Volcanic activity has the ability to affect global climate on still
longer time scales. Over periods of millions or even tens of millions of
years, increased volcanic activity can emit enormous volumes of greenhouse
gases, with the potential of substantial global warming (Pickering & Owen,
1994; Rampino & Volk, 1988). However, the global cooling effects of sulphur
dioxide emissions (Officer & Drake, 1983) will act to counter the greenhouse
warming, and the resultant climate changes remain uncertain. Much will
depend upon the nature of volcanic activity. Basaltic outpourings release
far less sulphur dioxide and ash, proportionally, than do the more explosive
(silicic) eruptions......