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The blueschist

The blueschist on which the Marquis of Anglesey’s Column stands is one of the most iconic rocks on Anglesey. It was formed some 570 million years ago when two of the tectonic plates that made up the Earth’s crust collided. At this time, what is now Anglesey was on the margin of an extensive ocean in the Southern Hemisphere quite close to the South Pole.

The rock which became the blueschist was originally formed by volcanic eruptions on the seabed near the middle of the ocean. The eruptions created distinctive masses of volcanic rock like inflated balloons. These are called pillow lavas. The pillow lavas were then carried towards the edge of the ocean on the moving tectonic plate until they reached the collision zone (technically called a subduction zone). The plate carrying the pillow lavas was then pushed down towards the Earth’s interior into regions of high temperature and pressure. When they had descended to about 35 kilometres below the Earth’s surface the rocks reached a temperature of 500°C where they were altered to blueschist. The heat and pressure at this depth changed the minerals that made up the rock (including the mineral glaucophane which gives the rock its blue colour) and created its platy, sheet-like structure.

Much of the blueschist was then pushed further down below the Earth’s crust where it finally melted and was destroyed. But some was pushed upwards towards the surface and was preserved. Eventually, after hundreds of millions of years, uplift and erosion of the rocks above exposed the blueschist at the surface where we see it now. Incredibly, despite its long, dramatic journey, this rare blueschist still displays the balloon-like shapes of the original pillow lavas.

The rock beneath the Column is probably the best place to study the rock but excellent exposures are also to be found in road cuttings along the A55 stretch through Llanfair PG and inland outcrops around Pentraeth, Llansadwrn, Llanddona and Wern y Wylan.

The Marquis of Anglesey’s column was not constructed from blueschist. It was made of blocks of limestone, as were the two bridges across the Menai Strait,which is found on the northeast coast of Anglesey. This limestone was formed some 330 million years ago from sediments laid down in a shallow sea. Anglesey had by then drifted north and was at the Equator, so its climate was tropical. The tropical waters were full of coral reefs and diverse fauna and flora – much like the coasts of Florida and the Bahamas today – so the limestone is rich in fossils which you can see on the coast at Moelfre.

Professor Colin Jago Dr Margaret Wood