Glen Coe, Lochaber: Scene of an ancient caldera collapse

Perhaps Scotland’s most (in)famous glen, Glen Coe cradles evidence of a series of volcanic eruptions and an ancient caldera collapse

Glen Coe, from the north-west, across Loch Leven. Image: Jim Blair.

Glen Coe, the ‘glen of weeping’, has a terrible history of inter-clan hostility and murder, not least the infamous massacre of 1692. But for geologists, this is where the idea of ‘cauldron subsidence’, now called ‘caldera collapse’, was born over 100 years ago.

The Glen Coe volcano was erupted through and onto a land surface of Dalradian metamorphic rocks about 420 million years ago. The volcanic episode began when hot, molten magma was injected sideways into layers of wet, sandy sediment, probably a lake bed, to form horizontal layers of igneous rocks known as sills. Later, the same magma poured out onto the surface to form lava flows. These lavas are composed of a rock type called ‘andesite’ (named after the Andes mountains of South American) and are well displayed on the steep slopes of Aonach Dubh.

Lying on top of the andesites is a great pile of other volcanic rocks reaching to the tops of the highest mountains. These later volcanic rocks include ‘rhyolite’ lava which erupted as a slow-moving sticky flow, volcanic ash known as ‘tuff’ and a rock known as ‘ignimbrite’, which formed from the debris produced by explosive ‘glowing cloud’ eruptions of extremely hot gas and rock (pyroclastic flows).

A series of faults controlled the development of the volcano. There were often long periods between eruptions, allowing rivers to develop and erode the tops of some of the lava flows. As the eruptions continued, the roof of the magma chamber underneath the volcano started to collapse along fault lines, in a ‘piecemeal’ fashion, to form a surface crater, called a ‘caldera’. Glen Coe was the first place in the world where ‘cauldron subsidence’ was recognised in ancient volcanic rocks.

When first formed, the caldera was bounded by a roughly circular ‘ring fault’, but due to crustal extension since then, the caldera now measures about 13 kilometres by 8 kilometres. You can actually put your hand on the fault near Stob Mhic Mhartuin!

Throughout the current Ice Age, over the last 2 million years, glaciers have cut down through the volcanic rocks in the glen to form the dramatic landscape of corries and arête ridges, and giving geologists excellent cross-sections through the lava flows.

Text contributed by Jim Blair, Lochaber Geopark

Find out more
Access: The main A82 trunk road runs through Glen Coe. There are several large car parks. The major footpaths are for experienced hill-walkers, the rest is for serious rock climbers! Most of the area is managed by the National Trust for Scotland which has an excellent Visitor Centre at NM 112 575 on the A82, 2 kms south-east of Glencoe village.

“Glen Coe”, Lochaber Geotrails leaflet, Lochaber Geopark.

“Ben Nevis and Glencoe”, ‘A Landscape Fashioned by Geology’ series, Stephenson, D and Goodenough, K, British Geological Survey (BGS) and Scottish Natural Heritage (SNH), 2007. Print or pdf versions available from SNH. A good introduction.

British Geological Survey; “Glencoe caldera volcano, Scotland”, ‘Classical areas of British geology’ series, Kokelaar, BP, Moore, ID, Bradwell, T and Stephenson, D, 2006, and map “Glen Coe”, bedrock 1:25,000 Geology Series, 2005. A comprehensive description.

Glen Coe, a glaciated, mountain landscape, photograph taken over the ‘Three Sisters of Glen Coe’, with the arête ridge of Aonach Dubh in the foreground. Image: © Neil Slinger.

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