Ben Nevis is popular with walkers and climbers for the range of opportunities it offers, from a relatively gentle ascent via its southern slopes to much more complex climbing routes on the north.
Britain’s highest mountain, with its peak reaching just 1,344 metres, is small mountain in global terms. But Ben Nevis is actually a remnant of a much higher range of mountains which were formed through the process of a continental collision that occurred over 400 million years ago.
The Surrounding Rocks
Ben Nevis lies on the south-western side of the Great Glen. The rocks surrounding the mountain and its associated peaks were laid down many millions of years ago on the floor of a now-vanished ocean, the Iapetus Ocean. They were originally sedimentary rocks, deposited on the ocean floor by rivers and currents and comprised largely mudstones and limestones.
Over time, tectonic movement of the Earth’s plates led to the closure of the Iapetus. As part of this process, the sedimentary rocks on the ocean floor were dragged beneath the surface (subducted) and buried up to 20 kilometres beneath a newly created continent. Subjected to enormous pressures and temperatures they were changed (metamorphosed). Sandstones became a rock called quartzite and mudstones were transformed into schists.
The Ben Nevis Volcano
As rocks were subducted, increased pressure leads to extreme heat that brings many forms of rock too the melting point. The resultant molten rock, known as magma, is more buoyant than its surroundings and rises, either solidifying within the surrounding rock as an intrusion or reaching the surface and erupting as lava. This process is typical of plate margins and is evident at many points on the Earth’s surface today.
Around 425 million years ago, exactly such a process occurred at what is now Ben Nevis. A large magma chamber formed within the surrounding metamorphosed rocks (the contact between the two rock types is visible at various locations in Glen Nevis) and fed a highly active volcano which threw out copious quantities of lava onto the Earth’s surface.
The process was highly complex and involved at least four separate intrusions; two intrusions involved a type of rock known as granite and two intrusions involved a related rock known as diorite. Both of these cooled slowly within the Earth and contain relatively large crystals. Overlying them, the lava cooled rapidly at the surface and, as a result, this matter was composed of smaller crystals.
The volcano was large and active – so active that it eventually "blew its top" in a way similar to the explosion of Krakatoa in 1883, and collapsed inwards. Large slabs of the overlying lava flows – as much as 2.5 km, according to McKirdy et al – collapsed into the underlying magma chamber. Here, within the cooling and solidifying rock, they were preserved.
The Exposure of the Ben Nevis Granite
The formation of the magma chamber took place deep underground and the massive volcano which was to become Ben Nevis has long disappeared. The scale is difficult to comprehend but at its maximum it may have been comparable in height to the present day Himalayas, according to a Geotrails leaflet.
Erosive processes over 400 million years ago, including many cycles of glaciation and erosion through rain, wind and frost, have removed the top layer, including almost all of the lavas other than that section which collapsed into the magma chamber and has, as a result, been preserved. What is visible today is effectively the roots of the volcano, with many kilometres of the overlying rock having been eroded away.
Glaciation has also modified the landscape by carving the glens which surround the Ben Nevis complex (including Glen Nevis to the south and Glen Spean to the north) depositing loose material on slopes and along the valley bottom. But the geology of Ben Nevis remains clear – the granite roots of an ancient volcano intruded into a landscape of heavily metamorphosed rock, with a summit composed of preserved lava flows.
Sources
- Lochaber Geopark Geotrails leaflet Glen Nevis.
- McKirdy, Alan et al Land of Mountain and Flood: The Geology and Landforms of Scotland Birlinn and Scottish Natural Heritage, 2009.
- Miller, Suzanne “Mountain Geology – Starting at the top: Ben Nevis” The Edinburgh Geologist, 2004.
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