2014

NM4684 : Triangulation Pillar on An Sgùrr

taken 10 years ago, near to Galmisdale, Eigg, Scotland

Triangulation Pillar on An Sgùrr
Triangulation Pillar on An Sgùrr
The trig point is at 393 metres, close to the highest point which is a metre higher!
Sgùrr of Eigg

The distinctive shape of An Sgùrr of Eigg is instantly recognisable from almost all directions. From the east it appears as a dramatic pillar of rock, while from north and south it appears as a long ridge with a sharp culmination.

Superficially the rock of An Sgùrr resembles the rock columns of Staffa and the Giant's Causeway. However closer inspection of the rocks reveals that they are quite different. Staffa and the Giant's Causeway are made up of smooth, fine-grained rock, while the rock of An Sgùrr consists of a glassy matrix with angular crystals in it. These crystals are relatively resistant to erosion, so they stand proud when the matrix is eroded away, resulting in a rough knobbly surface which is wonderful to walk on.

An Sgùrr is in fact a unique geological feature, the last result of the volcanic activity which created much of the Inner Hebrides as movement of the tectonic plates led to the opening of the Atlantic Ocean some 60 million years ago.

About 60 or 61 million years ago large amounts of basalt lava erupted from volcanoes along the rift that was developing between the European and American tectonic plates. Basaltic lava is low in silica, and therefore runny, and it flows readily, flooding large areas. These basalts blanketed much of what are now the Inner Hebrides and Northern Ireland with multiple layers of lava, forming a plateau of basalt rocks several hundred metres deep. Beinn Buidhe, on the north and east of Eigg, is a remnant of this plateau.

Over the next couple of million years, erosion by water and weather removed some of the basalt, and, in particular, a river carved a deep valley in the plateau where An Sgùrr now is. Eventually this river deposited material including trees and boulders in the river bed, resulting in the formation of a conglomerate in the valley bottom.

About 58 million years ago the youngest volcano in the Hebrides erupted catastrophically. The magma forming this volcano was chemically very different from the older basalts. It had a high silica content, so it was sticky and did not flow easily. Over time it gummed up the volcano, the pressure of more magma rising through the crust, and eventually the volcano exploded, rather like Mount St Helens in 1980.

Volcanic explosions like this produce nuées ardentes, 'glowing clouds', which are made up of hot gas, ash and rock fragments. They behave like fluids, and when they flow are termed pyroclastic flows. They travel at high speeds over long distances and can even climb over some hills. This eruption filled in the valley, and rapidly cooled to form pitchstone, with a glassy matrix and embedded crystals.

After this, erosion continued to remove the basalt plateau, but the pitchstone filling the valley is much more resistant to erosion than the basalt, so over millions of years the pitchstone ridge remained, standing proud above the surrounding landscape, and resulting in the distinctive feature we now recognise as An Sgùrr.

Further reading

'The Geology of Eigg', by John Hudson and Ann Allwright, Department of Geology, University of Leicester, 2003.

'The emplacement of a large, chemically zoned, rheomorphic, lava-like ignimbrite: the Sgurr of Eigg Pitchstone, NW Scotland', David J Brown and Brian R Bell, Journal of the Geological Society, Vol 170, pages 753-767. LinkExternal link




Creative Commons Licence [Some Rights Reserved]   © Copyright Anne Burgess and licensed for reuse under this Creative Commons Licence.
This photo is linked from: Automatic Clusters: · Trig Point [6] · Triangulation Pillar [2] ·
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NM4684, 51 images   (more nearby 🔍)
Photographer
Anne Burgess   (more nearby)
Date Taken
Monday, 28 April, 2014   (more nearby)
Submitted
Tuesday, 6 May, 2014
Subject Location
OSGB36: geotagged! NM 4633 8470 [10m precision]
WGS84: 56:53.0447N 6:9.9497W
Camera Location
OSGB36: geotagged! NM 4634 8469
View Direction
Northwest (about 315 degrees)
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SW S SE
Image classification(about): Geograph
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