On the trail of Shetland’s Volcano

For a land area of just 1,468km2, yet within a staggering 2,731km of coastline, Shetland has probably the most complex and diverse geology and geomorphology to be found anywhere in the World. Part of Shetland’s Geopark plan was a suggestion from the community of Northmavine that a geological gateway be established to their area at Mavis Grind, and a volcano trail be set up around the dramatically beautiful Eshaness. This project is well underway and should be completed later in 2007.

Although it is hard to imagine today, some 350mya ago, the peninsula of Eshaness was a fire and lava-belching volcano. In fact, the name “Esha Ness” comes from the Old Norse language and means the “Headland of Volcanic Ashes”. The beaches and cliffs of Eshaness show many fine examples of the rocks that formed in this ancient volcano and tell us something of the environment in which the volcano grew.

Eshaness peninsula

Setting the scene

Eshaness’ story begins some 400mya (in the Devonian period) when three of the Earth’s tectonic plates converged and eventually formed a huge continent now referred to as Pangaea. This collision threw up the Caledonian Mountain chain that was originally of Himalayan proportions but which rapidly began to erode. Rivers carried the erosion products (sediments) into lakes that formed in valleys between the mountains and on the plains below the foothills of the mountain chain. At this time ‘Britain’ lay in equatorial latitudes so the rocks we see exposed today were often laid down in environments that varied from semi-arid to desert. The Eshaness volcano (or volcanoes) probably formed at the north end of a broad valley containing a great lake (referred to as Lake Orcadie).

Glacial till Braewick beach

The volcano

Composite cone volcanoes are the most dangerous type on the planet because the magma that creates them has moderately high viscosity and a highly volatile content (water and carbon dioxide gas) that often decompresses explosively on reaching the surface. A typical cone-shaped volcano (a “stratocone volcano”) builds up, partly from the solid products of explosive eruptions and partly from molten lava flows. Eshaness was a volcano of this type and would have had this classic shape. We know this because, in the cliffs, we can see wonderful examples of the products of both violent eruptions and lava flows.

In the millions of years after the Eshaness volcanic province became extinct, erosion continued, Pangaea drifted north and broke up when the Atlantic formed, sea levels rose and fell, and ice ages came and went. Unfortunately, the central cone of the Eshaness volcano is now eroded away but the rocks exposed along the coast show us, in wonderful cross section, how the volcano was built up and the nature of the environments in which it grew.

Vesicular basalt Braewick

On the volcano trail: Braewick to Stenness

We start our tour of Eshaness with a visit to Shetland’s best pebble beach on the south side of the peninsula at Braewick where there is access from the excellent Braewick Café and campsite. The beach is banked up at its eastern end by a thick deposit of glacial till. The till is studded with a variety of cobbles and boulders scraped from the bedrock by a glacier flowing from Shetland’s own ice cap between 20,000 and 10,000 years ago.

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