HOSPERS, JAN

(b. Veendam, Netherlands, 19 June 1925; d. Göteborg, Sweden, 17 December 2006),

geocentric axial dipole, geophysics, geomagnetic reversal timescale, geomagnetism, paleomagnetism.

Hospers made several major contributions to paleo-magnetism. He presented significant arguments in support of the sequential reversals in polarity of Earth’s magnetic field (field reversals) and argued against the view that reversely magnetized rocks could be explained in terms of self-reversals. He also provided the first extensive empirical support for the geocentric axial dipole (GAD) hypothesis, the hypothesis that the axis of the Earth’s magnetic field, a dipole, when averaged over several thousands of years coincides with Earth’s axis of rotation. The empirical support he marshaled in favor of GAD, which extended back through the Miocene, provided a firm base on which Kenneth M. Creer, Edward Irving, and Stanley Keith Runcorn at Cambridge, and other paleomagnetists could present arguments favoring mobilism. Hospers also constructed a rudimentary polarity (reversal) timescale, and estimated the time taken for the geomagnetic field to reverse. His work was crucial for the later development of the radiometric polarity (reversal) timescale. He accomplished all of this in just three years (1950–1953) while working on his PhD at Cambridge University.

The Pre-Cambridge Years . Hospers’ father, Johannes Hospers, was a teacher; his mother, Marchiena Heilina Slim, a nurse. Hospers had one sister, Hanni Jansen Hospers, who was four years older than he. Hospers received his primary and secondary education at Veendam. He attended the University of Groningen (1945–1948), where he took courses in geology and physics, a somewhat unusual combination at the time. Hospers was most influenced by Philip Henry Kuenen, then head of the Department of Geology at Groningen. Kuenen instilled him with a lifelong interest in sedimentology. Hospers also spent three months in 1946 studying with the geologist Eugene Wegmann at the University of Neuchâtel, Switzerland. After receiving his BSc in geology from Groningen, he began studying at the University of Utrecht, where he worked with four prominent figures: Felix Andries Vening Meinesz (geophysics), Martin G. Rutten (general geology), Reinout Willem van Bemmelen (economic geology), and Wijnand Otto Jan Nieuwenkamp (mineralogy and petrology).

While at Utrecht he was invited on an expedition to Iceland in summer 1950, and was admitted to Cambridge University to work on a PhD in geophysics. Rutten, planning to study Iceland’s volcanics and tectonics during summer 1950, invited van Bemmelen to accompany him, and they then invited Hospers to go with them. Icelandic lava flows were notoriously difficult to correlate, and van Bemmelen, who knew a little bit about remanent magnetism, suggested that Hospers should measure the intensity of their remanent magnetization and use it for correlation. Meanwhile, Hospers, an indigent student, needed funding to pursue a PhD. He applied for and was awarded a Royal Dutch/Shell scholarship, which allowed him to study geophysics at Cambridge in the Department of Geodesy and Geophysics.

The Cambridge Years: Hospers’s Major Contributions to Paleomagnetism . Receiving the scholarship, Hospers entered Cambridge in fall 1949, still planning to use magnetization intensity as a stratigraphic tool to correlate Icelandic lava flows during summer 1950. He did not plan to study reversal of the geomagnetic field or use paleomagnetistism to test continental drift or polar wandering.

Ben C. Browne, then head of the Department of Geodesy and Geophysics at Cambridge, became and remained Hospers’s official dissertation supervisor, even though he never worked in paleomagnetism. Of crucial importance was that, before leaving for Iceland, Hospers became friends with Runcorn. Runcorn was interested in paleomagnetism and had just arrived in January 1950 as assistant director of research in the department; he influenced Hospers most at Cambridge. He encouraged Hospers to return to Iceland in 1951, introduced him to Sir Ronald Aylmer Fisher, and helped with other contacts. In return, Hospers convinced Runcorn of the reality of geo-magnetic reversals.

Hospers returned from his first collecting trip to Iceland with twenty-two samples. He found that there were significant groupings and approximately half were reversely magnetized. But there was considerable scattering of directions and he realized that to analyze them he needed a statistical procedure. Runcorn asked Fisher to help. Fisher developed the statistical analysis and even did the calculations for Hospers’s first paper (1951). Fisher’s statistical method was not published until 1953, but was made available to Cambridge workers in 1950. It is difficult to overestimate the importance of his method for the development of paleomagnetism.

Hospers submitted his first paper in June of 1951. Samples from Quaternary flows were normally magnetized; those from younger Tertiary lava flows were reversely magnetized, and those from older Tertiary lava flows were normally magnetized. He argued for reversals of Earth’s magnetic field, proposing the geomagnetic field had been reversed during the later Tertiary. He found no macroscopic and microscopic differences between normal and reversed samples. He eliminated local phenomena such as pressure, reheating, recrystallization, and lightning, because he thought it inconceivable that such processes should affect only the reversed uppermost Tertiary lava flows and not the overlying normal Quaternary flows. He ruled out tectonics; if the reversely magnetized younger Tertiary lava flows had flipped over, the older Tertiary ones would have done so too, but they were normally not reversely magnetized; tectonic inversion was no explanation. Given the estimate that the average time between successive Icelandic lava flows was a thousand years, Hospers proposed that it took five thousand years for a reversal to occur and that the geomagnetic field had been reversed for at least twenty-five thousand years; there were four lava flows between the oldest reversed Tertiary lava flow and youngest normally magnetized Tertiary lava flow, and twenty-six lava flows that had formed while the field was reversed during the later Tertiary. He also found that regardless of polarity the differences between the mean direction of magnetization of his samples and the theoretical dipole field (GAD) were not significant and were closer to the GAD than the present field.

Runcorn encouraged Hospers to return to Iceland in summer 1951 and also to obtain samples from the Eocene Antrim basalts of Northern Ireland. Hospers returned from Iceland with six hundred samples. Using Fisher’s statistics, Hospers calculated the mean direction of the magnetization of volcanic units, including several others studied by A. Roche, Stanislaw A. Vincenz, and G. M. Bruckshaw and E. I. Robertson, and compared them with the direction of the present field and that of GAD. Seven out of eight units agreed better with the GAD than with the present field. The eight units, listed from youngest to oldest were Early Quaternary lava flows from western Iceland (Hospers); Plio-Pleistocene, and Mio-Pliocene lava flows from the Plateau Central, France (Roche); Miocene lava flows from northern and southwestern Iceland (Hospers); tholeiite dikes of northern England of Oligocene or Miocene age (Bruckshaw and Robertson); Oligocene dikes, necks, and sills from Plateau Central, France (Roche); Eocene lavas from Northern Ireland (Hospers and H. A. K. Charlesworth); and Eocene lava flows from Scotland (Vincenz). The result that did not fall, within error, along the GAD field was the one from the older Eocene rocks of Northern Ireland. Hospers concluded that the “measurements show that taken over periods of several thousands of years the magnetic pole centres on the geographic pole. This has been so since Miocene times (approximately 20 x 10⁶ years ago)” (1954a, p. 119).

Hospers identified four normal and four reversed periods from the present through the Miocene, and proposed a polarity timescale. He estimated that normal and reversed periods lasted from 250 to 500 thousand years, and that it took 5 to 10 thousand years for a the geomagnetic field to undergo a reversal.

In arguing for field reversals, Hospers had to contend with the possibility that some rocks might become magnetized in a direction opposite to that of the ambient field; they may be self-reversed. The French physicist Louis-Eugène-Félix Néel had been asked in 1949 by John Graham, a paleomagnetist from the United States, if rocks could undergo such a self-reversal; Graham was unhappy about the idea of field reversals. Néel proposed four self-reversing mechanisms, and within a year the Japanese paleomagnetists Takesi Nagata and Seiya Uyeda found a self-reversing rock. Hospers first noted that his normal and reversed samples were each associated with definite stratigraphic levels, precisely what should be expected to give field reversals. He then showed that none of Néel’s proposed mechanisms applied to his Icelandic rocks. Obtaining a sample of Nagata and Uyeda’s self-reversing rock, he heated it up in the laboratory, and it did become magnetized antiparallel to the ambient field. He then did the same with his Icelandic rocks; none acquired a reversed magnetization; all became magnetized parallel to the ambient field.

The Post-Cambridge Years . Hospers obtained his PhD from Cambridge in 1953, and took a job with Royal Dutch Shell, where he remained until 1963, working in Turkey (1953–1954); Venezuela (1955–1959); Nigeria (1958–1962); and Rijswijk, Netherlands (1962–1963). He returned to academe in 1962, taking a part-time position as an associate professor in geophysics at the University of Utrecht. He then took a position as a full professor for two years in the Department of Physics, University of Alberta, Edmonton, Canada, then moved to the University of Amsterdam in 1965, where he remained as a full professor of solid-earth geophysics until 1975. He then became a full professor of applied geophysics, the first in Norway, at the Norwegian Institute of Technology, University of Trondheim. Much of his work there pertained to the North Sea, studying its basement thickness to help locate oil and gas. Interestingly, Hospers here returned to sedimentary studies, his first love.

Hospers was married to Kerstin Barbro Helga Tunbäck in 1954. They had three sons, Michiel, Martin, and Peter. Hospers was eighty-one when he died on 17 December 2006 after suffering two strokes within fourteen days. He was survived by his wife, sister, and Martin and Peter.

BIBLIOGRAPHY

WORKS BY HOSPERS

“Remanent Magnetism of Rocks and the History of the Geomagnetic Field.” Nature 168 (1951): 1111–1112.

“Reversals of the Main Geomagnetic Field, Part I.” Proceedings of the Royal Netherlands Academy of Sciences, Amsterdam, ser. B, 56 (1953a): 467–476.

“Reversals of the Main Geomagnetic Field, Part II.” Proceedings of the Royal Netherlands Academy of Sciences, Amsterdam, ser. B, 56 (1953b): 477–491.

“Reversals of the Main Geomagnetic Field, Part III.” Proceedings of the Royal Netherlands Academy of Sciences, Amsterdam, ser. B, 57 (1954a): 112–121.

“De Naturlijke Magnetiztie van Ijslandse Gesteenten.” Geologie en Munbouw 16 (1954b): 48–51.

“Rock Magnetism and Polar Wandering.” Nature 173 (1954c): 1183–1184.

“Magnetic Correlation in Volcanic Districts.” Geological Magazine 91 (1954d): 352–360.

“Rock Magnetism and Polar Wandering.” Journal of Geology 63 (1955): 59–74.

With H. A. K. Charlesworth. “The Natural Permanent Magnetization of the Lower Basalts of Northern Ireland.” Monthly Notices Royal Astronomical Society Geophysical Supplement 7 (1954): 32–43.

OTHER SOURCES

Frankel, Henry. “Jan Hospers and the Rise of Paleomagnetism.” Eos68, no. 24 (1987): 577, 579–581.

Irving, E. “The Paleomagnetic Confirmation of Continental Drift.” Eos69, no. 44 (1988): 994–1014.

Langeland, Helge. “Jan Hospers (1925–2006)—Minneord.” Department of Petroleum Engineering and Applied Geophysics, Norwegian University of Science and Technology, 2007. Available from http://www.ipt.ntnu.no/hospers.pdf.

Henry Frankel