GeoMon geosite : Marquess of Anglesey’s column

GeoMon geosite : Marquess of Anglesey’s column is the best exposure of the Anglesey Blueschist Belt; a complex zone of rocks comprising mainly blueschist, greenschist and metabasite (amphibolite). It stetches from Ynys Llanddwyn across to Pentraeth and as far south as the Menai Strait. These rocks were formed at a destructive plate margin. Although some seafloor rocks were scraped off the descending plate to form the melange, much continued down the subduction zone.

The rocks are metamorphosed equivalents of the famous basaltic pillow lavas and seafloor sediments seen at Ynys Llanddwyn with the degree of metamorphism reflecting their relative depths of burial. It is estimated that the basaltic pillow lava was converted to blueschist at a depth of around 35 km. This blueschist gets its name from the diagnositc mineral, glaucophane, which is formed under high pressure but relatively low temperature (<500C which is low for high grade metamorphic rocks). Note that the blue colour is the mineral rather than the rock and is only seen in thin section under the microscope. In contrast the greenschist formed from seafloor sediments that had travelled about 15km down and a temperature of only ~300C. (the greenschist gets its name and colour from chlorite, a mica-related mineral).

These rocks are an integral part of the Precambrian story of Anglesey. More details can be found in our publication Footsteps Through Time.

These rocks have become internationally famous. The GeoMon geosite : Marquess of Anglesey’s column is the best exposure of the Anglesey Blueschist Belt. The Marquess of Anglesey’s Column at Llanfairpwllgwyn has recently been remnovated and reopens tomorrow (1 March 2024)

(All photographs by Gavin Rowlands)

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