GEOTHERMAL GRADIENT DETERMINATIONS IN 
THE LAKE SUPERIOR COPPER MINES 
By L. R. InNGEerRsoiyi 
UNIVERSITY OF WISCONSIN 
(Received February 10, 1932) 
ABSTRACT 
The Michigan College of Mining and Technology, in cooperation with the 
Calumet and Hecla Copper Company and the author, is carrying out a program of 
temperature measurements in the deep copper mines of Northern Michigan, extending 
the previous work of Agassiz and others. Temperatures are measured with mercury 
thermometers mounted in bakelite tubes, placed in drill holes in mine workings where 
the rock has been freshly exposed, special attention being given the effects of drilling, 
blasting, and other heat conduction considerations. Present results give as the average 
gradient from the surface to 5679 feet below (temp. 95.3°F), 1°F in 108.5 feet (0.0168° 
C/meter). The gradient is more nearly uniform than has sometimes been supposed. A 
preliminary attempt has been made at calculating the previous “thermal history” of 
this region. Diffusivity of specimens of the rock measures 0.0075 c.g.s., and on this 
basis caiculations of theoretical temperature-depth curves have been made for 25 
different assumptions of previous temperature conditions, and compared with the 
actual curve. Results as yet are inconclusive but indicate that at least 30,000 years 
have elapsed since the last glacial epoch, a longer period than usually assumed. ~ 
| Pica since the days of Kelvin’s theoretical discussion of the age of the 
earth there has been interest in measurements of the earth’s temperature 
gradient, and while radioactive discoveries have considerably diminished the 
importance of calculations on this basis there are nevertheless many other 
interests—some of them new—attaching to geothermal measurements. As 
a result a mass of thermal data has been and is being collected from deep 
mine, well and drill hole measurements in all parts of the world. The copper 
mines of the Keweenaw peninsula in northern Michigan offer a particularly 
inviting opportunity for such study because of their great depth and freedom 
from loca] heat sources. The pioneer measurements of Agassiz,! which re- 
sulted in an apparently unusually low gradient, are too well known to require 
comment, and Lane,? Van Orstrand? and others have also made valuable con- 
tributions along this line. 
Certain favorable opportunities, depending partly on the increasing depth 
of the mines, have for some time called for a reopening of this study and ac- 
cordingly Professor James Fisher of the Michigan College of Mining and 
Technology and Mr. Harry Vivian, Chief Engineer of the Calumet and Hecla 
1 Alexander Agassiz, Am. Jour. Sci. (3) 50, 503 (1895): Brit. Assoc. Adv. Sci. Report of 
71st meeting, p. 65 (1901). 
2 A.C. Lane, Bull. Geol. Soc. of Am. 34, 703 (1923). 
*C. E. Van Orstrand, Am. Jour. Sci. 15, 495 (1928). See also N. H. Darton, U. S. Geol. 
Survey, Bulletin 701, p. 50. 
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