Flow of Heat in the Front Range, Colorado

Modern conceptions of the structure of the earth9s crust and of the distribution of radioactivity lead to an expectation of a greater flow of heat to the surface in mountains than in lowlands. An exceptional opportunity for testing this expectation is provided by the data obtained by the geologists of the Bureau of Reclamation during the construction of the Alva B. Adams Tunnel under Rocky Mountain National Park. This tunnel, 13 miles long, at a mean altitude of 8300 feet, passes under the Continental Divide, more than 12,000 feet above sea level. Some 70 observations of temperature have been reduced with the purpose of finding the flow of heat. Corrections have been applied for the topography on several different hypotheses regarding the physiographic history. The corrected vertical gradient of temperature lies between 24°C/km, on the assumption that the present topography has persisted indefinitely, and 20°C/km, on the assumption that the surface features have been derived from an old-age surface by erosion and by uplift of 7000 feet uniformly distributed over the last million years. If the time of evolution is taken as 4 million years, the corrected gradient is 22°C/km. An uncertainty of about 1°C/km results from lack of reliable data concerning the surface temperature. Thermal conductivity has been measured in the laboratory for 123 samples of rock from the tunnel; these rocks are chiefly granites, gneisses, and schists. The variations of conductivity with rock type and with position along the tunnel are found to be insignificant; the mean value of conductivity is 0.008 cal/cm·sec·deg. The heat flow is then computed as between 1.6 and 1.9 microcal/cm 2 ·sec, with a “best value” of 1.7 microcal/cm 2 ·sec. This is believed to differ significantly from the best values for a “normal” sea-level crust, which fall close to 1.1 microcal/cm 2 ·sec. With conventional assumptions as to thickness and density of the layers of the normal crust, the difference of heat flow may be accounted for in terms of mountain roots having a mean radioactivity of the same order as that of granites or intermediate rocks. The observed heat flow is consistent with the doctrine of mountain roots and with an approximately uniform distribution of radioactivity throughout the “granitic” layer. A few of the other possible interpretations are briefly discussed.