Ben Nevis – remnant of a lost volcanic landscape (lecture)

For three summers between 2014 and 2016, field geologists worked alongside professional climbers and botanists to undertake a new survey of the geology and Alpine flora found on the North Face of Ben Nevis, Britain’s highest mountain. The survey was co-ordinated by the Nevis Landscape Partnership, a charitable trust established in 2003 to help guide and manage opportunities for visitor enjoyment and appreciation of the wider Nevis area. Data on the geology and botany was gathered on iPhones using a digital compass clinometer FieldMove Clino, and the data was then transferred to the software application Move for further analysis and model building.

In 2015, geological field mapping was extended to include the whole of the late Caledonian Ben Nevis Igneous Complex (~430 Ma) and the late Precambrian Dalradian metasedimentary country rocks. The results of the new field mapping and 3D model building have provided important insights into the geometry, emplacement and preservation of the plutonic and volcanic rocks in this classic area of world renowned geology.

Structural data indicate that the plutonic rocks forming the Ben Nevis Igneous Complex have a laccolithic (blister like) form with a gently domed roof and was fed by magma rising up steep-sided NE-SW trending fissures in the core of the Appin Syncline. The summit region of Ben Nevis consists of late Silurian to Early Devonian age volcanic rocks originally interpreted as a thick sequence (>600m) of andesite lavas and agglomerates that were down-faulted during caldera subsidence. New field mapping has revealed that the volcanic rocks consist largely of volcaniclastic debris flows, and extensive block and ash flow deposits with minor air-fall tuff units. There is no evidence of any andesite lava flows or a volcanic vent. The volcanic detritus was derived from a volcanic centre situated to the NW of Ben Nevis, perhaps several tens of kilometres away. The rocks forming the summit region of the mountain have been re-interpreted as a large roof pendant or keel of the former late Silurian to Early Devonian volcanic land surface that once covered much of the SW Highlands of Scotland. Without the granites of the Ben Nevis Igneous Complex surrounding and protecting the volcanic rocks from recent glacial erosion, there would have been no evidence for the remnant landscape now preserved at the summit of the highest mountain in Britain.