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Elgol Minor Intrusions

Skye, Inner Hebrides, Scotland

Elgol

There is considerable current interest in the emplacement of magma into sedimentary basins, not least because of oil exploration in extreme settings such as the NW continental shelf of Britain where basalt is an important constituent of the subsurface geology. 

Elgol minor Intrusions

The coast of SW Skye near the village of Elgol is a great place to gain understanding of intrusion mechanisms where there are arrays of basaltic dykes and sills (so-called “minor intrusions”) within the Jurassic strata. 

These rocks have been baked by the nearby Cuillin igneous centre. However, the rock relationships at Elgol tell of a magmatic history that pre-dates the baking. 

SW of Elgol pier most of the Jurassic strata are sandstones. They contain a swarm of dykes, many of which are composite indicating multiple intrusions along the same zone of weakness. Close to the pier there is a sill, intruded into shales. 

The strata are folded with entrained wings of basalt indicating considerable deformation caused by bulldozing of basaltic magma during intrusion. 

NE of Elgol pier, in a succession of interbedded shelly limestones and shales, are arrays of dykes and local sills. These show back injection of shale into the basalt and complex flamey intrusion margins. There’s even a plug of breccia agglomerate made up of Jurassic strata and basalt blocks. 

These relationships all indicate that the shales were not fully lithified at the time of intrusion. The agglomerate probably formed by a catastrophic steam explosion of formation waters super-heated as intrusions came in. 

As with so many of the best igneous outcrops in NW Scotland, the dykes at Elgol have been repeatedly targeted by indiscriminate and disfiguring rock coring of prominent outcrop faces – in the name of geological research. There are plenty of more discreet faces, especially in the intertidal area of the section, that could have been used.

Text: Professor Rob Butler

100 Great Geosites

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Both images © Rob Butler

Earth Science Week 2017

Earth Science Week
October 7-15
Theme: 'Our Restless Earth'