Giant’s Causeway (Part 1): An introduction
Dr Trevor Watts (UK)
This is the first of two articles on the volcanicity of the Giant’s Causeway and the surrounding area. The Causeway itself is an area of basalt columns, about 100m or so across, jutting into the Irish Sea. A remnant of a vast ancient lava flow, it is located in a coastal strip that is lavishly scattered with other superb volcanic features. The whole area is both beautiful and fascinating, and neither spoilt in any way, nor over-crowded out of season. We (my wife Chris and I) went there because I had a few days’ work in Northern Ireland and it seemed like a good idea to combine this with a short break during an October, half-term school holiday.
The geology of the Giant’s Causeway
The long-held theory that the Causeway was created by an Irish giant called Finn MacCool in Middle Earth times has – sadly – been discredited. Around 60mya (in early Tertiary times), great masses of molten rock were rising from the depth of the earth’s mantle, probably centred beneath present-day Greenland. These nation-sized ‘lava-lamps’ are collectively considered to be a ‘hot spot’, now known as the ‘Iceland Plume’. They split the earth-wide continent of Pangaea apart in great cracks that were aligned roughly northwest to southeast. This was sufficient to split the land apart on a vast scale, beginning the opening of the Atlantic Ocean and creating an increasingly wide gulf between, on the one side, the Americas and, on the other, Europe/Africa.
In many areas, the rising magma forced its way sideways between pre-existing layers of rock. Its choice then was either to find another route to the surface or to stay where it was, cool down, solidify and form a ‘sill’. Wherever the molten rock (magma) reached the surface, it spread out in massive floods of lava. Initially, the lava poured out along the whole length of each crack – often many miles long – in ‘fissure eruptions’. Eventually, the cracks partially sealed themselves off, leaving openings here and there along their former line.
As lava poured from these and built up, so volcanoes were formed in broken rows. As it spread further afield, thick flows buried the ancient countryside for many miles around, spreading over great areas of what is now Northern Ireland, Scotland and the Faroes. Molten floods from related cracks occurred across much of the present North Atlantic, including around Iceland and Greenland. Similar basaltic landscapes and columns occur around Fingal’s Cave, on the Isle of Staffa, 130km to the north. They are thought to be of approximately the same age, but they are not considered to be from the same eruptive vent, as there are small but significant differences in the chemical profiles of the lavas at the different sites.
Exactly where most of the magma reached the surface in Northern Ireland has not been pinpointed with certainty. There are differing views. However, there are several places near the Giant’s Causeway (and further away), where you can easily see volcanic vents. These could be the source of the Giant’s Causeway magma and are certainly part of the same, overall eruptive scene.
Periods of major or lesser lava floods alternated with calm periods, when tropical forest and swamp re-formed and clothed the landscape. Inevitably, the next lava flood buried these and this cycle continued for several million years, with the greatest flood basalts apparently being produced 59 and 55mya.
The first major phase erupted several lava flows, which buried the land in layers a few metres thick. Many of these flows contained a lot of dissolved gas, which often became trapped as bubbles (or vesicles) towards the upper part of each flow, as they cooled and solidified. Many bubbles became filled with zeolite minerals and are referred to as amygdales. Layers of these vesicles and amygdales can also form near the base of each flow, as the lava reacts with heated groundwater. Therefore, these layers can often mark both the bases and tops of the flows and can easily be seen along parts of the Causeway, such as close to Windy Gap, adjacent to the Causeway itself. These early flood basalts are piled up, one on top of another, to great depths forming the Antrim Plateau. This is now the most extensive lava plateau in Europe, and is referred to as the ‘Lower Basalts’.
After this first major phase, there was a long period of relative inactivity. Hot, wet tropical and sub-tropical climates took over. Forests, swamps, rivers and lakes were rich in life, and layers of sedimentary rocks, such as lignite, mudstones and siltstones, were also laid down in places. During this period, the underlying lava was altered by weathering, often to the depth of many metres.
This resulted in a thick layer of red rock that is resistant in some places and more like a crumbly soil in others. In some locations, the alteration and disintegration of the lava did not quite seep into the rock sufficiently to affect the whole of the rock mass and there are small rounded masses of dark lava left, surrounded by the red rock. These are known locally as ‘Giant’s Eyes’ and they can be over 30cm in diameter. This red layer is known as ‘laterite’, from the Latin ‘later’ meaning brick or tile. In many places along the Antrim coast, it is very obvious as a bright, brick-red filling in the basaltic sandwich of the cliffs. It is rich in iron, hence the red colour. It also contains large proportions of aluminium, titanium and a weathered-out clay mineral known as gibbsite. The laterite layer, between the two major flood basalt phases, is often referred to as the ‘Interbasaltic Layer’.
After hundreds of thousands of years, there was a renewal of major volcanic activity. The floods of lava were much deeper than before, with 30m of deposits being the norm, rather than the exception. The lava spread across the dense tropical forests and swamps, as well as deserts and areas of limestone. It filled the valleys and smoothed out the landscapes, creating an ‘unconformity’ where it settled over the previous scenery.
The ground where the Causeway now lies was then a valley. It was flooded by a huge amount of lava in a single flow that completely filled the valley to a depth of 100m. This exceptional depth meant that the lava cooled very slowly. As it did so, it shrank slightly, forming shrinkage cracks like mud in a dried-out lake bed. These cracks penetrated deeply and formed at right angles to the cooling surface (either the base or the upper surface of the flow, or both). When the cooling of such lavas is slow, cracks consistently go very deep and gradually create columns of dark, fine-grained lava known as basalt. The slower the cooling, the more regular and clear the formation of the columns. In this region, the columns are mainly five or six-sided, but some are four, seven or eight-sided. One is triangular.
The Main Causeway flow (see below) was buried by later flows, collectively known as the ‘Causeway Basalts’. Some of these were equally thick over most of their extent and they form the upper parts of the nearby cliffs, the great faces of columns and the Causeway itself. In some places, the spreading lava encountered lakes, where it formed delta-like ‘feet’ of fragmented, glassy or pillow-shaped lava. Examples are found about one kilometre east of the Giant’s Causeway, at Port na Spaniagh. Others, from about the same period, are found on the Isle of Arran in Scotland and they are being formed in modern times every time lava from the volcanoes in the Nyiragongo area of the Congo flows into Lake Kivu.
Since the most recent eruptive times, vast amounts of land have been eroded away by the sea, rain, rivers and ice. The volcanic cones have gone, but the stumps remain in some places. The Giant’s Causeway itself has been etched out from one section of basaltic lava that was especially deep. Hundreds of metres of similar columns all around it have been eroded away over the past millions of years, leaving this stump – albeit a beautiful one.
Higher up, the erosion has created cliffs that display layers of the bright red tropical soils and dark lava flows. As well as the Causeway, many other basalt columns are exposed, sometimes as huge vertical curtains of columns, one on top of another. Here and there, they are eroded out into separate ‘chimneys’ or as the horizontal columns that form dykes, such as the ‘Camel’s Back’ in the bay below the Causeway Hotel (Location 2 on the sketch map).
Although the volcanoes are extinct in this region, related activity continues further afield. The Iceland Plume is still there and is frequently active, as witnessed by the Earth’s two greatest lava floods in historic times – from Eldgja in Iceland in 935 and from nearby Laki in 1783. During the latter, 14km3 of lava poured from the vents in a mere eight months. Eventually, the Laki crack became partly sealed and a series of separate volcanoes took over the eruptive activity. More recently, Surtsey, Heimaey, Vatnajokul, Loki, Eyjafjallajokull and Katla have provided the Icelandic interest. (Katla probably erupted in 2011, but all the activity remained beneath the ice – see: http://www.katla-volcano.co.uk/.) It is very likely that similar scenarios occurred in the Antrim area millions of years ago.
Arriving at the Giant’s Causeway
The principal access to the Causeway is through the cluster of buildings at the top of the cliff. From here, there is a long walking trail that drops down to sea level at the Giant’s Causeway. Beyond that, the trail rises and continues at a level almost half-way up the cliff. This magnificent trail is, tragically, closed at Port na Spaniagh (see below), so you have to return the way you came. And then you have a choice – either to descend slowly back to the Causeway whence you came or to climb the steps to the top of the cliff. There, you have a fine overall view of the Causeway and, from here, you can also continue your walk along the top of the cliffs, eastward for about 6.5km to Dunseverick Castle. From there, you can catch a bus back to the car park.
The Camel’s Back
Before setting off down the hill to the Causeway, there is a diversion to the left, eastwards along the cliff-top, for a view of a huge wall of basaltic rock known as the ‘Camel’ or the ‘Camel’s Back’. It was formed when magma forced its way up through the ground, creating great cracks, which it then filled. The Camel’s Back was one of the vertical vents that cooled slowly, forming well-defined basalt columns, known as dykes. As the columns form at right angles to the cooling surface, those on the Camel’s Back are horizontal, like a store of winter logs outside a cabin. The view of the Camel’s Back from the top of the cliff is good, but it is better from the beach.
There is not really a path down the cliff, but it is possible to scramble across the steep, grassy slopes from the main path onto the beach. From here, the horizontal columns are very clear and well formed. The basalt they are formed from is particularly hard, hence its resistance to weathering and its prominence above the surrounding softer rocks. Above them in the cliff, there is a very clear and thick layer of red laterite rock.
Some of this magma would have flooded onto the ancient surface as lava flows. Some would have stayed under the ground, forcing its way between the near-horizontal rock layers, forming sills and they give rise to similar-looking exposures to lava flows when they are exposed in cliff faces.
About 200m further along the cliff to the west, there are rocky bays and headlands. The entrance to one steep-sided gorge is partly blocked by a tall sea stack, which has more horizontal columns of basalt. It is another dyke, like the Camel’s Back, and is called the ‘Sentinel’.
From the beach, it is easy to get back to the main path at different points. Whether going all the way down along the road from the car park or re-joining the road lower down, the main route is steep, but well surfaced. It is about a kilometre and a half downhill to the Causeway or it is possible to catch the shuttle bus from the car park if the weather is bad. However, it’s best to walk.
At the first corner along the trail to the Causeway, there is a small exposure in a cliff face that was created when the track was cut out. It displays unusual rock formations. Here, the magma cooled in blocks, rather than tall columns. Later exposed under desert conditions, they eroded in concentric layers (or skins) from the outside. The process is known as ‘onion weathering’ or ‘spheroidal weathering’. Over the millennia, each layer gradually began to peel away under the influence of daily temperature changes, freezing and thawing, and the chemical action of occasional rainwater. In stages, the weathering ate deeper into the blocks. This tended to round them off, giving the appearance of onion layers.
The great granite tors of Bodmin Moor and Dartmoor in England are examples of similar rock formations, as are the kopjes of southern Africa, and there are some excellent examples on the Palisades, along the Columbia Plateau in Washington State, USA. All around the ‘onion-weathered’ rocks, there are beautiful and very distinct sections of the vivid red laterite soil. Close to this rock face, there are information boards that describe and illustrate the formation of the onion-peeling.
The Giant’s Causeway
Beyond ‘Onion Corner’, the path runs parallel to the foreshore of a small bay called Sandy Port – or Port Ganny – until it reaches the Giant’s Causeway itself (Location 4 on the map). The shuttle bus terminates here.
The Causeway is in three sections – the Little, Middle and Grand Causeways – each stretching further into the sea – and each is a tremendous collection of beautifully formed columns. They are etched with exquisite beauty by the waves and weather, some worn and smoothed, others seemingly fresh and sharp-edged. Visitors are free to wander at will over the whole area. In high season and the middle of the day, it is a crowded place. But go in the morning before 9:00am, or in the evening, and it is tranquil. We had mornings and evenings there almost alone in October 2011. The low evening sun on the rocks brought out the warm colours and the bright morning sun etched out the shadows of the columns.
There are nearly level platforms as perfectly fitted as if designed by a great landscape artist or architect. They pile up in great steps and ledges – high stages that overlook the angular edges of the shore. Most columns are around 30cm across, but some are twice that size. Although some barely reach above the waves, others tower proudly over the whole scene in great curtains of lichen-covered rock. As well as the vertical cracks that form the columns, there are also horizontal cracks that divide each column into segments.
These are also part of the shrinkage process. Generally, they are not perfectly at right angles to the column’s length and each division is made up of a concave/convex joint. So, when they are exposed at the surface, as on the platform of the Giant’s Causeway, about half of them are rounded upwards like a shallow dome and the rest are scooped out like a shallow bowl, often containing rain or seawater.
For all its beauty, it is a small area. Some people we met said they had walked past it, not realising that this was it and they had had to retrace their steps. Is it small? Yes, but it still contains about 40,000 columns.
Eventually, you can walk beyond the Giant’s Causeway, following the wide, tarmac pathway through a narrow gap in a section of tall columns. This section is known as the ‘Colonnade’ and it is formed by straight, nearly vertical columns that are among the largest in the area. These wider columns are the result of extremely slow cooling.
Port Noffer and the Organ
Going eastwards, there is a path along the shore of another bay known as Port Noffer. The path is still well looked after and surfaced with tarmac for a short distance. Along the shore of Port Noffer, there are several small dykes jutting into the sea like breakwaters. They are more resistant to erosion than the surrounding rock, but not much, and are eroded down to stumps that are scarcely seen, especially when the tide is high.
The path becomes a well-maintained clay track, as it begins to rise and starts to traverse the face of the cliff. Soon, you will come to a cluster of basalt columns that form a small vertical face of the cliff (Location 6 on the map). This group of columns is known as the ‘Organ’. They are extremely regular, impressively tall and slender, and they merge abruptly with the rough basalt above, where the molten rock did not cool at the same rate.
The rough cap rock is known as an ‘entablature’ or ‘upper pediment’. It indicates that the surface layers of the lava flow cooled fairly quickly to form a jumble of irregular shapes and a mass of disorganised lava. It protected the hot lava below from cooling too quickly and gave it time to form the regular columnar shapes. Here, the junction between the columns and the mass of jumbled lava above is exceptionally sudden, probably the result of very rapid cooling of the upper part of the flow by large quantities of water, such as from a flood or monsoon-like rain.
Beyond this, the trail becomes narrower and narrower. Few tourists go beyond the Organ, because they are here only for the obvious highlights of the area. Many have arrived on a day trip or have only allowed half a day for the whole experience. This is such a pity. The pathway continues for less than a kilometre beyond the Organ and it is well worth spending some time exploring to the end of the trail. It winds round the headlands along the base of the red laterite layer, offering wonderful views all the way.
In places, the rock contains very eroded examples of the onion-weathered boulders, known here as Giant’s Eyes (Location 7 on the map). In these, the basalt has weathered to a uniformly fine texture, but this process has not yet reached the centre of the boulders and so is left as less altered, darker ovoids in the main mass of rock.
There are superb views of basalt columns, including one set known as the ‘Harp’, because the columns are curved and shaped a little like a harp. Perched high above, another set of columns, known as ‘Chimney Tops’, is a section of cliff that has been left isolated by erosion (Locations 8 and 9 on the map).
The trail cuts through a narrow notch in one particularly sharp headland. When you go through, another glorious vista of sea and cliffs awaits you. There are gorgeous views of great colonnades of columns, dykes and crashing waves. However, 100m further on, the trail is closed. You can see the reasoning behind this, but not the sense. The trail itself actually continues for about another 5km along the face of the cliff, about half way up.
At first glance, it looks to be in good condition, but it has not been maintained and there is probably some risk of falling blocks. Further on, the path is becoming overgrown as neglect sets in. There have been small landslips and the path has been allowed to decay into an unsafe condition. This must be the most incredible sea cliff walk in the world – and it is closed. The path can be seen trailing increasingly faintly across the face of the cliff until it rises precipitously to the cliff-top, well past Benbane Head.
We stood where the path was closed off for an age, wishing we could go further and admiring the wonderful vistas. However, eventually we had to retrace our steps, back past the Organ. Before reaching the Giant’s Causeway, there is a fork in the path. The right one drops down back to the beach and the Causeway. We took the left one, rising slowly at first up the cliff, then up a series of steps to a point where you have a good overview of the Causeway. There are also several information boards about the area.
Then there is another choice. You can walk along the cliff-top back towards the hotel or you can go eastwards, also along the cliff-top, towards Benbane Head, about 7km away. And that is what we did.
Here, the views are absolutely stunning, both on a panoramic vista-scale and in the detail of columns, dykes and sea stacks. The ‘Amphitheatre’ is a wide, open bay with great cliffs of basalt columns in two major flows. The ‘Horseshoe’ is the nearly flat remnant of a sea stack, surrounding a tiny circular inlet that may have been a tube where lava was extruded, but then sank back. The Harp can now be seen from above, as a section of curved columns part-way up a cliff face. The curves are the result of continuing slow movement, when the lava was almost cooled and solidified. Above the Harp, there are the Chimney Tops, with one particular column standing proudly alone. From here, there are many tantalising views of the abandoned trail, half-way down the cliff. We could only imagine how good the close-up views are from along its decaying length.
Eventually, this cliff-top walk passes the scenic wonderland of Benbane Head, Bengore Head and Contham. After a couple of extremely muddy fields, the B146 road is reached. While waiting for the bus back to the Causeway Hotel, there may be time to peek at the ruins of Dunseverick Castle. This is located where the track meets the road. It was supposedly visited several times by St Patrick in the fifth century, and it was old even then. Besieged by Vikings in the ninth century, it was renowned as the end point one of the near-mythical ‘Royal Roads’ from Tara, the ancient seat of the Kings of Ireland. Little remains now – mainly one wall that General Munroe’s troops were unable to demolish during the Irish rebellion in 1641. And across the road, is the bus stop.
|The parts of this article comprise:|
|Giant’s Causeway (Part 1): An introduction|
|Giant’s Causeway (Part 2): Other volcanic highlights|