Fenner, Clarence Norman

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Fenner, Clarence Norman

(b. near Clifton, New Jersey, 19 July 1870; d. near Clifton, 24 December 1949)

petrology, volcanology.

Fenner was the son of William Gruff Fenner and Elmina Jane Carpenter Fenner. He received the degree of Engineer of Mines from the School of Mines of Columbia University in 1892. After fifteen years of experience in the field he returned to Columbia to earn the M.A. in 1909 and the Ph.D. in 1910. On graduation he joined the staff of the Geophysical Laboratory, Carnegie Institution of Washington, remaining there until his retirement in 1938. Fenner then returned to his childhood home, where he lived with his brother Herbert and continued his petrological studies until his death.

Fenner’s principal contributions to petrology are his experimental determination of the thermal stability of the various polymorphs of silica; field description, chemical analysis, and structural and theoretical study of the great eruption of Mt. Katmai, in Alaska (1912); recognition of a type of basalt crystallization leading to iron enrichment; and a physicochemical theory of rock solution and of ore deposition by gaseous emanations. In addition he investigated uranium and thorium minerals bearing on the age of the earth and devised chemical methods for their separation and analysis. During World War I, Fenner was in charge of the optical glass plant of the Spencer Lens Company at Hamburg, New York. He succeeded in putting optical glass on a production basis, thereby helping to establish in the United States that industry which had formerly been reliant on Germany.

In the laboratory Fenner investigated silica, the principal constituent of silicates and of the earth’s crust. He showed that the lowest-temperature form of SiO2—quartz—inverts to tridymite at 870° ± 10°C. and that tridymite inverts to cristobalite at 1470° ± 10°C. He found that the velocity of transformation of one form of silica into another was very sluggish and that one stable form did not always pass directly into the next most stable form, but progressed through successive steps. Fenner attributed the appearance of tridymite and cristobalite in some natural occurrences to this process. The transformations in the laboratory were speeded up by the use of a sodium tungstate flux. Although it has been suggested that this technique may have led to contamination of the material and hence to incorrect values for pure SiO2, the values of transformation ascertained by Fenner have been confirmed and accepted. His concern with equilibrium, high precision, and detailed definition of products—as well as the application of laboratory data to natural occurrences—set a sound foundation for subsequent study of all silicate systems.

Fenner was a member of the 1919 National Geographic Society expedition to Mt. Katmai, Alaska, to study the violent eruption of 1912, believed to be the second largest energy release by a volcano in historic time. In 1923 he returned to the Katmai region as the leader of an expedition sponsored by the Geophysical Laboratory, Carnegie Institution of Washington.

The principal deduction that Fenner drew from these field studies and subsequent laboratory work was that assimilation of andesitic wall rock by rhyolitic magma had proceeded by means of escaping volatile substances whose exothermic reactions underwent continually increasing acceleration which finally led to explosion (1950, p. 604). The large volume of ejecta was laid down mainly in the Valley of Ten Thousand Smokes as dust and gas mixtures—incandescent tuff flows—issuing through fissures in the valley from a sill or very similar body of magma in the underlying sedimentary strata. The many fumaroles, after which the valley was named, were presumed by Fenner to be due to the continued evolution of gases from that body of magma.

Chemical analyses of the various rocks carried out by Fenner demonstrated that the quantitative variation of each chemical oxide is essentially linear for the series of intermediate rocks. He regarded the nearly linear relationship as evidence of the primary role of assimilation in the formation of rocks intermediate in composition between the end members andesite and rhyolite. On a much later occasion he agreed in part with Norman L. Bowen that the deviations from linearity for some of the rocks suggest that crystal differentiation had played some part in the production of the Katmai rocks.

Fenner’s emphasis on assimilative processes was also apparent in his study (1938) of a rhyolite flow that he described as following a valley in an eroded basalt surface on Gardiner River, Yellowstone Park, Wyoming. The older basalts were impregnated and mobilized, according to Fenner, by the rhyolite magma, and the compositions of the resulting soaked rocks were represented by straight lines between the basalt and rhyolite. It was his belief that in their origin in the depths of the earth, the two magmas would have formed a conjugate pair of immiscible liquids (1948, p. 500). (It is interesting to note that at an early stage of his field studies, in 1914, he had been impressed by the apparent chemical attack of magma injected into country rock in New Jersey.)

Fenner assigned volatiles a major role in the assimilation process, a view no doubt acquired from his experimental work with George W. Morey in water-containing systems (1917). He also assigned a primary role to gaseous emanations in transporting metals from a magma into surrounding rocks, thereby producing some types of ore deposits (1933). Further concern with volatiles led to studies (1934, 1936) in the geyser basins of Yellowstone Park, Wyoming, where he deduced the manner in which emanations modified the composition of the rocks through which they had passed and were in turn modified. He made a major contribution to the understanding of ash flow mechanics through his emphasis of their high mobility because of entrapped gases.

Fenner was a leading antagonist of Norman L. Bowen’s prevailing theory that magmatic fractionation and differentiation lead to silica and alkali enrichment. His own arguments led Fenner to believe that differentiation of magma proceeded in the direction of iron enrichment, producing ferrogabbro, not granite. Later work showed that both Bowen and Fenner were correct in their views, if the partial pressure of oxygen under which the magma crystallizes is taken into consideration. The “Fenner trend” obtains where the partial pressure of oxygen is relatively low in a magma, whereas the “Bowen trend” obtains if the same magma crystallizes under a relatively high partial pressure of oxygen. The “Fenner trend” was particularly well illustrated in the layered intrusion of Skaergaard, Greenland. Fortunately, both men remained friends; their discussion of the subject in debate was carried out only in print after thoughtful review.

Although Fenner was most experienced as a field petrologist, he had a deep understanding of the limitations of experimental studies and theory that purported to elucidate natural phenomena. He applied the data on simplified systems bearing on rocks and related theoretical deductions with great caution, relying mainly on geological field relations and petrographic observations. Even his own strong support of the roles played by gaseous emanations and by assimilation was tempered by the belief that other processes may be of quantitative importance.

Fenner was elected, with Norman L. Bowen and Joseph P. Iddings, to the Petrologists’ Club of Washington in the year of its founding, 1910. Although he was a quiet and unassuming man, the records of the club indicate that he was not reluctant to debate the issues with other petrological leaders such as Whitman Cross, Esper S. Larsen, Jr., Frederick E. Wright, Adolf Knopf, and Henry S. Washington.


Fenner published widely, on a variety of subjects. On silica polymorphs, see “The Stability Relations of the Silica Minerals,” in American Journal of Science, 4 (1913), 331–384; and “The Relations Between Tridymite and Cristobalite,” in Transactions of the Society of Glass Technology, 3 (1919), 116–125.

The Katmai area of Alaska is discussed in “The Katmai Region, Alaska, and the Great Eruption of 1912,” in Journal of Geology, 28 (1920), 569–606; “The Origin and Mode of Emplacement of the Great Tuff Deposit in the Valley of Ten Thousand Smokes,” in National Geographic Society, Contributed Technical Papers, 1 (1923), 1–74; “Earth Movements Accompanying the Katmai Eruption,” in Journal of Geology, 34 (1926), 673–772; and “The Chemical Kinetics of the Katmai Explosion,” in American Journal of Science, 248 (1950), 593–627, 697–725.

Two articles on the Gardiner River area of Wyoming are “Contact Relations Between Rhyolite and Basalt on Gardiner River, Yellowstone Park,” in Bulletin of the Geological Society of America, 49 (1938), 1441–1484; and “Rhyolite Basalt Complex on Gardiner River, Yellowstone Park, Wyoming: A Discussion,” ibid., 55 (1944), 1081–1096.

Additional field studies are “The Watchung Basalt and the Paragenesis of Its Zeolites and Other Secondary Minerals,” in Annals of the New York Academy of Sciences, 20 (1910), 93–187; “The Mode of Formation of Certain Gneisses in the Highlands of New Jersey,” in Journal of Geology, 22 (1914), 594–612, 694–702; and “Pleistocene Climate and Topography of the Arequipa Region, Peru,” in Bulletin of the Geological Society of America, 59 (1948), 895–917.

The basalt fractionation trend is discussed in “The Crystallization of Basalts,” in American Journal of Science, 18 (1929), 225–253; “The Residual Liquids of Crystallizing Magmas,” in Mining Magazine, 22 (1931), 539–560; and “A View of Magmatic Differentiation,” in Journal of Geology, 45 (1937), 158–168.

On immiscibility in magmas, see “Immiscibility in Igneous Magmas,” in American Journal of Science, 246 (1948), 465–502.

Articles on the role of volatiles include “The Ternary System H2O—K2SiO3—SiO2,” in Journal of the American Chemical Society, 39 (1917), 1173–1229, chemical study by George W. Morey, microscopic study by Fenner; “Hydrothermal Metamorphism in Geyser-Basins of Yellowstone Park, as Shown by Deep Drilling,” in Transactions of the American Geophysical Union, 15 (1934), 240–243; and “Bore-Hole Investigations in Yellowstone Park,” in Journal of Geology, 44 (1936), 225–315.

Ore deposits are discussed in “Study of a Contact Metamorphic Ore-Deposit. The Dolores Mine at Matehuala, S. L. P., Mexico,” in Economic Geology, 7 (1912), 444–484, written with J. E. Spurr and G. H. Garrey; “Pneumatolytic Processes in the Formation of Minerals and Ores,” in American Institute of Mining and Metallurgical Engineers, Ore Deposits of the Western States (New York, 1933), pp. 58–106; and “The Nature of the Ore-Forming Fluid: A Discussion,” in Economic Geology, 35 (1940), 883–904.

On the optical glass industry, see “The Technique of Optical Glass Smelting,” in Journal of the American Ceramic Society, 2 (1919), 102–145; and “The Use of Optical Pyrometers for Control of Optical Glass Furnaces,” in Bulletin of the American Institute of Mining and Metallurgical Engineers, no. 151 (1919), 1001–1011, and in Pyrometry (New York, 1920), pp. 495–505.

Hatten S. Yoder, Jr.