A Natural Order of Volcanic Succession: Midnineteenth-Century Field-based Models of Von Richthofen, King, and Dutton

During the latter half of the nineteenth century, pioneering studies of volcanic rocks were undertaken by Ferdinand von Richthofen, Clarence King, and Clarence Dutton in the Great Basin and California Cascades. They developed hypotheses on the petrogenesis of magmas in order to explain what they deemed to be a regular progression of volcanic rock types through time during the Cenozoic Era. There was general agreement that effusive volcanism resulted in an orderly sequence of magmas: propylitic, andesitic, trachytic, rhyolitic, and basaltic. Von Richthofen related this phenomenon to the tapping of magma sources at ever-greater depths during orogenic activity, the basic magmas being denser and occupying a lower level within the Earth. King felt that so-called ‘lakes of fusion’ developed within the Earth as a result of erosional unloading of its crust; and that within such lakes gravity separation resulted in silicic magmas floating upon basic. In the secular refrigeration of the globe these temporary lakes would necessarily occur successively at greater and greater depths and the secular changes that recorded themselves in the subtle petrographical distinctions by which the various acidic and basic members could be distinguished were in each case an expression of depth. Dutton, by contrast, saw no connection between unloading and volcanism and established a modified model for the genesis of magma, emphasizing the importance of magma provenance being controlled by the fusion of variable source rocks. All felt that such temporal changes in magma type were universal and provided a basis for a natural classification of volcanic rocks.