La Réunion has been shaped by volcanic activity and is characterized by its rugged orography. Two volcanoes form most of the island, the dormant Piton de Neiges, whose ~3070 masl high summit is the highest peak of the island and is located about 10 km east of Maïdo observatory, and the very active Piton de la Fournaise (~2630 masl), about 40 km south-east of the observatory. Piton de la Fournaise is a basaltic hot spot shield volcano with one effusive eruption every ~10 months on average . While no studies investigated the mercury degassing of Piton de la Fournaise, at least one study attributed high soil mercury concentrations on La Réunion Island to the island's volcanic history .