This book records the author’s experience in measuring physical processes at Hawaiian craters and in mapping changes at the craters themselves. Parts 1 and 3 are hitherto unpublished diaries of 1912–1913 and May 1924 at Kilauea Crater. Part 2 is a condensation of 26 years of Hawaiian volcano observations and experiments. Parts 4 and 5 are map studies of crater evolution. Part 6 is a review of crater classification and theory of magma, and the author’s geologic conclusions. Part 7 is a summary, with suggestions on earth structure, and the seismology, physics, and chemistry of volcanism, mainly limited to magma. The motive governing the assemblage of observations, maps, sections, diagrams, curves, photographs, reviews, and narrative histories in this discussion of craters is to model a statement for the first half of the twentieth century outlining the field volcano science that has grown up since James Dwight Dana published his Characteristics of volcanoes in 1891. Like that work this one is based on Hawaiian experience, with the method of permanent observatory work supplanting Dana’s excellent but partly vicarious recording. This has permitted more continuity than was possible to Dana, but it is likewise one man’s work studying volcanoes for half a century. Crater changes were the main interest of Professor Dana. That therefore is a reasonable theme whereon to base a new approach to field volcanology. Geophysics is growing and is the author’s main interest. He is frankly imaginative, radical, and experimental in his outlook. His model in scepticism was Clarence Edward Dutton, who made mistakes also, but none the less laid foundations whereon to build. The word crater is not used in any limited sense, and the title of the book follows the precedent of many authors, including Dana, in generalizing from Hawaii. The title places action before structure. Hawaii is typical of primitive process of crater development. Geographical and lunar processes of comparative volcanology are repeatedly referred to. In contrast to some geologists the author is guided by conviction from experience that volcanic action is alternately magmatic and phreatic. Hawaiian craters illustrate both processes. Tumefaction and engulfment alternate. The first is magmatic, the second is structural break-down admitting water. Any theory based on an open conduit is to the author inconceivable for a crusted globe restraining sun matter in its core.