The origins of these extinctions are still being debated, but scientists think that most involved large spates of volcanism—giant rifts opening in Earth’s surface and spewing lava over periods of thousands or even millions of years. The volcanism probably triggered many other changes that led directly to extinctions, including anoxia, or a lack of oxygen, in the oceans as a consequence of ocean warming; ozone depletion, which resulted from volatiles released by volcanism; and wildfires caused by a heated climate.