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Pourquoi-Pas? and Geomagnetism Leó Kristjánsson Institute of Earth Sciences University of Iceland (Presentation at a University of Iceland seminar, September 2006)
Sections Geomagnetism and paleomagnetism Raymond Chevallier Paul-Louis Mercanton Subsequent developments Conclusion Appendix: a note on scientific exchange Summary: I describe one aspect of the Pourquoi-Pas? expeditions to the North Atlantic. Research on permanent magnetization in lava flows which were sampled in 1925, 1929 and 1931, provided valuable information on the past history of the Earth´s main magnetic field.
Geomagnetism and paleomagnetism The presence of a geomagnetic field has for centuries been an indispensable aid for sailors and travellers, and also a intriguing phenomenon for scientists to study. In the 19th century, French expeditions made accurate maps of the field (especially its declination) in many parts of the world, both on shore and at sea. In Iceland and the surrounding ocean, much geomagnetic data was collected by scientific cruises (such as La Recherche, 1836) and by military vessels accompanying the French fishing fleet. The example on next slide shows an 1883 survey by J. Wallut.
Geomagnetism and paleomagnetism- continued One segment of the study of geomagnetism concerned the magnetic properties of rock formations, which could cause localized disturbances (anomalies) in the field. Some important discoveries regarding this topic were made in France, e.g. in 1900-10 by Bernard Brunhes at Clermont-Ferrand, in the volcanic region of Auvergne. Brunhes and others realized that a part of the geomagnetic disturbances was due to a permanent magnetization in the rocks.
Geomagnetism and paleomagnetism-continued Brunhes demonstrated that in some rocks, especially igneous ones, the direction of the permanent magnetization was that of the geomagnetic field at the time of emplacement of the rocks. He also concluded that when a certain Miocene lava flow in France was erupted, the field had a direction opposite to that of to-day.
R. Chevallier The work of Brunhes and others established the branch of science now known as paleomagnetism, but it developed rather slowly in the next few decades after 1910. One major study completed in 1925, was that on magnetic directions in several lavas from Etna, approx. AD 1200-1700, by Raymond Chevallier in Paris. Just as Chevallier’s results were being published, he joined the 1925 cruise of Pourquoi-Pas? in order to obtain samples from recent lava flows in Iceland and Jan Mayen for comparison. Annales de Physique 1925
R. Chevallier - cont. Chart of the 1925 cruise to the Faeroes, Jan Mayen, Scoresby Sound, Reykjavík and Rockall. The only lava near Reykjavik whose age was thought to be historical and known, was the Thurá lava of AD 1000 (later shown to be in fact ca. 1900 years old).
R. Chevallier - continued Thurá lava, the first lava sampled in Iceland for paleomagnetic measurements. The lava flowed down from the Hellisheiði plateau and spread out. Conditions are not ideal, but Chevallier collected four oriented samples from an undisturbed outcrop at the top where the lava is beginning to flow down the slope. Map: K. Saemundsson 1995 x
R. Chevallier - continued Chevallier described his results from the Iceland and Jan Mayen samples. Their magnetic directions were internally consistent and fairly similar to the field direction of to-day. Chevallier also published a paper in Comptes Rendus on his samples from three locations in the Faeroes, mostly with similar directions. Comptes Rendus, 1930
R. Chevallier – cont. Chevallier wrote a 108- page book describing the 1925 cruise. It includes photos from various parts of Iceland. This book was published in 1927 (reprinted 1953). Chevallier did not continue measuring directions in rocks, but studied the magnetic properties of various iron minerals and compounds.
Paul-Louis Mercanton Mercanton was one of the foremost glaciologists of the 20th century. He was active in research and science administration from 1896 to 1959, mostly in Lausanne. Mercanton wanted to follow up the work of B. Brunhes by sampling rocks of different ages for magnetic studies. In 1910 he sampled Mesozoic diabases in Spitzbergen, in 1914 early Tertiary basalts in West Greenland, and in 1921 basalts in Jan Mayen. Mercanton also arranged for expeditions e.g. to Australia to collect rock samples. Some of the samples were magnetized in directions more or less opposite to the present geomagnetic field.
P.-L. Mercanton - continued Mercanton took part in the 1929 and 1931 cruises of Pourquoi-Pas?, collecting basalt samples in Mull (W-Scotland), the Faeroes, and Iceland for paleomagnetic studies. His Icelandic sites were as follows: Two in mt. Hólmatindur opposite Eskifjörður (slide) One at Akureyri and another one at the top of the Vaðlaheiði ridge across the fjord One in a mountainside in Patreksfjörður (slide) One in the Almannagjá ravine, Thingvellir One in Hvammsey island, Hvalfjörður Mercanton also measured the present geomagnetic declination near several of his sampling sites.
P.L. Mercanton - continued Mercanton obtained somewhat ambiguous results in Mull. In the Faeroes he found rocks of both polarities, whereas Chevallier had only observed magnetization directions like those to-day. In the Iceland Tertiary lavas which were at that time thought to reach to a similar age as those in the Faeroes and Scotland, Mercanton found ordinary or intermediate directions, not reversed ones. Comptes Rendus, 1932
Subsequent developments Mercanton did not continue paleomagnetic studies after 1932. In the International Polar Year 1932-33 he was a chief organizer of the operation of a Swiss/Danish meteorological observatory at Snæfellsjökull in West Iceland, while France ran a station at Scoresby Sound. As far as I know, no paleomagnetic sampling was carried out in Pourquoi-Pas? cruises after 1931. Not much progress was made in paleomagnetism until around 1950. Many scientists even did not believe that reversals of the main geomagnetic field had occurred: rather, that minerals in rocks might sometimes become magnetized in a direction opposite to that of the field.
Subsequent developments- continued Major advances in paleomagnetism began in the early 1950’s, by the studies of e.g. J. Hospers in Iceland and A. Roche who continued Brunhes’ work in Auvergne. Paleomagnetism contributed greatly to the revolution in earth science in the latter part of the 20th century. Hospers’ research in Iceland was soon followed up by Icelandic and foreign scientists and it has been important for both local and global studies;over 7000 lavas have been sampled here for laboratory measurements. Reversals of magnetic polarity are observed frequently in the Icelandic lava pile. The geomagnetic field may have reversed >100 times during its buildup in the last 15 Ma.
Conclusion In addition to the valuable work done in other disciplines by the Pourquoi-Pas? cruises in the North Atlantic, the studies by R. Chevallier and P.-L. Mercanton on lava flows in Iceland and elsewhere were well known and often referred to. Their results furthered the development of paleomagnetism as a branch of earth science. However, Mercanton missed discovering reverse magnetic directions in the Iceland lava pile. This was largely a matter of chance but perhaps in part due to an interfering phenomenon (“viscous magnetization”), not understood until investigations by L. Néel and others c.1950. Such a discovery might have generated new sampling efforts here earlier than was actually the case.
Appendix: A brief note on aspects of scientific exchange between Iceland and France The Pourquoi-Pas?–expeditions and other activities by French scientists in the Iceland area in past centuries indeed provided important information on the nature and resources of the area. Iceland in return provided materials of value for French science in the 19th and early 20th centuries, especially transparent crystals of calcite (spath d’Islande) for optical research. These crystals were used by: R.-J. Haüy - J.B. Biot - F. Arago - E.L. Malus - A. Fresnel A. Cauchy - A. DesCloizeaux - H. Fizeau - A. Cornu - E. Becquerel L. Pasteur - F. Fouqué - H.Becquerel - A. Michel-Lévy - J. Jamin A. Lacroix - J.A. Le Bel - E. Mascart - A. Cotton - J. Cabannes.......as well as thousands of other scientists. This research often influenced their careers- and sometimes world science- decisively. The crystals from Iceland were also essential for the work of many makers of optical equipment in France. This equipment had wide applications, e.g. in sections of French agriculture and industries.
Thank you Hand-colored photograph of Pourquoi-Pas? in Ísafjörður harbour, Aug./Sept. 1936 (Kr. Leós)