On the trail of Shetland’s volcano
Allen Fraser (UK)
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.
Although it is hard to imagine today, some 350Ma 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.
Setting the scene
Eshaness’ story begins some 400Ma (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).
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.
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.
The beach itself is composed of bright red cobbles and shingle of Ronas Hill granite. It is speckled with occasional grey gneiss pebbles washed out of the glacial till, and also by dark brown and black cobbles. If we look closely at these dark brown and black cobbles, we notice small holes or cavities on the surface – some are empty and some are filled by a milky white substance.
These cobbles began life as basalt and andesite lavas spewing out of the Eshaness volcano. The cavities, known as “vesicles”, represent bubbles of gas that exsolved from the molten lava. Sometimes, hot watery solutions (100oC), which percolated through the cooling lavas, formed crystals within the vesicles. These milky-white crystals mostly belong to the mineral family of zeolites and are called “amygdales”. When hotter solutions (300oC to 400oC) percolate through rock, it carries the mineral silica in solution and, if you are really lucky, you will find this filling in the vesicles as agate.
At the west end of the beach, we make our first encounter with a lava flow – a narrow outcrop of black basalt filled with vesicles looking almost as fresh as the day it erupted. Further along the shore, we encounter a different type of rock. This dark red sandstone outcrops on the beach at Braewick where it is in contact with what was once a hot lava flow. A thin coating of iron oxide (rust) gives rise to the dark red colour on individual sand grains and tells us that this sand was once in a desert environment. If you look closely, you can see the sand grains form faint stripes across the rock. This is current bedding and indicates that this sandstone was deposited in a riverbed by flowing water. Although it was in a desert environment, the erupting volcano would have caused massive thunderstorms to build up. Downpours from these would cause rivers and streams to flow down from the volcano.
Close by, we find a very fine-grained sandstone laid down as alternating dark red and light grey bands. From this, we deduce that this sandstone was laid down in a (perhaps ephemeral) lake that often changed nature from iron oxidising to iron reducing. A little further along the coast below the high water mark, notice how the grey river-borne sandstone is sprinkled with angular fragments ranging in size from fingernail to fist. These angular fragments are airfall deposits from our volcano that landed in the wet sand during eruption phases.
As we walk back to the beach, notice that the rock sequences we have examined have been truncated by a jumbled mass consisting of many of the same rock types we have already examined. This conglomerate is an avalanche or “lahar” deposit and is probably due to the collapse of part of a volcanic slope. Here is more evidence of a stratocone volcano since these have inherently unstable sides and are prone to avalanche.
Not far from the café is the fascinating community museum of Tangwick Haa. As well as having wonderful exhibits of local history and culture, the museum has an interpretation of the geology of the area and more information on the Volcano Trail.
To get a closer look at how the Eshaness volcano built up, we head west to Stenness where there is a remarkably sheltered bay and beach. The shelter is due to the more resistant (to erosion) lava flows that make up the arms of the bay and the Isle of Stenness that lies across its entrance. Take a walk across the beach to the headland at the far side of the bay. This is formed from vesicular andesite lavas. Notice the ruins of the old fishing or “haaf” station that was active up until the end of the 19th century. The geology here provided a sheltered harbour for this type of fishing, which was carried out from open boats, and an excellent beach for splitting, salting and drying the fish for export.
On the Volcano Trail: the Lighthouse to Grind o’ da Navir
From Stenness, we head onto the north side of the Eshaness peninsula to the lighthouse. A good place to start the rest of our volcano tour is on the cliff top, at the point where the fence ends south of the lighthouse. During the last glacial period, which ended some 10,000 years ago, Shetland had its own ice cap that spawned glaciers grinding their way north-west across Eshaness. These have left much loose red and grey erratic material that contrasts quite markedly with the dark grey and brown lavas. Look out also for deep scratches (glacial striae) on the bedrock.
The sequence of rocks we see today have been gently folded into a NE-SW syncline (that is, U-shaped) that is tilted to the south-west. Therefore, as we walk north, we cross progressively older lava flows. The whole sequence has become jointed and faulted due to the cooling of the lava, and the sea has forced apart these joints and eroded the rock into geos, stacks and blowholes.
A walk along the north-west coast of Eshaness takes us through a suite of extrusive igneous rocks. These are sheets of lava that poured out of the crater, side vents and fissures to build the volcano upwards. Generally speaking, there are three types of lava at Eshaness: basalt (black to grey), andesite (brown) and rhyolite (buff to red). Among these lavas are air-fall deposits (pyroclastic rocks) piled up to form agglomerate. Pyroclastic rocks are mix of solid, semi-solid and molten material torn from the inside of the volcano and blasted high into the air from volcanic vents to land as volcanic ash and bombs onto the lava flows.
The top of the low cliffs around the lighthouse provides an excellent exposure of pyroclastic rocks showing us how they fell after being blasted from a volcanic vent. Sometimes, these would fall onto molten lava flows or back into a lava pond or lake in the volcanic vent. There are fine examples of this near the lighthouse. This area could well have been a side vent (called a “parasitic cone”) that grew on the side of the volcano.
(The Kirn O’ Slettans, below the lighthouse, is a deep blowhole in the cliffs; during storms, jets of water are forced high into the air through the blowhole.)
If we continue to walk along the cliff tops, north-east from the car park, the deep inlet of Calders Geo shows us how the volcano built up from successive layers of lava flows and pyroclastic rocks. In many cliff faces and sea-stacks, we can count individual lava flows stacked one upon the other. Lava shrinks as it cools, often forming columnar joints and cracks in the thicker flows. Jointing and partial collapse of a section of the lava has allowed the sea to open an underground passage leading to an inland cave that collapsed forming the Hols o’ Scraada.
Near the Blackhead of Breigeo, thin lava flows overlie a thick band of soft green material. This is a layer of extremely weathered lava that now provides a zone of weakness and preferential erosion allowing wave action to undercut and collapse the cliff face. The thin lava flows above are likely to be the result of fissure eruptions on the flank of the volcano. The headland beyond these flows is formed out of a thick, rhyolite lava flow. Rhyolite is a much more acid lava (with a higher silica content) than the others and is associated with highly explosive eruptions.
We end our cliff-top walk at the most dramatic coastal site in Shetland. The headland of the Grind o’ da Navir is built up from a distinctive type of volcanic rock known as “ignimbrite”. Ignimbrite forms as a result of deposition from incandescent hot ash flows (nuée ardente – literally “glowing cloud”) or pyroclastic flows (see above) that sweep down steep volcanic slopes. The momentum of a pyroclastic flow, once it is launched, can cause it to sweep up and over hills, covering huge areas, at speeds in excess of 100km per hour making this phenomenon the most lethal part of a volcanic eruption.
Pyroclastic flows are due to the gravitational collapse of part of the eruption column of the volcano or laterally directed explosive eruptions. These flows are not liquid but are a searingly hot, turbulent mixture of shards of volcanic glass or pumice and other volcanic fragments suspended in a gas matrix. Close examination of the ignimbrite rock shows that it is made up of lenticular fragments, often resembling a candle flame (“fiamme”). These fragments represent former hot, ductile, glassy blobs of pumice that were flattened and deformed by compaction as the avalanche left a hot blanket, many metres deep in its wake. There are classic examples of fiamme exposed at the Grind o’ da Navir.
The drama of the Grind o’ da Navir did not end with the extinction of the volcano but is continuing today, with another powerful force of nature now in the ascendant. The large amphitheatre in the headland, with its central pond, has been hewn out of ignimbrite by wave action. At the seaward side, part of the cliff has collapsed forming a gate 10m wide and 13m high, some 15m above the sea. The Old Norse name for this remarkable headland translates roughly as “The Gateway of the Borer”, a reference to the gateway that “da Grind” provides to the Atlantic between its twin bastions and also to the way in which storm waves thunder through the portal.
This staircase gateway allows the funnelling of huge storm waves that are thrown forward as fast-moving, green water. The velocity of the water has caused the rock to split along joints into large blocks. Some blocks measure 2.5m by 1.2m by 1.2m. These have been ripped up and ‘floated’ about 100m inland by gigantic waves to form an impressive beach that ridges up to 3.5m in height. The most recently moved blocks are on the seaward face of the first ridge, many of which were moved during the great storms of 1992 and 1993. The second and third ridges to landward have a scatter of fresh blocks but include large numbers of blackened and lichen-covered blocks. Comparison with photos held by the Shetland Museum shows that the bases of these ridges have been stable for the last century.
The most recent episode of wave quarrying took place during the storm of 12 January 2005 and the fresh scars from block removal are still clear in the notch on the seaward end of the pond. Fresh scars on the northern bastion show where large slabs were lost in the 1993 storm. This, together with the opening of the tunnel and geo to landward, suggests that ‘da Grind’ may suffer major changes in major storms in the 21st century including the draining of the pond.
Eshaness and its ‘Grind’ were, and still are, places of great beauty and high drama. The following is a nineteenth century eyewitness account of a winter storm:
“It is winter! The voice of the tempest is heard – no other sound. The sea-birds are cowering in their rocky homes – the fisherman has sought the shelter of his hut – the cattle have fled inland. The blinding spray is sent far over the Villions – the waves of the mighty Atlantic are hurrying towards the iron-bound coast. See that tall billow! It rises to the skies – now with the noise of thunder it falls upon the Grind, uptearing and upheaving vast masses of rock, which it carries, like so many pebbles, to the savage shore behind, where they rest with the spoils of other storms, a shapeless heap thrown together as by the hands of Titans” (Andrew McCrae, 1860).
References
Adrian Hall in Shetland Landscapes at www.fettes.com/shetland.
Andrew McCrae in Art Rambles in Shetland by John T Reid published by Edmonston and Douglas, 1869).