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Geologists Keep Finding 'Impossible' Rocks on This Remote Tropical Island
Geologists Keep Finding 'Impossible' Rocks on This Remote Tropical Island
PETER DOCKRILL 19 MAR 2019
Halfway between the east coast of Africa and Madagascar lies the remote island of Anjouan – a volcanic landmass born in ancient eruptions some four million years ago.
Because of Anjouan's fiery origins, the entire island should be uniformly composed of volcanic basalt: a dark-coloured, lava-derived rock.
. . .
For over a century, geologists have been documenting evidence of a different kind of rock that can be found on Anjouan. It's a kind that doesn't make sense, given the entire landmass ought to have spewed out of a volcanic rift in the ocean floor.
Varying reports of this lighter-coloured 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.
More:
https://www.sciencealert.com/impossible-rocks-on-this-remote-volcanic-island-are-a-massive-geological-mystery
1 replies
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https://en.wikipedia.org/wiki/Porphyry_%28geology%29
Porphyry is a textural term for an igneous rock consisting of large-grained crystals such as feldspar or quartz dispersed in a fine-grained silicate rich, generally aphanitic matrix or groundmass. The larger crystals are called phenocrysts. In its non-geologic, traditional use, the term porphyry refers to the purple-red form of this stone, valued for its appearance.