NM4884 : Poll nam Partan

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

This is 1 of 5 images, with title Poll nam Partan in this square
Poll nam Partan
Poll nam Partan
In the foreground is a fine beach of white shell sand with ripples of dark volcanic sand. This is part of the shore of Poll nam Partan, 'pool of the edible crabs'. In the background is An Sgrr.
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

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NM4884, 71 images   (more nearby search)
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Date Taken
Thursday, 1 May, 2014   (more nearby)
Friday, 13 June, 2014
Geographical Context
Coastal  Geological interest 
Subject Location
OSGB36: geotagged! NM 4890 8489 [10m precision]
WGS84: 56:53.2308N 6:7.4371W
Camera Location
OSGB36: geotagged! NM 4899 8488
View Direction
WEST (about 270 degrees)
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Image classification(about): Geograph
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