Transcript Document

Pourquoi-Pas? and Geomagnetism
Leó Kristjánsson
Institute of Earth Sciences
University of Iceland
(Presentation at a University of Iceland seminar, September 2006)
Sections
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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.
Compass deviations
(mag. declination), deg.
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.
Annales de Physique
1925
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.
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).
Thurá lava, the first
lava sampled in
Iceland for
paleomagnetic
measurements.
R. Chevallier - continued
The lava flowed
down from the
Hellisheiði plateau
and spread out.
x
Map: K. Saemundsson 1995
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.
R. Chevallier - continued
Comptes Rendus, 1930
• 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.
R. Chevallier – cont.
Chevallier wrote a 108page 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.
Hólmatindur
Patreksfjörður
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)