George Patrick Leonard Walker (1926–2005) was a key figure in the development of volcanic geology and volcanology through his exceptional and broad-reaching contributions. The early part of his career was dominated by an innovative long-term study of the volcanic geology of eastern Iceland and he continued to contribute to the volcanic geology of many areas worldwide. The second part of his career was pivotal in turning volcanology from its previous descriptive style into a modern quantitative science through his quantitative studies of pyroclastic deposits, lava flows, hypabyssal intrusions and volcanic processes. His success rested on his flair for meticulous observation, insistence on measurement, and keen intuition to generate major advances in understanding. He had the ability to merge systematic and comprehensive data sets with novel conceptual models to yield fundamentally new insights. His work was characterized by its extreme originality and broad scope, and forms the underpinning of much of our modern understanding of how volcanoes erupt. He was an inspiration to many colleagues and students. He influenced volcanology not only by research, but also by his genius as a teacher.