[Taxacom] Quartzite on a volcanic island in the Comoros

John Grehan calabar.john at gmail.com
Tue Mar 19 08:32:25 CDT 2019

Anjouan island is part of the Comoros and supposedly the result of volcanic
eruptions about 4 Ma. Apparently geologists have been documenting evidence
of a  rock formation that doesn't make sense in this context. Varying
reports of a lighter-coloued sedimentary rock – once described as sandstone
in the 1960s – have been emerging since about 1900. In more recent decades,
the specific mineral was identified as quartzite. One suggestion so far is
that when the Madagascar separation occurred a chunk of continental rock
(including quartzite) split off from either Africa or Madagascar, and came
to rest on the ocean basin that formed between them and subsequently
intertwined with the basalt during volcanic activity.

Whatever the geological cause, which appears to be unresolved at present,
its an indication of the potential geological complexity of interpreting
the biogeographic significance of volcanic islands. Most often the age of
volcanic islands is predicated on the most recent and accessible layers,
discounting the possibility of older origin which may go back to a
considerably earlier age when different continental configurations were at
play. Sometimes it is only the biogeographic pattern that is visible and
accessible to give evidence of such origins. The present assumption by
geologists studying this particular case is that the continental chunk was
on the seafloor, but of course how big that chunk and whether ever
subaerial and for how long after rifting is something that geology may
never be able to answer.

John Grehan

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