Half-Mountain Quartzite Found in Young Volcanic Island Anjouan, Scientists Puzzled

How a half-mountain of quartzite was discovered on the young volcanic island of Anjouan baffles scientists.

Geological Puzzle on the Volcanic Island of Anjouan

Anjouan, a volcanic island in the southwest Indian Ocean, is the location of an unusual geological riddle. Residents and geologists on the island constantly discovering a particular kind of rock that is not supposed to be there.

When tectonic plates moved apart, magma rose up and cooled to produce the basalt that makes up the island as it developed on an ocean basin. It is not expected to find quartzite on Anjouan, yet scientists have claimed to have discovered a lot of the sedimentary rock there for maybe more than a century.

The Unusual Occurrence of Half a Mountain of Quartzite

Geologists discovered odd rocks that may have been quartzite around 1900, but the documentation was inadequate to confirm this. Geologists discovered a sizable deposit of what they thought was "sandstone" in 1969 on the island close to Tsembehou; however, it turned out to be quartzite. Then in 2017, a neighboring ridge was where French geologist Patrique Bachèlery discovered further quartzite.

A few years later, geochemist Cornelia Class of Columbia University's Lamont-Doherty Earth Observatory began her investigation. Within minutes, she and her colleagues discovered quartzite, and there was plenty of it up along the ridge.

The quartzite's origin is still unknown with certainty. However, Class told Live Science that the chemical makeup of the island's basalt rocks doesn't show a connection with the continental crust, calling the find a feat that is considered impossible. Instead, she hypothesized that a piece of quartzite from the continental crust must have been deposited in the ocean basin and then taken 13,120 feet above the seabed by the igneous basalt.

Class, on the other hand, asserted that because quartzite was discovered, they, as specialists, should come up with an explanation.

The quartzite's age will help scientists identify when it was deposited and, perhaps, how it became the sole volcanic island in the world with a piece of continent resting on it, probably from the fragmentation of the extinct continent Gondwana, IFL Science reports.

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Anjouan and Quartzite

One of the Comoro Islands, Anjouan Island, is a small island situated between Madagascar and Africa's east coast. Nearly two kilometers of lava were heaped up during volcanic eruptions on the seafloor around 4 million years ago before it burst through the waves. Then it continued to build up, creating what is now the rocky, heavily forested island of Anjouan.

Sandstone, a form of sedimentary rock, is the precursor of quartzite, which is created from pure quartz sand grains gathered in a river delta. The grains join together and metamorphose into incredibly hard, thick quartzite when they are further buried and squeezed. Due to the absence of quartz in the basalt that created the island of Anjouan, the rock couldn't have possibly grown there.

The landscape of Anjouan is too young and treacherous to have ever produced much of a river delta; instead, its streams rush through chasms and down the steep hills to its rocky coasts, Columbia University explained in a press release.