).
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
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
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