The island of Réunion has been studied using data from airborne and shipborne magnetic surveys. The subaerial history of Réunion spans the last 2.1 million years. The Brunhes‐Matuyama geomagnetic reversal enables differentiation of volcanic rocks older and younger than 0.78 Ma. The lower submarine flanks are poorly magnetized and are interpreted as landslide deposits. The core of the island is composed of highly magnetized rocks. Piton des Neiges volcano is composed mostly of rocks older than 0.78 Ma. Only its western flank and central area include thick piles of younger rocks. Piton de la Fournaise is a highly and normally magnetized edifice, but its northern and eastern flanks are underlain at shallow depth by reversely magnetized formations. The latter are regarded as remnants of Les Alizés volcano, associated with the Grand‐Brûlé hypovolcanic complex. At the Matuyama‐Brunhes transition the island was composed of at least two main volcanoes (Piton des Neiges and Les Alizés) and perhaps also of a third old volcano (Takamaka) at the center‐north of the island. Piton de la Fournaise grows on the flank of Les Alizés and is a relatively young focus of volcanism. These volcanoes have had successive phases of construction and destruction. The analysis of magnetic anomalies over Réunion was decisive in defining a new coherent model for the evolution of the island.

The catastrophic lahars triggered by the 13 November 1985 eruption of the ice-clad Nevado del Ruiz volcano, Colombia, demonstrate that the interaction of hot pyroclasts with snow and ice can release 30–50 millionm3 of meltwater in 30–90 minutes. The 1985 eruption caused a 16% loss in area and a 9% loss in volume of snow, firn and ice. Turbulent pyroclastic density currents mechanically mixed with snow and produced meltwater at a rate of 0.5–1.6mms–1. Laboratory experiments suggest that turbulent, fluidized pyroclastic density currents exert mechanical and thermal scour, thereby efficiently transferring heat from hot pyroclasts to snow. Ice cap loss at Nevado del Ruiz continued between 1985 and 2000, representing a ∽52% decline in area and a ∽30% fall in volume. Ice 60–190m thick caps the east and southeast summit plateau, whereas an ice field < 30m thick and devoid of snow is retreating on the north, northeast and west edges. This asymmetrical distribution of ice reflects combined long-term effects of the 1985 eruption and of the post-1985 ice cap retreat. Should volcanic activity resume, steep-sided glaciers can fail and pyroclastic flows and surges can sweep the snowpack and generate mixed avalanches and lahars. Although the potential source of meltwater has decreased since 1985, extensive debris at the ice cap margins can be incorporated to future lahars.