The papers presented in this thematic set evolved from the 1993 annual two day meeting of the Volcanic Studies Group, held at the Open University. The papers are dedicated, as was the meeting, to the memory of Richard Thorpe who tragically died whilst on fieldwork on Lundy during August 1991. The meeting shared the title of this thematic set and included a session on magmatic processes at destructive plate margins and one on within-plate magmatism. These two broad subjects were chosen to reflect Richard's wide-ranging interests in petrological and geochemical problems and this is also reflected in the subjects of the following six papers.

An obituary to Richard Thorpe appeared in the 1991 Annual Report of the Geological Society and it is not my purpose to repeat what was said about Richard there. However, he will probably be best remembered for his edited book entitled Andesites and related rocks and an unusual but topical aspect of the petrology of andesites is the subject of the first paper in this thematic set. Matthewset al. investigate the influence of sulphur redox reactions on the oxygen fugacity of calc-alkaline magmas from Lascar in Chile and Pinatubo in the Phillipines. Both of these volcanoes are known for the presence of primary igneous anhydrite, implying unusually oxidizing conditions during crystallization and high concentrations of sulphur in the parental basaltic magma. They conclude that large quantities of SO2 gas are liberated during mixing between basaltic and more silica-rich magmas. Observed T-fo2 relationships in both volcanoes

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