Commerson Crater, Volcanic caldera in Saint-Joseph, Réunion, France

Commerson Crater is a broad, shallow caldera in the Piton de la Fournaise massif on Réunion, within Saint-Joseph, outside the Enclos Fouqué. Its inner slopes are gentle, built of reddish-brown and grey rock in horizontal bands of solidified lava, and the uneven rim rises against the sky. Low scrub grows on the floor and lower slopes.

The caldera is named after Philibert Commerson, the French explorer and naturalist. Its eruption produced several hundred million cubic metres of lava, far more than historical Piton de la Fournaise eruptions such as 15 million m³ at Saint-Philippe in 1986. That flow partly filled the bed of the Rivière des Remparts, which runs south to Saint-Joseph, 22 km away.

The crater is named after Philibert Commerson, the French explorer and naturalist who sailed with Bougainville; the name was given in his honour. Its lava is described as "primitive": the magma rose from a deep chamber without evolving in a shallower reservoir, and carries olivine cumulates (dunite) picked up on the way, found in the flows and sometimes in the ejecta.

Commerson Crater sits outside the Enclos Fouqué on the Piton de la Fournaise massif, in the municipal territory of Saint-Joseph. The rim is reached on foot; the photo shows a broad bowl with gently sloping inner sides, not a sheer drop. Rainfall is heavy during tropical storms, so check conditions with local authorities before setting out.

Cyclone Hyacinthe in January 1980 dropped 6.43 metres of rain on Commerson Crater in 15 days, the heaviest rainfall from a single tropical cyclone recorded anywhere. The crater also holds the second-place figure: Cyclone Gamede brought 5.51 metres in 2007. No other place has received even 3.50 metres from one cyclone.