NM4885 : Druim an Aoinidh and An Sgrr

taken 8 years ago, near to Kildonnan, Highland, Great Britain

Druim an Aoinidh and An Sgùrr
Druim an Aoinidh and An Sgrr
The nearer cliffs are columns of basalt, and An Sgrr is formed of pitchstone with a similar jointing pattern.
Sgrr of Eigg

The distinctive shape of An Sgrr 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 Sgrr 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 Sgrr 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 Sgrr 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 Sgrr 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 nues 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 Sgrr.

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



Columnar Basalt

Natural columns of basalt are a fairly familiar sight in views of Staffa and the Giant's Causeway, but they occur in many other places as well.

When molten rock, called magma, is erupted on the surface of the earth, it is termed lava. Lava cools relatively quickly, so there is no time for large crystals to develop, and the resulting rock is fine-grained.

Basalt forms from cooling of lava with a low silica content and a high content of heavy elements like iron and magnesium. Such lava is very runny, and spreads out to cover large areas with successive flows, building up a flat-topped pile of layers that fills in and covers over the pre-existing landscape.

As it cools, the rock contracts, and it develops cracks. It forms hexagons because hexagons are the only shape that can completely fill a void without sharp corners. If you like, they are Nature's compromise between circles, where all parts of the surface are equidistant from the centre, and squares, which can also fill voids but have corners much further from their centres than the middles of their sides are.

The cracks propagate at right angles to the cooling surface, resulting in the formation of hexagonal columns. As the cracks go deeper into the lava flow, they can be deflected, for example if they lie on top of an uneven cold surface, forming curves and complicated patterns of columns.

Not every column is exactly hexagonal, but if you counted the number of sides per column in any given flow, the average number of sides would be six.

Creative Commons Licence [Some Rights Reserved]   © Copyright Anne Burgess and licensed for reuse under this Creative Commons Licence.
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NM4885, 70 images   (more nearby search)
Photographer
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Date Taken
Thursday, 1 May, 2014   (more nearby)
Submitted
Friday, 13 June, 2014
Geographical Context
Geological interest 
Subject Location
OSGB36: geotagged! NM 4864 8510 [10m precision]
WGS84: 56:53.3352N 6:7.7049W
Camera Location
OSGB36: geotagged! NM 4891 8523
View Direction
West-southwest (about 247 degrees)
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