PIONEERING VOLCANOLOGY IN THE WESTERN GREAT BASIN AND CALIFORNIA CASCADES

In the 19th century, pioneering studies of volcanic rocks in the western Great Basin and California Cascades were undertaken by Josiah Whitney, Ferdinand von Richthofen, Clarence King, Ferdinand Zirkel, Israel Russell, Joseph LeConte and J. S. Diller. Whitney, Russell, LeConte and Diller focused on mapping and field description. Von Richthofen and King theorized on petrogenesis of magmas to explain what they deemed to be a regular progression of volcanic rock types through time during the Cenozoic Era. Zirkel was employed by King to undertake petrographic study of rocks collected during his survey, a first application of such techniques in America. There was general agreement that effusive volcanism resulted in the orderly production of magma: “propyllitic”, andesitic, trachytic, rhyolitic, basaltic. This von Richthofen related to tapping of magma sources at ever-greater depths [the basic magmas being denser and occupying a lower level within the earth] during orogenic activity. King felt that within the earth 'lakes of fusion' developed as a result of erosional unloading of the earth's crust, and that within such lakes gravity-separation resulted in silicic magmas floating upon mafic: "In the secular refrigeration of the globe these temporary lakes .. would necessarily occur at greater and greater successive depths. .. the secular changes that recorded themselves in the subtle petrographical distinctions by which the various acidic and basic members can be distinguished .. are in each case an expression of depth .. since with the secular recession of temperature the critical shell in which fusion would be induced by erosion must constantly retire from the surface downward". The tapping of such lakes which themselves evolved through time could explain von Richthofen's orders. Both felt that temporal changes in magma type provided a basis for a natural classification of volcanic rocks. While their speculations proved to be untrue, their willingness to search for causes to explain volcanic successions in the American West constituted an advance in theoretical petrology. Shortly after, Hague & Iddings (1885) determined petrographically that: "..the so-called Pre-Tertiary and the Tertiary rocks pass by insensible gradations into one another, and that evidence of a difference in geological age is wholly wanting."