Flood basalt eruptions are a type of large-scale volcanic activity, both in terms of extent and duration, that can occur on land or on the ocean floor. A flood basalt may continue to erupt for tens of thousands - possibly millions - of years and the lava can cover hundreds of thousands of kilometres. Large plateaux and mountains can result from the huge volume of newly surfaced rock. The huge volume of lava is accompanied by a similarly large release of volcanic gases such as sulphur dioxide and carbon dioxide. These can affect climate and cause acid rain, so flood basalts are thought to be a potential cause of mass extinctions.