The Earth Inside and Out phần 7 ppt

Conclusions While Heezen and Tharp helped revive Wegener's mobilism, and their physiographic diagrams reflected the latest findings by leaders in the field of oceanographic research, vis

Fig 3 Section of a colour-coded province map of the Mid-Atlantic Ridge in the equatorial Atlantic The red, yellow and green areas show highest elevation Reproduced from a map worksheet in the Heezen Collection, Library of Congress (photographer Gary North, authors Bruce C Heezen, Marie Tharp, Date 1960) Fig 4 The World Ocean Floor Panorama, authors Bruce C Heezen and Marie Tharp, Date 1977 and copyright by Marie Tharp 1977 Reproduced by permission of Marie Tharp, 1 Washington Ave., South Nyack,

NY 10960.

sequential or 'genetic' development of oceans

from mid-ocean ridges The Indian and Atlantic

Oceans supposedly began as continental rifts

and slowly grew, and the rifts in East Africa, the

Red Sea and the Gulf of Aden were comparable

features at different stages of development

(Heezen 1962)

Physiographic diagrams: a reflection of

changing scientific attitudes

Many scientific disciplines, not least geology,

frequently proceed by the use of visual thinking

and aesthetic considerations rather than

deduc-tively through logic or inducdeduc-tively from

empiri-cal data (Miller 1981) This 'aesthetic' method

contributed significantly to the reintroduction of

the notion of continental drift The rifted

Mid-Atlantic Ridge suggested that the Earth's crust

had moved laterally and the diagrams

con-tributed to the demise of geology's old

perma-nence theory But Heezen and Tharp did not

propose that continental drift caused the rift.

Rather, the diagrams attracted the attention of

other geoscientists who made an acceptable case

for continental drift and later for plate tectonic

theory, which incorporated 'drift' (Le Grand

1988) In 1958, Heezen acknowledged that

palaeomagnetic studies and other new data

implied lateral continental motion, but he

advo-cated expansionism, not drift Expansion had

been proposed previously in the twentieth

century.27

In 1960, Heezen and Tharp promoted

expan-sion while Harry Hess (1906-1969) proposed

what became, after modifications, an acceptable

model for continental drift By that time,

tec-tonics had begun to play a key role in the

col-laborators' research programme and Heezen

used maps of the Earth's major tectonic features

to bolster his argument for expansion

Accord-ing to Heezen, the most important tectonic

factors influencing sea-floor topography were

crustal extension, strike-slip faulting, normal

faulting, and subsidence (Heezen 1962) The

work of the Australian geologist S Warren

Carey of the University of Tasmania, a staunch

expansionist, influenced Heezen In 1956, Carey

organized a major conference on continental

drift and Heezen participated (Carey 1956) He

and Carey advocated a relatively rapid rate of

expansion (Le Grand 1988) The Columbia

structural geologist Walter Bucher (1889-1965),who in 1933 had proposed that the Earth under-went alternating periods of expansion and con-traction, also advised Ewing and his students atLament.28 The collaborators believed thatmantle material welled up between the separ-ating continents, which were pushed aside as theEarth expanded, and produced the mid-oceanicridges by a form of sea-floor spreading A promi-nent factor in Heezen's advocacy of expansionwas that he, and most of his Lamont colleagues,believed that oceanic rifts and trenches weresimilar crustal features produced by tension Inaddition, he did not accept that excess crustcould be subducted back into the Earth's inte-rior at the oceanic trenches Heezen rejectedsubduction as he could not visualize the geome-try of convection cells or currents as being such

as to cause compression at the trenches (Heezen1962) The early physiographic diagramsreflected his tensional hypothesis: the oceanbasins were supposedly stretched apart as theEarth expanded, with rifts opening in all direc-

tions (Heezen et al 1959; Menard 1986) The

col-laborators believed that an ocean basin wasstructurally one unit, with all major features,including the mid-ocean ridge system and thecontinental margins, being minor splinters andfissures in the floor of one 'grand crack': theocean basin

According to Felix Vening-Meinesz(1887-1966), another pioneer of sea-floorresearch, the presence of negative gravityanomalies over deep-sea troughs, accompanied

by seismic activity in these regions, indicateddown-buckling of the Earth's crust Vening-Meinesz, Harry Hess and Ewing had accompa-nied the 1936-1937 cruise of the submarine

Barracuda to the Caribbean, with Ewing

collect-ing trench gravity data (Bowin 1972) After thisexpedition, he concentrated on equipment andthe technical aspects of data collection ratherthan the development of theories Hess,however, immediately began to develop Vening-Meinesz's ideas on down-buckling and convec-tion currents (Le Grand 1988) He called theseregions of crustal compression 'tectogenes'(Oreskes 1999) Lamont scientists, however,especially Ewing, did not accept crustal com-pression at the trenches, as they believed thatthe gravity data collected over trenches wereinconclusive (Heezen 1962) The 'confusion' atLamont continued until the evidence for

27 Heezen was familiar with the literature on expansion He cited Taylor (1910) and Eyged (1957) in his most descriptive work on the topic, the paper The deep-sea floor' (Heezen 1962).

Marie Tharp, pers comm., October 1999.

Fig 5 Physiographic diagram of the South Atlantic, authors Bruce C Heezen and Marie Tharp, Date 1961 and copyright by Marie Tharp 1961 Reproduced by permission of Marie Tharp, 1 Washington Ave., South Nyack, NY 10960.

continental drift became overwhelming in the

mid-1960s

The Heezen and Tharp physiographic

dia-grams, especially of the Atlantic, reflected the

rapidly changing comprehension of processes

shaping the ocean basins, during a brief period

of revolutionary scientific activity as defined by

Thomas Kuhn (Kuhn 1962) In 1961, the

diagram of the South Atlantic, with the tion of a few equatorial fracture zones, illus-trated few departures from the theories thatprevailed during the 1950s (see Fig 5) The earlyphysiographic diagrams had land-like features,while later versions pictured a different world.Late in the 1960s, continental drift was becom-ing acceptable and plate tectonic theory was

excep-quickly developing as a viable alternative to the

old paradigm The appearance of the diagrams

changed drastically in the 1968 Geological

Society of America's edition of the North

Atlan-tic after decisive geomagneAtlan-tic core studies had

convinced the majority at Lamont of continental

drift, although Heezen and Tharp did not

com-pletely abandon expansion (Heezen & Hollister

1971) The most significant change in the

appearance of their diagrams was the style of

topographic symbolism The edges of many

fea-tures, especially mountain peaks, 'became'

jagged and sharp (see Fig 6) The pronounced

angularity, emphasized by thicker black lines,

and the ordered definition of the later editions

contrasts with the more random, 'softer' edges

of peaks in the early maps On the rifted

Mid-Atlantic Ridge, the incorporation of many

regu-larly spaced offsets gave this great feature a new

and forbidding appearance Offsets along the

ridge were exaggerated to emphasize the extent

of displacement.29 Eventually, the Mid-Atlantic

Ridge region became filled with sharp, closely

spaced peaks

The appearance of light and shadow in the

diagrams also changed as the new paradigm was

adopted In the early diagrams, the shadowing

was not as strong as in later editions and there

were no sharply defined sources of light In the

1968 diagram, the floor of the central Atlantic

Ocean was represented as if it were brightly

illu-minated from a point to the south, with light

impinging on the face of the peaks, and

high-lighting the appearance of the fracture zones

The sketched lines and shading of these features

was dark, accentuating their angularity and

depth These changes reflected the significance

of these features in the development of plate

tec-tonics theory and the understanding of the

Earth's behaviour, even though the

collabora-tors were not themselves supporters of plate

tec-tonics

The collaborators' years of research and

analysis of offset fracture zones intersecting the

Mid-Atlantic Ridge inspired others, such as J

Tuzo Wilson, to consider their origin and the

direction of crustal motion at these features

While fracture zones had been discovered on the

floor of the eastern Pacific by H W Menard of

the Scripps Institution, in California, Heezen

and Tharp established their existence in the

Atlantic During the drafting process, Tharp

noticed trends and the collaborators looked for

Fig 6 Section of a physiographic diagram of theNorth Atlantic ocean floor, showing the jaggednature of the offset fracture zones on the Mid-

Atlantic Ridge, c 1968 Reproduced from the

Heezen Collection, Library of Congress(photographed by Gary North, authors Bruce C.Heezen and Marie Tharp)

additional fracture zones using the earthquake

data (Heezen et al I964b) They discovered

irregular patterns along the central rift valley,which led them to believe that offsets on theridge occurred at angular breaks of between 80and 100° (Heezen & Tharp 1965) Tuzo Wilsoncited this and other data, when he proposed thatthese features were not ordinary offset faults,but a new class of faults that occurred on mid-ocean ridges and are locally transformed intozones of crustal movement According toWilson, the motion along these faults was oppo-site to that of the usual strike-slip faults The'new' type of fault did not extend across theridge, but joined the next segment of the riftedridge He named these features 'transformfaults' (Wilson 1965) and they were soon incor-porated into the emerging new theory of globaltectonics.30

29 Archival source: Tharp 19990, Tanya Levin interview, 24 May 1997,142

30 In this important paper Wilson cited Bucher (1933), Carey (1956), Heezen (1962) and several other works thatlaid the foundation for the concept of transform faults

The collaborators' analysis of the data

col-lected during the International Indian Ocean

Expedition illustrates that scientific

obser-vations are theory-laden (cf Hanson 1961);

theoretical assumptions can dictate what is

dis-cerned in or inferred from data While the

rela-tive symmetry of the Mid-Atlantic Ridge had

inspired Heezen and Tharp to consider

expan-sion, the complex and asymmetric nature of the

Indian Ocean topography failed to change their

expansion model Rather, the new data

strength-ened their belief that continental drift, utilizing

a simple pattern of convection currents inside

the Earth, was not a feasible option and they

continued to advocate expansion (Heezen &

Tharp 1965) According to Tharp, Heezen was

essentially a uniformitarian, who believed that

observable processes could be used to explain

the geological record (Tharp 1982a) Many

scientific disciplines employ analogy (Hesse

1981) and as geologists, Heezen and Tharp

gath-ered data and often used analogies to help in

their analysis However, to comprehend all the

forces shaping ocean-basin topography, these

tools were not sufficient as: (1) subduction does

not occur on the continents; (2) the movement of

the offset fracture zones intersecting the

Mid-Atlantic Ridge differed from existing examples

of fault systems on land; and (3) it was necessary

to consider physical laws and go beyond

geo-logical fieldwork at the surface in order to

understand how the Earth's crust behaves, as

Wilson (1951) had suggested In addition,

researchers who were not deeply involved in

data collection were able to distance themselves

from specific research problems and propose

broad explanatory theories (Menard 1986)

Conclusions

While Heezen and Tharp helped revive

Wegener's mobilism, and their physiographic

diagrams reflected the latest findings by leaders

in the field of oceanographic research, visual

representation and analogy could take the

col-laborators only so far and they were not

involved in establishing plate tectonics per se

(Le Grand 1988) The collaborators'

carto-graphic endeavours stimulated scientific change

by revealing critical elements of sea-floor

topog-raphy and behaviour These included: (1) the

rifted Mid-Atlantic Ridge; (2) the extension of

the mid-oceanic ridges around the planet; (3) the

idea of the sequential or genetic development of

oceans from continental rifts; and (4) the angled

nature of the faults intersecting the

Mid-Atlan-tic Ridge But this newly documented

know-ledge, no matter how essential, could not in itself

propel ideas beyond a certain point and scend assumptions that were firmly established

tran-in the collaborators' mtran-inds Nevertheless,Heezen and Tharp successfully continued theirmapping and data gathering into the 1970s.Their efforts were vital to scientific change, eventhough, after the plate tectonics revolution, theirmethod remained the same and their theoreticalideas did not change radically Even if the col-laborators are not usually acknowledged forsubstantial theoretical contributions to therevolution in the Earth sciences, their physio-graphic diagrams, globes and related artifactsmay well be considered milestones in the history

of cartography, and their work undoubtedly tributed to the eventual grand change in geo-logical theory that occurred in the 1960s.Perhaps one might say that Heezen and Tharpwere (together) the Tycho Brahe of the Earthsciences revolution, providing essential empiri-cal information but not able to break free ofolder ways of thinking Or insofar as they did so,they pursued an idea that (so far as most geolo-gists are concerned) led to a dead-end

con-I wish to thank S Herbert for her invaluable ance; G Fitzpatrick, at the Library of Congress, for encouraging my interest in the physiographic dia- grams; G North, at the Library of Congress, for taking the photographs of the diagrams in the Heezen Col- lection; and Marie Tharp for her encouragement and giving me the opportunity to interview her.

assist-Appendix

Archival sources

THARP, M 1997 Interview conducted by Gary North,

21 November One session, one video-cassette; preliminary transcript The Heezen Collection, The Library of Congress, Washington DC THARP, M 19990 Reminiscences of Marie Tharp: interviews conducted in four sessions by Ronald Doel on 14 December 1995 and 18 December

1996, and by Tanya Levin on 24 May 1997 and 28 June 1997 Preliminary transcript (These are part

of the Lamont-Doherty Earth Observatory Oral History Project Oral History Research Office, Columbia University On file at the American Institute of Physics Neils Bohr Library, College Park, MD.)

THARP, M 1999b Interviews conducted by the author

on 25-26 October Three sessions, three tapes; preliminary transcript The Heezen Collec- tion, The Library of Congress, Washington DC.

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transformation in the twentieth century

GREGORY A GOOD

History Department, West Virginia University, Morgantown, WV 26506-6303, USA

Abstract: In 1900, researchers interested in Earth's magnetism generally proclaimed all

facets of magnetic phenomena to be within their purview Most researchers in this field

referred to themselves as 'magneticians' first and physicists or geologists second After

World War II, specialization increased A number of distinct research areas appeared over

several decades: the geodynamo theory and the study of the core-mantle boundary;

palaeo-magnetism and its growing connection to geology; the production of induced fields in

Earth's crust; and, among others, the electromagnetic phenomena of the upper atmosphere

and near space The former unity dissolved and the field fragmented One result of

frag-mentation has been a loss of memory and a consequent misinterpretation of an important

part of the history of geoscience This paper relates the challenges of recovering a history

obscured by later events.

When most geologists think of studies of Earth's

magnetism in the twentieth century, they think

of palaeomagnetism, and with good reason The

investigation of, for example, reversals of

direc-tion of Earth's magnetism played a critical role

in the acceptance of continental drift and plate

tectonics, one of the central developments in

geology during the century (Le Grand 1998;

Frankel 1998) It's a dramatic story, and

mag-netic reversals themselves, seeming

simul-taneously unexpected and unsettling, have

caught the imagination of a broader public The

many facets of the story of continental drift and

plate tectonics, including the role of

palaeomag-netism, are thoroughly analysed in Naomi

Oreskes' The Rejection of Continental Drift

(1999, pp 263-267)

One must remember, however, that there is

much more to Earth's magnetism than

palaeo-magnetism's importance in plate tectonics

When most geophysicists think of this broad

phenomenon, they think toward one of two

extremes of Earth's environment: the depths of

the core-mantle boundary where the main

geo-magnetic field is produced, or the heights of the

magnetosphere where the planet's magnetic

field interacts with the solar wind and begins the

chains of events that lead to magnetic

disturb-ances and the aurora polaris Investigations of

the phenomena of these two realms relate to two

other critically important stories of

twentieth-century geoscience

Interestingly, all the major streams of

geomag-netic research explored in this paper -

palaeo-magnetism, the origin of Earth's main magnetic

field, fields induced in Earth's crust and mantle,

and ionosphericmagnetospheric phenomena

-witnessed their great periods of dramatic successsimultaneously in the mid-twentieth century.From roughly the end of World War II until thelanding on the Moon in 1969, one dramatic dis-covery followed another In palaeomagnetism,the work in the 1950s and 1960s of Allan Cox,Richard Doel, Brent Dalrymple, Donald H.Tarling and Ian McDougall, among others,established a timescale for reversals in the maingeomagnetic field This ultimately supported thefamous Vine-Matthews-Morley hypothesis,which linked palaeomagnetism firmly to sea-floor spreading and plate tectonics Concerningthe origin of the main geomagnetic field, import-ant developments in this story occurred in the1940s and 1950s, with the first theories of a self-sustaining dynamo, proposed by Walter Elsasserand Edward Bullard, and the rotational theory of

P M S Blackett These theories, while notimmediately successful, started a new direction

in geomagnetic research The third major stream,

of fields induced in Earth's crust and of tivity, included both global and local investi-gations (Parkinson 1998) The fourth stream, thestudy of near-Earth space, included the investi-gation of the interaction of Earth's magnetic field

conduc-in that region with the solar wconduc-ind by EugeneParker, the discovery of polar substorms and

aurora polaris, of whistlers, and more (see, for

example, Akasofu 1996; Hufbauer 1998; Stern1989,1996; Van Allen 1983; Cliver 1998).All these areas of magnetic research,however, are much larger than these descrip-tions imply Scientists and historians alike tend

to be blinded by the bright lights of successfulresearch The successes of mid-twentiethcentury geomagnetic research helped drive the

From: OLDROYD, D R (ed.) 2002 The Earth Inside and Out: Some Major Contributions to Geology in the Twentieth Century Geological Society, London, Special Publications, 192, 229-239 0305-8719/02/$15.00

© The Geological Society of London 2002.

historical process of specialization

Palaeomag-netic researchers, main field theorists,

conduc-tivity/induced field researchers, and space

scientists began moving in progressively more

independent directions Despite some

continu-ing overlap in instrumentation and/or theory,

the rigours of their respective fields demanded

ever more concentration As this specialization

has continued, people looking back have had a

hard time seeing past the bright lights of the

mid-century to a time before, when researchers in

geomagnetism conducted their research for

reasons unconnected with plate tectonics, the

geodynamo, crustal conductivity or

magneto-spheric interactions This does not imply that the

different specializations were or are mutually

irrelevant, since indeed palaeomagneticians (to

borrow a word from W D Parkinson), for

example, have placed significant constraints on

viable theories of the origin of the main field

And while a palaeomagnetician quite likely

would not understand the calculations of flow

patterns in the outer core, the results would still

be of interest Specialisation has been partial

and primarily methodological and institutional

This paper tells a straightforward story of these

events and places the drama of

mid-twentieth-century geomagnetism in the context of the

longer story of generations of successful and

interesting research (cf Parkinson 1998)

The story told, moreover, concentrates on

research questions and methods, leaving aside

crucial social, institutional and cultural issues

related to the development of geophysics

Unde-niably, industrial/economic interests, the

mili-tary use and support of geophysics, and the

politics of the World Wars and the Cold War all

played important roles in this history The

Inter-national Geophysical Year, the establishment of

World Data Centres and space exploration also

influenced the development of geophysics in

many ways Doel (1997), among others, has

begun investigation of the history of these

matters, which require much more vigorous

pursuit

Terrestrial magnetism

In 1900, investigations of Earth's magnetism

flourished as never before All the major

Euro-pean powers and their colonial empires, along

with the United States, Japan, and several other

nations, established magnetic observatories and

sent out teams of researchers to map magnetic

declination and other variables (Merlin &

Somville 1910; Chapman & Bartels 1940,

pp 955-957) In 1904, Louis Agricola Bauer

established the Department of International

Research in Terrestrial Magnetism, betterknown as the DTM, at the Carnegie Institution

of Washington, to fill in the gaps in these surveysaround the world The DTM also broughtregular observations of the changing magneticfield to places that were previously bereft ofobservatories (Good 1994; Bauer 1912-1927).The explicit goal of this frenetic global activitywas to understand Earth's magnetism in itsentirety Many geomagnetic scientists at thebeginning of the twentieth century were inspired

by the two nineteenth-century giants, Alexandervon Humboldt (Rupke 1997) and Carl FriedrichGauss (Dunnington 1955) Humboldt hadattempted the impractical: to grasp the dynamicphenomena of the Earth and the Cosmos in onemind, and to reveal and revel in their intercon-nections This included both magnetic and elec-tric phenomena From his work sprang theinstitutionalization of 'terrestrial physics' and'cosmic physics', which continued as well recog-nized branches of physics into the early twenti-eth century (Walker 1866; Conrad 1938).Magnetic researchers in 1900 saw their chosenphenomena in this context and directed theirresearch in such a way as to endeavour to fulfilHumboldt's vision

Researchers in 1900 had a critically importantpractical advantage over Humboldt Geomag-netic research, with its requirements of observa-tories, instruments and international activity,was expensive During the half century sinceHumboldt's last magnetic researches, the fiscaland organizational vitality of many nations hadincreased significantly They could now affordnot only to survey their home territories, buttheir extensive colonial empires Germany,France, the Netherlands, Britain and the UnitedStates, in particular, did this (Pyenson 1985,1989) A few nations and ambitious individualslike Roald Amundsen and Robert F Scott, inthe rush for the polar regions, likewise equippedtheir expeditions for magnetic research (e.g.Chree 1903; Good 1991) The DTM took advan-tage of the largess of Andrew Carnegie's privatefortune to launch the most far-ranging and sys-tematic of these global enterprises

Magnetic researchers in the early twentiethcentury tended to be interested in all aspects ofEarth's magnetic phenomena Consider two ofthe more important theorists: Adolf Schmidtand Arthur Schuster Schmidt, who directed thePrussian magnetic observatory in Potsdam, fol-lowed in the footsteps of Edward Sabine by pub-lishing an extensive compendium of magneticdata Whereas Sabine's numerous 'Contri-butions to Terrestrial Magnetism' assembleddata from magnetic surveys of many countries

and individuals, Schmidt collected tables of data

from many observatories for the systematic

study of time variations (Schmidt 1903-1926)

Schmidt assembled these and other data to

answer diverse theoretical questions What were

the causes of magnetic storms (Schmidt 1899)?

Did electric currents flow through the surface of

the Earth (Schmidt 1939)? What caused secular

variation (Schmidt 1932)?

Schuster, trained as a physicist by Helmholtz

and Maxwell, published his first important

mag-netic work in 1889: an application of Gauss's

spherical harmonics to the problem of the

diurnal variation of Earth's magnetism

(Schus-ter 1889) This work provided the basis for

future studies of induced electromagnetic fields

in the crust and in the upper atmosphere This

gave rise to two apparently independent, yet

closely related, areas of research: electrical

con-ductivity of the crust and electrical currents in

the ionosphere and beyond Schuster also

pub-lished on the causes of magnetic storms

(Schus-ter 1911) and on the causes of Earth's main

magnetic field (Schuster 1912) This inclusivity

was common to leading researchers around

1900

Sydney Chapman presented a most useful

guide to geomagnetic research in the early

twen-tieth century in his acceptance speech for the

first Chree Medal in 1941 Charles Chree, who

died in 1928, had been Director of the Kew

(meteorological and magnetic) Observatory

from 1893 to 1925 In his Chree Address,

Chapman related the research careers of Chree,

Schmidt and Bauer, saying that these three

-plus the Dutch Willem van Bemmelen, the

Indian N A F Moos, and Edward Walter

Maunder and Arthur Schuster in England

-'epitomise the progress of earth magnetic

science during nearly half a century' (Chapman

1941, p 630) He characterized the different

'gifts' that each researcher brought to the

science: Moos and Chree's mathematical ability

and indefatigable treatment of data; Bauer's

'fiery enthusiasm' and 'wide views'; Maunder's

familiarity with events on the surface of the Sun,

and Schuster's 'brilliant sorties' and 'striking

theoretical conclusions' (Chapman 1941,

pp 632-633) Chapman divided the rest of his

discussion into the consideration of time

relationships and distribution of geomagnetism

over space, a traditional division that closely

parallels the studies of solar-terrestrial

relation-ships and deep-Earth magnetic phenomena

today These traditions of terrestrial and cosmic

physics relate intimately to the multifaceted

development of geophysics and space physics

This, however, is not the place to pursue the

story of the physical study of the Earth in extenso

(Doel 1997; Good 2000)

As the cases of Schmidt and Schuster indicate,geomagnetic research in 1900 was not merely'Baconian' or inductive, as, indeed, it was not inearlier centuries either That is, scientists werenot aimlessly collecting reams of data (Thispopular characterization of 'Baconianism' doesnot do justice to Francis Bacon, but this issueneed not be entered into here.) Their data col-lection was directed by theory Even the activity

of the Carnegie's DTM - with its dozens of nician-expeditionaries off around the world,with its magnetic survey vessels cruising theoceans, and with its observatories in Peru andAustralia automatically generating extensivedata relating to numerous types of phenomena -was undertaken to answer questions Bauer hadwritten his dissertation at Berlin on the analysis

tech-of the main magnetic field and secular variation(Good 1994; Bauer 1895) The data available, helamented, were inadequate to the theoreticalstudies that needed to be undertaken In order

to explain the production of the main field, thecause of secular variation, the diurnal variationsand magnetic storms, data collection guided bytheory was required

Primarily, the theories of Carl Friedrich Gaussand James C Maxwell (Garland 1979; Harman1998; Hunt 1991) provided that guidance Geo-magnetic research from the 1890s onwards was

in the hands of investigators trained in physics.They exploited the data obtained during the'Magnetic Crusade' (Morrell & Thackray 1981)and the first International Polar Year (Mill-brooke 1998) They applied Gauss's sphericalharmonic analysis with ever-greater sophisti-cation Schmidt, Bauer, Schuster and othersfirmly entrenched the habit of treating geomag-netism and geoelectricity exclusively in terms offield theory; and they made it clear that thefuture of explaining the main field and disturb-ance fields lay in this direction

Geomagnetism

We no longer remember, and it seems unlikelytoday, but in 1938 'geomagnetism' was a new

word in English Germans had written of

Erd-magnetismus for nearly a century, but to

anglo-phone and francoanglo-phone researchers the subject

had long been 'terrestrial magnetism' and

'mag-netisme terrestre\ Sydney Chapman suggested

the change Although his reasons were linguisticand pragmatic, numerous changes were sweep-ing through this research community, whichmade the change more than a matter of linguis-tic convenience

A publishing event in 1940 marked a critical

period in the history of geomagnetic research:

after a decade of collaboration, Sydney

Chapman (then of Imperial College London)

and Julius Bartels (then the Director of the

Geo-physical Institute, Potsdam) published their

monumental treatise with the simple title, using

the new word: Geomagnetism (Chapman &

Bartels 1940) The authors noted in the preface

(Vol 1, p vii) that no general treatments had

been published on the subject since Edward

Walker (1866) and Eleuthere Mascart (1900)

and that these works answered 'few of the

ques-tions which most interest modern workers on

geomagnetism' Researchers were now

investi-gating solar and cosmic ray physics, geophysical

prospecting and radio communication Perhaps

more importantly, these researchers employed

new methods of physical and mathematical

analysis

Geomagnetism, according to Chapman &

Bartels (1940, Vol 1, pp vii-viii), stood

'between solar physics and the mainly more

local terrestrial science of meteorology, on the

one hand, and on the other, the universal

science of physics' Indeed, it encompassed

parts of each of the neighbouring fields The

topics covered in the book reflected this,

includ-ing for example: Earth's main field; secular

vari-ation; magnetic anomalies and geological

prospecting; periodic variations due to the Sun

and Moon; magnetic disturbances;

solar-terres-trial connections; earth currents; aurora polaris\

atmospheric conductivity and the ionosphere;

statistical and harmonic analysis of periodic

phenomena and the main field; physical

theories of the main field; electromagnetic

induction within the Earth; and much more

Nothing, it seems, was omitted

As with all watershed works, however,

Chapman & Bartels's Geomagnetism did turn its

back on part of the history of its subject It began

a movement in new directions Their

com-pendium not only incorporated the

accomplish-ments of Gauss and Maxwell and the data of the

expeditions and observatories of the nineteenth

and early twentieth centuries, it also

incorpor-ated elements of the 'new physics' Many

theories based in older natural philosophy did

not merit discussion even in the final historical

chapter of the book (Chapman & Bartels 1940,

Vol 2, pp 898-937) The authors faced the

future and their book provided the platform for

launching the next generation of researchers

These new researchers carried their

investi-gations along diverging trajectories:

palaeomag-netism; theories of the main geomagnetic field;

investigations of induced fields and currents

(and conductivity); and studies of the upperatmosphere and near space

This paper recounts the 'stories' of magnetism, the origin of the main magneticfield, and of induced fields/crustal conductivity

palaeo-in the twentieth century (The history of palaeo-gations of near-space researches will mostly bereserved for another publication.) These storiesare worth recalling because the development ofplate tectonics and of dynamo theories com-pletely changed the reasons for investigating theEarth's magnetism The contexts of magneticresearch before 1950 seldom even merit mention

investi-in histories written investi-in recent years, because thesehistories have often focused explicitly on howplate tectonics came to be accepted or becausethey have seen the pre-dynamo days as non-theoretical and essentially uninteresting Hence,although earlier contexts of investigations ofrock magnetism have been seen - legitimately -

as irrelevant to plate tectonics and so havelargely been omitted or forgotten, they do in factembody a significant part of the history of geo-magnetic research in the twentieth century.Likewise, although interest in the causes of themain geomagnetic field motivated generations

of researchers before 1950, that early work hasbeen seemingly eclipsed by the development ofdynamo theories

Rock magnetism

The study of 'rock magnetism' is larger than thestudy of palaeomagnetism That is, the subject isnot just about what the magnetism of rocks cantell us about the past condition of the Earth This

is certainly part of the story, but so are two othermain topics: the connection of remanent mag-netism to local anomalies and the study of rockmagnetism as a subject in its own right Never-theless, consider the pre-1950s history of palaeo-magnetism first

The utility of rock magnetism for revealingthe history of the main field and secular vari-ation far pre-dates its connection to the researchquestions of polar wander and plate tectonics.Since the discovery of secular variation in 1634

by Henry Gellibrand, researchers had beentrying to explain the slow variation of decima-tion and had placed it in the context of numer-ous research agendas Edmond Halley famouslysought to explain secular variation by hypothe-sizing the existence of a magnetic shell sur-rounding a magnetic 'nucleus' inside the Earth,the two revolving at slightly different speeds.The 'four-pole theory' of Halley was revived inthe early nineteenth century by ChristopherHansteen and encouraged consideration of a

secular periodicity even into the early twentieth

century In the late nineteenth century,

investi-gators were critically aware that little was known

of the history of Earth's magnetism If one were

generous, good data then extended back

perhaps three hundred years (now four

hundred) to the 1580s (with the work of Robert

Norman), and that only for declination and dip

(Jackson et al 2000) Good data for magnetic

intensity were available only since the work of

Gauss and Wilhelm Weber in the 1830s, with less

useful data going back to Humboldt and Jean

Charles Borda in the 1790s While such a short

reach might have seemed acceptable when the

planet was thought to be only a few thousand

years old, by 1900 Earth's history was generally

accepted to be much longer While a few

nine-teenth-century scientists contemplated an Earth

as young as a few tens of millions of years, most

thought in terms of hundreds of millions, from

the 1820s until the discovery of radioactivity

(Thomas 1998, pp 13-16)

The most general motivator, then, for

palaeo-magnetic research in the early twentieth century

was to provide better data for the explanation of

secular variation It was also thought that these

data might help to explain the production of the

main field Bauer, certainly, saw the need to

extend the palaeomagnetic data-set in these

terms He combined physical palaeomagnetic

research with what might be called 'archival

palaeomagnetism' That is, he searched old

publications, sea captains' logs, etc., trying to

wring the best information possible from a

'barely damp rag' (Jackson & Barraclough 1998;

Good 1994; Bauer 1908) From the 1930s

onwards, investigators at the DTM began

sys-tematic research on remanent magnetism

largely to fulfil the Department's original remit,

related to the main field and secular variation

(McNish 1937; Johnson & McNish 1938;

Graham 1949; Le Grand 1994, 1998) Another

important question that motivated investigation

of palaeomagnetism around 1950 was whether

remanent magnetism really reflects the field

when a rock formed (Le Grand 1994, 1998)

Rock magnetism transformed

A comparison of how rock magnetism and its

history were treated in three landmark books

spanning 1940 to 1964 will give some idea of how

they were transformed during these 25 years

(Other publications were no less important

notably T Nagata's Rock Magnetism (1953)

-but these will suffice to make a few important

points.)

The three books here compared are Chapman

& Bartels's (1940) Geomagnetism, P M S Blackett's (1956) Lectures on Rock Magnetism, and Edward (Ted) Irving's (1964) Paleomagnet-

ism and its Application to Geological and physical Problems The points these three

Geo-examples make are deceptively simple First, thecontext of a research problem area changed overtime Second, the changes in that context affectthe history that we select to write

Consider Chapman & Bartels's chapter netism and geology: magnetic prospecting'(Chapman & Bartels 1940, Vol 1, pp 137-158)

'Mag-In all of their massive two-volume study, thisshort chapter is the only one to discuss rock mag-netism Granted, both Chapman and Bartelsfaced more toward the cosmos than toward thesolid Earth Even so, most of this chapter dis-cussed mapping of magnetic anomalies andrelated it to the mapping of gravitational anoma-lies and the locating of ore bodies Chapman andBartels described Schmidt's field balance, theuse of local variometers, and the reduction ofobservations They did not ask what rock mag-netism had to say about any large theoreticalmatter - not the history of the magnetic field; notdrift or polar wander; not even theories of mag-netization in general The one question they didask was: Can magnetic anomalies tell us howthese crustal rocks were magnetized? Were'highly susceptible rocks' magnetized by induc-tion by the present geomagnetic field (Chapman

& Bartels 1940, Vol 1, pp 145-146)? They cluded that the existence of strong negativeanomalies indicates 'that magnetic rocks may be

con-permanently magnetized in directions differing

from that of the present field' They explicitlyavoided deciding between 'whether this perma-nent magnetization was produced by the generalfield of the earth at the time of congelation ormetamorphosis, or whether other causes must

be considered' (Chapman & Bartels 1940, Vol 1,

sal of the magnetic field in that region [emphasis

added] at the time of the formation' (p 156).Chapman & Bartels wrote later in the book (p.701) that, nevertheless, the outright rejection of

evidence of geomagnetic reversals as unreliable

was 'perhaps too dogmatic'

By the time Blackett wrote his book, much

had changed At the DTM, Johnson, Murphy &

Torreson (1948) had published their 'Pre-history

of the Earth's magnetic field' John Graham had

completed much of his research on rock

mag-netism (e.g Graham 1949, 1955) Most

import-antly, Blackett himself had developed and

ultimately rejected a 'fundamental' theory that

attributed magnetism generally to massive

rotat-ing bodies, which will be discussed later in this

paper Also in the late 1940s, the first rough

attempts at dynamo theories were made by

Elsasser and Bullard (Nye 1999) Blackett came

at rock magnetism with the history of the Earth's

field prominently in mind He reviewed all the

useful literature back to the 1890s

Such information [about past geomagnetic

conditions] would be of great importance for

its own sake but would be of immense value in

an attempt to understand the physical

mechanism giving rise to the field Without

the study of rock magnetism we had no

possi-bility of knowing whether the field might not

have been vastly different in the distant past,

perhaps a thousand or more times greater or

smaller (Blackett 1956, p 5)

While Blackett studied magnetization itself as a

phenomenon, he was more interested in the

1950s to connect rock magnetism to global

prob-lems, especially to the cause of the main field

(Blackett 1956, p 7)

Blackett raised another critical perspective in

which rock magnetism had great importance:

the possibility that the Earth's magnetic dipole

had reversed itself suddenly and repeatedly

(Blackett 1956, pp 6-8) At first, this larger

implication was, for Blackett, mainly part of

dis-covering the history of Earth's main magnetic

field But he conducted this research during the

early 1950s, when Keith Runcorn and Kenneth

Creer were proposing polar drift as a way to

explain 'odd' palaeomagnetic readings Arthur

Holmes was still discussing continental drift and

the work of Wegener in his Physical Geology

(Holmes 1944), which Blackett read while

working on rock magnetism in the early 1950s

Blackett saw possible reversals, and the

mapping of a drift, as an indication, if not a

proof, that his own theory connecting

geonetism and rotation was wrong If Earth's

mag-netism were tied fundamentally to its rotation,

this magnetism could not reverse unless Earth's

rotation did - a wholly unlikely scenario

Blackett's experimental investigation

con-tributed to the rapid specialization of

magne-tometers in the 1950s, some more accurate andprecise, others more portable Although histheory failed, his instrument design helped ulti-mately to validate plate tectonics theory (Nye1999) Likewise, the work of Packard & Varian(1954) on the proton-precession magnetometerproved effective in the mapping of magneticallystriped oceanic crust Fluxgate magnetometers(useful in aeromagnetic surveys), Zeeman-effect magnetometers (extremely sensitive anduseful in space probes) and cryogenic magne-tometers (useful in palaeomagnetic work),taken altogether, represented the broad-rangingeffects of new technologies, based on electronicsand physical research, on geophysics in the mid-twentieth century (Parkinson 1983, pp 44-59).They also demonstrate that specialization hasbeen embodied in instrumentation

Edward Irving's book presented a formed view of rock magnetism His shift to theterm palaeomagnetism was purposeful, indicat-ing that he was mainly interested in rock mag-netism as evidence of past conditions Heparticularly highlighted 'the hypothesis of conti-nental drift' and pointed out that palaeomag-netic measurements provided 'numerical tests'that could refute it, with evidence of a typedifferent from the data originally used by AlfredWegener (Irving 1964, pp vi-vii) AlthoughIrving was certainly aware of the connection ofpalaeomagnetic data to theories of the origin ofthe main field, this had dropped out of his story(Irving 1964, p 4) His historical section dis-cussed measurements of reversed magnetization

trans-as early trans-as Alexander von Humboldt Curiously,Irving went on from Humboldt to state that anumber of other intensely anomalous rock out-croppings had been found in the nineteenth

century and that these were termed punti distinti

or points isoles His history had the same

char-acter, with the clear application of a selectioncriterion Irving isolated these scientific actsfrom their historical contexts and saw them inthe light of his own concerns (Irving 1964,

pp 6-8)

This selective approach to history is notunusual and I do not mean to criticize Irving.Indeed, this kind of selectivity is inherent in allhistory - not just the history of science written byscientists One is easily drawn to tracing out a'family tree' when looking backward from astrong preoccupation, such as one's currentresearch Even reflecting back on one's own life,

it is common to forget or gloss over the fusion, or the dead-end project, or the onesimply left behind Oral histories and memoirssubstantiate this repeatedly

con-In the case of palaeomagnetism, its history

was unselfconsciously rewritten with succeeding

generations, as the focus of palaeomagnetic

research itself shifted In the end, even

his-torians looking back at the history of

palaeo-magnetism have written mainly about its

importance in the plate tectonics story But in

the 1940s and earlier, that was not generally the

context of research in rock magnetism The

utility of palaeomagnetic data in evaluating the

hypothesis of continental drift was a connection

that few imagined before the 1950s, although

Paul Mercanton apparently first suggested this

connection in 1926 (Mercanton 1926)

The main field and the geodynamo

The drive to explain the origin of the main field

(and secondarily of secular variation)

necess-arily intersected with interest in

palaeomagnet-ism, as noted above One of the main reasons for

this was that researchers felt that information

provided by historical observations did not

cover a long enough stretch of Earth's record

Much research on the origin of the main

mag-netic field, however, had nothing to do with rock

magnetism Indeed, most developments in this

line were driven by the capabilities of physics

and mathematics

Arthur Schuster outlined the available

expla-nations in his 'Critical examination of the

poss-ible causes of terrestrial magnetism' in 1912

(Good 1998, pp 355-356) He thought it was

premature to rule out permanent magnetization

since the effect of very high pressures on

mag-netization were not understood The possibility

of an inductive effect from electrical currents

inside the Earth was, he thought, overrated The

first of these explanations had roots in William

Gilbert and Edmond Halley's ideas, and the

second in those of Andre-Marie Ampere

Schus-ter rejected another idea, popular in the

nine-teenth century, that Earth's main field was

induced by external, cosmic causes, and

cluded that one of the most promising ideas

con-nected Earth's magnetism to its rotation If

molecules were magnetic or if they carried an

electric charge, rotation could produce the main

field But Schuster drew no firm conclusion

Interest in this possible explanation persisted

throughout the 1920s, 1930s and 1940s, when it

was picked up again by Blackett As outlined by

Mary Jo Nye (1999, pp 74-76), S J Barnett,

Albert Einstein, Johannes de Haas, H A

Wilson, W F G Swann and A Longacre all

explored the issue Barnett, Swann and

Lon-gacre attempted to measure the magnetism of

rotating bodies Einstein and de Haas

con-sidered the matter theoretically When Blackett

conceived the idea himself in the late 1940s, hequickly tracked down this literature throughChapman & Bartels (1940, Vol 2, p 705) In

1947, Blackett 'splashed' this revived theoryacross the world's headlines and began a multi-faceted effort to test the idea once and for all.Objections against it arose from diverse quar-ters: evidence of magnetic reversals in stars wasdiscouraging, as were arguments from quantumelectrodynamics raised by Wolfgang Pauli andothers Most telling, however, were the results ofgeophysical tests Following a suggestion ofEdward Bullard, Keith Runcorn measuredEarth's magnetic field deep in mine shafts.Bullard had noted that if magnetism were due to

a distributed cause, such as Blackett's, then thefield should be less within Earth's surface.Results, initially equivocal, ultimately tendedagainst Blackett Blackett's own elaboratelaboratory experiments went against a funda-mental relation between rotation and magnet-ism and in 1952 he published his negative results(Nye 1999, pp 78-87) He shifted his energy toapplying his sensitive magnetometers to palaeo-magnetism and to testing continental drift

A second and quite different type of theoryemerged alongside Blackett's In 1919, JosephLarmor published a short article that discussedthe possibility that dynamo action inside the Sunproduces its magnetic field Despite ThomasCowling's argument in 1934 that an axially sym-metrical field cannot be maintained by adynamo, Walter Elsasser began exploring thistype of theory in 1939, in 'On the origin of theEarth's magnetic field' (Brush & Bannerjee

1996, pp 223-224) Elsasser wrote:

The terrestrial field is traced here to the ence of thermoelectric currents in the metallicinterior of the earth The currents owe theirexistence to inhomogeneities continuallycreated by turbulent convective motions(Elsasser 1939, p 489)

exist-As Elsasser later related, this was not strictly adynamo theory Nevertheless, it is wheredynamo theory began According to Stephen G.Brush and S K Banerjee, Elsasser's contri-bution to dynamo theory laid 'the foundation ofthe modern theory of terrestrial magnetism'(Brush & Banerjee 1996, p 224) After the inter-ruption due to World War II, Elsasser returned

to this problem in 1946 with 'Induction effects interrestrial magnetism' His recognition thattoroidal fields can exist in Earth's core providedthe basis for a self-exciting dynamo, namely one

in which induction effects reinforce the existingmagnetic field (Brush & Banerjee 1996, p 225).Bullard stepped into this picture in 1948 and

1954, supplying a more detailed theory of how a

self-exciting dynamo might work

Although the general idea of a self-sustaining

dynamo in the outer core quickly gained general

(but not universal) acceptance, some aspects

remained controversial for years G E Backus

and others pointed out defects in the earlier

theories and developed improved dynamo

models in the late 1950s Others involved in this

included Stephen Childress and Glynn Roberts,

A Herzenberg and E N Parker In the 1960s,

Raymond Hide developed the

'magnetohydro-dynamic wave hypothesis' as an alternative to

motion of the outer core relative to the mantle

In this hypothesis, waves oscillating through the

liquid outer core caused secular variation P H

Roberts and S Scott, meanwhile, worked on the

idea of 'frozen flux', in which the magnetic field

moves with the fluid core material This critically

important area of geomagnetic research

con-tinued through the rest of the twentieth century,

with important work being undertaken by David

Gubbins, Jeremy Bloxham, and others (Brush &

Banerjee 1996, pp 227-231)

Geologists might be forgiven if they wonder

what this had to do with their work There was,

however, one main area of intersection The

magnetic polarity reversals on which so much

geochronology and so much research in plate

tectonics now depend are in principle

explain-able only with a geodynamo (Gubbins et al.

2000) That the details of the physical and

math-ematical analysis of that dynamo might be

beyond the reach of many geologists shows how

far the partitioning of geomagnetic research

developed during the twentieth century

From crustal conductivity to the equatorial

electrojet

Two lines of research could scarcely appear less

related than variations in electrical conductivity

of the crust and mantle and the existence of

elec-trical currents in the ionosphere or beyond

However, in 1889 Schuster demonstrated that

electrical currents in the upper atmosphere

cause the daily variations in geomagnetic

measurements (Schuster 1889) Schuster

discov-ered that external currents produce internal

fields and currents, and that these can be used to

study conductivity of the crust Sydney

Chapman proposed a simple model of global

conductivity dependent only on depth in 1919,

which was further developed by Albert Price

and B N Lahiri (1939) and by Chapman &

Bartels (1940) (Parkinson 1998, p 362) The

broad outlines of global crustal conductivity

established, R Banks pushed the idea to greaterdepths around 1970

Studies of local variation in crustal ity also took off The Carnegie Institution'smagnetic observatories began systematicmeasurements of Earth currents and conductiv-ity in the 1920s Around 1950, Andrei Niko-laivich Tikhonov and Louis Cagniard developedthe methods of 'magnetotellurics', in whichmeasurements of potential differences betweenprobes are combined with readings of appropri-ate magnetic variations to study conductivity atvarious depths (Parkinson 1998, p 363) In the1960s, Albert Price extended this method toinclude horizontal variation The direction ofgeomagnetic changes can indicate the local gra-dient of conductivity Walter Jones, UlrichSchmucker, Peter Weidelt, John Weaver, IanGough, and others, extended this work to thedetailed study of bodies of various types and thedevelopment of new instruments Robert Parkerapplied inversion methods to magnetotellurics

conductiv-in 1970, as this study was pushed deeper conductiv-into theEarth Studies of conductivity variation on theocean floor and of electromagnetic induction inthe oceans was pursued extensively from the1970s into the 1990s (Parkinson 1998,

pp 363-364) These research topics representone of the most significant interaction zonesbetween geology and geophysics in the latetwentieth century

It should also be remembered, though, thatstudies of currents and fields in the ionosphereand near space followed paths that were largelyignored by geologists, and rightly so The

investigations included studies of the aurora

polaris (Silverman & Egeland 1998), magnetic

substorms (Akasofu 1998), the interaction of thesolar wind and the magnetosphere (Akasofu1983), the electrical ring-current in the ionos-phere called the equatorial electrojet (Chapman1951), and other important phenomena of theupper atmosphere and near space The picture

of the history of geomagnetic research in thetwentieth century must ultimately encompass all

of these various investigations and the munities of scientists involved, but that is a taskfar beyond the scope of this paper

com-The transformation of disciplines

This paper has emphasized the fragmentation ofterrestrial magnetism into several diverging spe-cializations: palaeomagnetism, work on the geo-dynamo, crustal conductivity, and ionosphericand magnetospheric research As J A Jacobs

writes in his recent text Geomagnetism (1987):

Chapman & Bartels surveyed about 100 000

pages of literature to write their compendium.

When Matsushita and Campbell wrote theirs in

1967, they faced a much more daunting

prospect By 1987, Jacobs felt compelled to call

on a long series of experts to each write about a

single specialization (Jacobs 1987, Vol 1, p vii).

There is an element of truth to this story, but

it is important not to carry this thought too far.

While certainly geomagnetic researchers in the

late twentieth century have tended to specialize

more than their colleagues did a century earlier,

the process has been necessary There are,

despite the tight focus on separated problem

areas, significant continuities across the whole of

geomagnetic research Frequently, the

instru-mentation is the same All draw on similar

physi-cal theory and mathematiphysi-cal techniques There

are connections among phenomena Indeed,

there are many examples of individuals who

continue to work in more than one of the

problem areas Nevertheless, career imperatives

and institutions enforce specialization And,

when looking back, writers seldom seem aware

of the joint kinship shared by researchers in

palaeomagnetism, the main field, and space

physics - let alone geophysical prospecting,

radio physics and cosmic ray studies Historical

investigations must at least acknowledge how

specialization has affected our ability to see this

past, or the writing of this history will be

'pre-sentist' in the worst sense Major developments

in science sometimes induce a sort of amnesia,

which we must constantly fight against if we are

to write histories faithful to the contexts of their

times.

Special thanks are due to all of the participants in the

Rio sessions for discussions that broadened and

deep-ened this investigation, and to R E Doel, W D.

Parkinson, and A Jonkers for their critiques of the

draft manuscript I didn't address all of their

recommendations in the final revision, but I have

taken them to heart and will be considering their other

suggestions in future writing I also thank S Solomon,

director of the Department of Terrestrial Magnetism,

Carnegie Institution of Washington, for graciously

hosting and supporting my historical research during a

sabbatical year in 1998-1999.

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