Sgrr of Eigg :: Shared Description

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

by Anne Burgess
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51 images use this description. Preview sample shown below:

NM4684 : Triangulation Pillar on An Sgùrr by Anne Burgess
NM4584 : Puddles in the Pitchstone by Anne Burgess
NM4485 : Sgùrr of Eigg by Anne Burgess
NM4884 : Poll nam Partan by Anne Burgess
NM4684 : Sgùrr of Eigg by Anne Burgess
NM4884 : Shore Cottage and An Sgùrr by Anne Burgess
NM4584 : Gully on An Sgùrr by Anne Burgess
NM4787 : Blàr Dubh by Anne Burgess
NM4786 : Blàr Dubh by Anne Burgess
NM4584 : Celandine Carpet by Anne Burgess
NM4584 : Carpet of Celandine by Anne Burgess
NM4584 : Sgùrr of Eigg by Anne Burgess
NM4885 : An Sgùrr and Old Manse by Anne Burgess
NM4684 : An Sgurr, Eigg from the entrance to the Sound of Mull by Michael Jagger
NM4885 : The Glebe by Anne Burgess
NM4584 : Pitchstone on the Ridge of An Sgùrr by Anne Burgess
NM4584 : A Steep Scramble by Anne Burgess
NM4784 : Galmisdale House and An Sgùrr by Anne Burgess
NM4684 : The Nose by Anne Burgess
NM4584 : Sgùrr of Eigg by Anne Burgess
NM4885 : Druim an Aoinidh and An Sgùrr by Anne Burgess
NM4784 : Galmisdale Farm by Anne Burgess
NM4485 : Sgùrr of Eigg by Anne Burgess
NM4584 : Fallen Rocks at Grulin Uachdrach by Anne Burgess
NM4784 : Waymarker by Anne Burgess

... and 26 more images.

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Created: Sun, 4 May 2014, Updated: Thu, 5 Jun 2014

The 'Shared Description' text on this page is Copyright 2014 Anne Burgess, however it is specifically licensed so that contributors can reuse it on their own images without restriction.

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