The volcanic island of Anjouan in the southwest Indian Ocean is home to a strange geological mystery. The inhabitants of the island and geologists are more than surprised to find a type of rock that should not be on this island.
The island emerged in an ocean basin when tectonic plates moved apart and magma rose and cooled, turning into basalt that formed the island. Therefore, sedimentary rock quartzite, a type of sandstone made from grains of quartz sand collected in river deltas before the quartzite is compressed over time, is not expected to be found in Anjouan. The island’s basalt does not contain quartz, and the island itself is too young to form a large river delta. Still, geologists report finding abundant quartzite on the island for perhaps more than a century.
Geologists have reported unusual rocks that may have been quartzite in 1900, though with poor documentation to know for sure. In 1969, a large “sandstone” formation was found on the island near the town of Tsembehou and was later determined to be quartzite. Then in 2017, French geologist Patrique Bachèlery found more quartzite in a nearby ridge. A few years later, Columbia University’s Lamont-Doherty Earth Observatory geochemist Cornelia Class began her research. Class and his team did indeed find quartzite in just minutes and confirmed that there was an abundance of quartzite along the ridge.
“This goes against plate tectonics,” Class said in a Columbia University press release, adding: “Quartzite bodies do not belong to volcanic islands.”
Islanders told Class that they always found these rocks, used them to sharpen their knives, and showed them where they could find more quartzite. By creating a map of the quartzite on the island, Class found that the amount of quartzite was almost half a mountain.
There is no definite explanation yet as to how the quartzite was found on this island. A piece of quartzite from the continental crust may have deposited itself in the ocean basin and then risen 4,000 meters above the seafloor by igneous basalt, but Class said in an interview with Live Science that the chemistry of the island’s basalt rocks did not show a relationship with the continental crust and found the find “something we thought was impossible”. but we really need to explain it here and when we find something like this.”
Tests to determine the age of the quartzite will help determine when it has accumulated. It may be possible that this island is the only volcanic island in the world that awaited with a continent after the collapse of the ancient continent Gondwana.
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