Geology · Granite hearts, exhumed

Benasque

Granite roots of a lost supercontinent, crowned by a dying glacier

Variscan intrusion (Maladeta batholith), exhumed by the Pyrenean orogenyGranite emplacement ~307–298 Ma (Late Carboniferous–Early Permian; U-Pb zircon 298.0 ± 2.4 Ma on associated dikes); exhumation during Pyrenean collision ~85–20 MaGondwana–Laurussia convergence (Variscan); later Iberia–Eurasia (Pyrenean)🌋 none — exhumed Variscan batholith — No active volcanoes. IGN (Instituto Geográfico Nacional), Spain's volcano-monitoring authority, monitors active volcanism only in the Canary Islands; the Pyrenees, including the Maladeta massif, have no Holocene volcanism. The Maladeta granite is a plutonic body that cooled at depth ~300 Ma — it was never a volcano.

The granite of the Benasque skyline is far older than the Pyrenees themselves. Around 300 million years ago, as the supercontinent Pangaea was being welded together in the Variscan orogeny, huge volumes of magma rose into the crumpled Paleozoic sediments of what is now the Axial Zone and froze at depth as the Maladeta batholith. Radiometric dating places its emplacement in the Late Carboniferous to Early Permian — U-Pb zircon ages of about 298 million years on porphyry dikes tied to the batholith pin the end of the episode. The intrusion is not one body but a complex of juxtaposed plutons of granodiorite and granite — the Aneto unit among them — each chemically zoned, cooking the surrounding slates and limestones into a metamorphic aureole. Those deep roots then sat buried for a quarter of a billion years. It took a second mountain-building event to resurrect them: the collision of Iberia with Europe between roughly 85 and 20 million years ago stacked and lifted the old Variscan basement into the Axial Zone, the crystalline spine of the Pyrenees. Erosion stripped away kilometres of cover, and because granite resists weathering better than the slates around it, the exhumed batholith now holds the highest summits of the entire range: Aneto at 3,404 m, with the Maladeta crest beside it and Posets (3,375 m), the second summit of the Pyrenees, across the valley. Ice finished the sculpture. Pleistocene glaciers ground cirques and staircases of ibones — bedrock tarns — into the granite, and the Little Ice Age left fresh moraine ridges that the trail to La Renclusa still crosses. What remains today is precious and vanishing: the Aneto glacier, the largest in the Pyrenees and among the southernmost in Europe, is in terminal decline. A 2023 study in The Cryosphere (Vidaller et al.), comparing 1981 photogrammetry with modern surveys, found the glacier lost 64.7% of its area in 41 years, thinning from a mean 32.9 m of ice to just 11.9 m by 2022, with no surviving accumulation zone and imminent fragmentation into three ice bodies. Trekkers on the Portillón route are watching the last act of Pyrenean glaciation. The massif saves its best trick for water. At the Forau de Aigualluts, a short walk from La Besurta, the entire meltwater stream of the Aneto glacier thunders into a karstic sinkhole and vanishes into a band of soluble Paleozoic limestone pinched against the granite. The naturalist Ramond de Carbonnières guessed in 1787 where it went; in 1931 the caver Norbert Casteret proved it by pouring fluorescein dye into the sinkhole. Hours later the water reappeared green — 4 km away and across the main Pyrenean divide, at the Uelhs deth Joèu springs in the Val d'Aran. Aneto's melt, born on the Mediterranean side of the watershed, secretly feeds the Garonne and reaches the Atlantic at Bordeaux.

What you can see on the trail

The Aneto glacier — the largest in the Pyrenees, in terminal retreat

📍 Aneto normal route above Refugio de La Renclusa, from the Portillón Superior

From the Portillón you overlook the biggest glacier left in the Pyrenees — and one of Europe's southernmost. Published surveys show it lost about two-thirds of its area between 1981 and 2022 and now averages only ~12 m of ice; the bare grey granite below the ice marks how far it reached within living memory.

Forau de Aigualluts — a river swallowed whole by karst

📍 Plan d'Aigualluts, an easy walk from La Besurta trailhead

The meltwater of the Aneto glacier plunges into this sinkhole and vanishes into a band of soluble limestone squeezed against the granite. Dye tracing in 1931 (Norbert Casteret) proved it re-emerges 4 km away in the Val d'Aran — as a source of the Garonne, bound for the Atlantic.

Little Ice Age moraines and glacier-polished granite slabs

📍 Trail from La Besurta up to Refugio de La Renclusa and toward the Portillón

The sharp-crested ridges of loose boulders you cross are moraines dumped by the Aneto and Maladeta glaciers at their recent maximum, only a few centuries ago; the smooth, scratched granite slabs between them were polished by the ice itself.

The ~300-million-year-old granite itself — from valley boulders to the Paso de Mahoma

📍 Throughout the upper Ésera valley; most dramatically on the Aneto summit ridge (Paso de Mahoma)

Every pale, speckled boulder here crystallized from magma some 300 million years ago, during the assembly of Pangaea — long before the Pyrenees existed. The airy granite knife-edge of the Paso de Mahoma is the exhumed heart of that ancient magma chamber.

Fun fact

Meltwater from the Aneto glacier, born on the Mediterranean side of the divide, secretly ends up in the Atlantic: it sinks into the Forau de Aigualluts and re-emerges 4 km away in the Val d'Aran as a source of the Garonne — proven on 19 July 1931, when Norbert Casteret poured some 60 kg of fluorescein dye into the sinkhole and watched the Uelhs deth Joèu springs turn green.

Rocks: granodiorite and granite — Maladeta batholith (incl. the Aneto unit) · Paleozoic metasedimentary host rocks — slates and schists with a contact-metamorphic aureole · bands of Paleozoic limestone/marble — the karst of Aigualluts · Quaternary glacial deposits — Little Ice Age moraines below the Aneto and Maladeta glaciers