Bore-Hole Investigations in Yellowstone Park

Abstract

In connection with the study of thermal phenomena of Yellowstone Park carried out by the Geophysical Laboratory for several years, two bore holes were drilled in geyser basins. During the progress of this work, temperatures and pressures were measured and observations were made on underground structures shown by the cores. Subsequently the cores were studied in the laboratory by chemical and petrologic methods to ascertain the mineral transformations effected by hydrothermal circulation, and their significance. Structural features, such as porosity and fractures, together with thermal conditions found, help to explain the underground circulation. The data enable us to picture the manner in which meteoric waters, falling upon adjacent plateau areas, penetrate to deep levels, are met by magmatic exhalations at high temperature, are diverted upward, and appear at the surface as hot springs and geysers. Light is thrown upon the mechanism of geyser action itself, which is found to be a rather more complicated process than has been supposed. Metasomatic alteration of the rocks has been important. A considerable variety of new minerals has been formed, and the composition has been much changed. The most notable effect found in the cores has been the addition of silica and the molar replacement of soda and lime of the feldspars by potash. The results of this study supplement and confirm those obtained by E. T. Allen in his study of the composition of the thermal springs, carried out as part of the same project. From this assembly of concordant data we are enabled to trace out with considerable confidence a series of reactions that apply not only to the rocks rendered accessible by the drill holes but to those at greater depth, and to deduce the manner in which the original magmatic emanations, dissolved in meteoric waters, have modified the composition of the rocks through which they have passed, and in turn have been modified.