Geology of Glen Coe
An ancient supervolcano, sculpted by ice. The violent geological forces that created Scotland's most dramatic landscape.
The Supervolcano
420 million years ago, Glen Coe was the site of a massive volcanic complex — a supervolcano that dwarfed anything erupting on Earth today. The eruptions were colossal, producing thick layers of lava, ash, and pyroclastic flows that built up a mountain range rivalling the Andes.
Then the roof caved in. The magma chamber beneath the volcano emptied during eruptions, and the entire mountain collapsed inward, creating a cauldron subsidence — a vast ring-shaped depression. Glen Coe is one of the best-preserved examples of this geological phenomenon anywhere in the world.
The ring fault — the fracture line where the mountain collapsed — can still be traced around the glen. It's visible in the rock faces of the Three Sisters and the Aonach Eagach ridge. You're literally walking inside a collapsed volcano.
The Rocks
Rhyolite
The pink-grey rock that forms Buachaille Etive Mòr and much of the Three Sisters. A volcanic rock formed from silica-rich lava. Excellent for rock climbing due to its rough texture and solid holds.
Andesite
Dark volcanic rock found throughout the glen. Named after the Andes, though Glen Coe's andesite predates South America's mountains by hundreds of millions of years.
Quartzite
The pale, glittering rock on the summit of the Buachaille and other peaks. Incredibly hard — it resists erosion, which is why these mountains still stand while softer rocks have worn away.
Granite
The grey granite of Rannoch Moor formed deep underground as magma cooled slowly. Exposed by hundreds of millions of years of erosion.
The Ice Ages
If the volcano created the raw material, the ice ages sculpted it into the landscape we see today. Over the last 2.5 million years, repeated glaciations carved the mountains into their current dramatic forms.
- The U-shaped valley: Glen Coe's broad, flat-bottomed valley was gouged by a glacier over a kilometre thick.
- Corries: The bowl-shaped hollows high on the mountainsides (like the Lost Valley) were carved by smaller glaciers that clung to the peaks.
- Arêtes: The razor-sharp Aonach Eagach ridge was formed as glaciers carved corries on both sides, leaving a knife-edge between them.
- Rannoch Moor: The vast, flat moor to the east was scraped clean by the ice sheet, leaving a landscape of bare rock, peat, and thousands of small lochans.
- Erratics: The huge isolated boulders scattered around the glen were dropped by retreating glaciers around 11,000 years ago.
See It For Yourself
The NTS visitor centre in Glen Coe has excellent geological displays and a 3D model of the cauldron subsidence. The Geological Trail — a waymarked walk from the visitor centre — takes you past key rock formations with interpretation panels.
For the best overview of the volcanic ring fault, climb to the Lost Valley (Coire Gabhail) and look back at the surrounding ridgelines. You're standing inside the caldera of a 420-million-year-old supervolcano.