Observation of magnetic chronic drag over geologic time

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Observation of magnetic chronic drag over geologic time

Naoto Kawai, Shoichi Kume

To cite this version:

Naoto Kawai, Shoichi Kume. Observation of magnetic chronic drag over geologic time. J. Phys.

Radium, 1959, 20 (2-3), pp.258-261. �10.1051/jphysrad:01959002002-3025800�. �jpa-00236030�

OBSERVATION OF MAGNETIC CHRONIC DRAG OVER GEOLOGIC TIME

By ^{NAOTO KAWAI} and SHOICHI KUME,

Geological ^and Mineralogical Institute, Kyoto University, Japan.

Résumé.

On a étudié le traînage magnétique ^à température ambiante dans des champs faibles, ^{en mesurant} l’évolution de l’aimantation d’échantillons de roches maintenues pendant plusieurs années dans le champ terrestre. Ces expériences ^ont été étendues à de très longues ^durées

en étudiant des échantillons de roches qui ^ont été réorientés à une certaine époque de leur histoire.

On donne une interprétation qualitative ^{des résultats.}

Abstract. 2014 To study ^the magnetic after-effect in low fields and at room temperature, ^rock specimens ^were kept for several years in a fixed position in the earth’s field, and the change ^of magnetization measured. The measurements were extended _{to very} long period ^{of time} by studying ^rock samples that had been reoriented at a previous ^{date in their} history. ^A qualitative interpretation ^is given.

LE JOURNAL DE PHYSIQUE ^RADIUM 20, FÉVRIER-MARS 1959,

Twenty ^{years have} passed ^{since Thellier} [1]

discovered rocks whose magnetism ^{shows a}

remarkable change ^{with time in the} geomagnetic

fiel d.

Nearly ^ten years ago Néel [2], [3] established ^a

theory ^{of the} magnetic fluctuation after-effect,

based on Thellier’s [1] and Nagata’s [4] experiments,

and an élégant ^{solution was} given ^{to the} problems

of rock-magnetism ^{as well as to those} concerning

what is called domain fixing..

The théories and the experiments ^wl1ich appear to have been plausibly established still leave much to be examined and confirmed. One of tlie first

questions ^{that have come to our} Jninds is in what materials this kind of after-effect prevails. ^Bar-

bier’s [5] ^and Lliboutry’s [6] experiments, ^{can be}

considered as the first answers.

Néel, ^on theoretical considerations, ^distin-

guishes ^the fluctuation after-effect from that of

diffusion, ^{in that the latter tends to a finite value}

within a finite time, whereas the former does not,

the effect bqing prolonged indefinitely. Despite

this statement, ^his theory ^{itself is} constructed on the basis of experimental ^data (1) ^{which were obtained}

in a very limited ^{time scale of the order of the} length

of a day. Many authors, ⁱⁿ measuring ^{the after-} effect, ^have applied ^{a rather} strong magnetic ^field

or raised the température ^{of their} specimens ^to quicken the otherwise _{very slow} domain fixing.

ive still need experimental ^{results obtained in}

weak magnetic ^{fields and at low} temperaturc,

as this is important in the field of rock-magnetism

as well as to inquire ^{whether or not Néel’s} theory

is satisfied in an almost frozen state of the after- eflect.

About eight years ago ^{we set} up the following

Fie. 1. - - Tirii(,,-clialige ^of niagnetization observed iu the laboratory.

experiments ^to oiEelve the fluctuation after-effeet mentioned above. Several sedimentary ^{rocks with}

unstable remanent magnetism ^have carefully been

stored in the laboratory, keeping ^{them at each}

constant orientation and ^constant temperature ⁱⁿ

such ^a _way that the geomagnetic ^{field was} applied

to theHi at a laige angle ^{with their own} rr agnetic

vectors. During ^the storage ^{cach rock} spécimen

was takcn out from tinie to time and its magnetism

was tested by ^{a sensitive astatic} inagnetoineter.

It was found from this repeated observation that the direction and the intensity ^{of the} magnetism change slowly ^and steadily ^{with time as shown in} Figure 1, ^{in which the arrows show the} geomagnetic field, ^{the vectors the} repeatedly determined rema-

(1) ^Further experimental results have been obtained in connection with the study ^of palaeoTnagnctism (cf. ^ref. [7], [8)).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphysrad:01959002002-3025800

259 nent magnetism ^{of the} specimen ^{rocks and the}

numerals suffixed ^to each, ^the year in which the measurements were made. A preliminary report announcing ^{this same} change ^was published by ^the present writers in 1953 [9].

In general, ^the time-change ^of magnetism ^shows

that the initial ^vector of remanent magnetism ^tends

to approach ^the geomagnetic ^{vector. In the first}

two or three years of the experiments, ^the change

was conspicuous ^and proceeded ^{at a rate} nearly proportional ^{to the} logarithm of the duration.

However, ^the change subsequently ^{became more}

and more slow as time passed, ^{and at} present ^it

seems as if the change ^were frozen. It seemed that our efforts, therefore, ^{would be in} vain, ^even

if further observations were to be made.

However, ^{we have} clung ^{to a} ray ^of hope ^{to find}

at least one method to clear _up this difficulty.

The method is based on the following ^considera-

tion : One ^can find natural samples ^{which have} long ^been kept in situ under conditions similar to that under which ^we have carried out our experi-

ments. For instance, pebbles ^{embedded in} _strata, i

stones embodied into old edifices or statues and

Fie. 2 (1).

^Itemanent magnetism ^of reoriented specimens ^whose posi-

tive poles ^are plotted ^{on Schmidt’s} projections.

A) Specimens collected from conglomerate.

B) Specimens ^collected

from conglomerate.

C) Specimens ^collected from statues and edi- fices.

D) Specimens collected from pavements.

E) Specimens

collected from pavements. ^Hollow circle : Positive magnetic pole plot-

ted on lower hemi-sphere. ^Full circle : Positive magnetic pole plot-

ted on upper hemi-sphere. Cross : Position of the present geomagnetic

field.

(II).

Retrospective consideration of magneticafter-effect.

0 : Mean deviation angle ^{from thé} geomagnetic ^field (degree).

T : Time (year).

blocks of rock paved ^{on roads are favorable} subjects

of experiment, ^as individual pieces ^{of these mate-}

rials have been long exposed ^to the local geoma-

gnetic field, ^{and the} original ^remanent magnetism, therefore, ^{must have} undergone ^a change ^{like the}

one we could observe in the laboratory, ^{but with}

far longer ^a duration, covering ^the time from the initial reorientation of the specimens up ^to the

present. By measuring ^their magnetism ^{and so} asking ^{them their} past, ^{it is} possible ^{to draw out at} present ^the history ^{of the} chronic change.

At Hota in Boso Peninsula, Japan, ^{one can col-}

lect a number of rock samples ^{which have been}

detached from the same mass of sedimentary ^rock

and reoriented at various times. Some of these

samples ^{were cut off} by natural erosion from this stratum and embedded into a conglomerate ^about

ten million years ^{ago and some were} carried away from it at a quarry ^and paved ^{on a road some} fifty

years ago. The reorientation of the rocks, ^whe-

ther naturally ^or artificially, ^{have left the rémanent} magnetic ^{vectors scattered at random.}

It was found from the measurements that the rocks whose time of reorientation is latest still have their magnetization ^{vectors scattered at randorn}

whereas those whose time of reorientation is ^ear- liest hâve their ^vectors all nearly parallel ^{to the}

local geomagnetic ^{field. The} samples with inter- mediate age of reorientation show a distribution of vectors intermediate between the above-mentioned

two extremes, namely, ^{between utter} irregularity 2

and perfect order. Positive magnetic poles ^{of the}

rock samples whose reorientations ^are considered

to have taken place contemporaneously ^are plotted

on a Schmidt’s projection ^{and five diagrams of}

(2) It should be remenbered that Graham [10] ^has pro-

posed ^{a test to} determine the stability ^{of the remanent}

magnetism of rocks from the degree ^of randomness.

varying age of reorientation (A), (B), (C), (D) ^and (E), ^{are shown in} figure 2(1).

We can see that the angle ^{of deviation from the} time, ^{as is} demonstrated in Figure ² (II). ^On

the ordinate ^are plotted ^{the mean} angles ^calcu-

lated ^for each _age. It should be pointed ^{out that}

the angle ^of deviation, ^however long ^{the duration} may be, ^{does not reach} zero, ^{as even} at the point A, representing the samples ^{of the remotest} antiquity,

the angle is about 15° ^{or even} larger, ^{which is too} largue ^{for it to} be considered as an error of measu-

rement, ^{3° at most. This} suggests ^{in turn that}

our after-effect does not come to end, despite ^the lapse of time of more than ten million _years. It is to be noticed that 4 X 109 _years is thé age of the earth which is shown by point ^{Ron the} logarith-

mic abscissa, ^{so that one can conclude that the entire}

chronic drag ^cannot be traced back even in geologic

time.

FiG. 3.

Deviation angle ^vs length relation of mean

vectors obtained at individual diagrams (A), (B), (C), (D)

and (E) ⁱⁿ Fig. ^2.

0 : : Mean déviation angle (degree).

L : Length of ^vector (e. ^m. u./cc).

Again, ^thé intensity ^{of remanent} magnetistn

and the angular ^{deviation are} averaged ^{on each} diagrarn ^of figure ² (1), ^{and their} relation in the

course of the change ^{is shown in} figure ³ (1). By IJepres-enting ^{these mean} values of the intenrity

and the ang1e of devi,,ition as a vert or, ^{as shown in}

figure 3 (II), ^{it is} possible ^to infer how the chronic change ^has actually occurred in the natural state,

From figure ^{1 and} figure ³ (II) ^{we also see that}

the top ^{of the} vector traces ^a line (see ^{the dotted} lines) ^which together with the initial and the final

vectors of _remanence makes an isosceles triangle.

According ^{to this} model, ^tbe magnetic ^vector

shortens its length ^{at first as the} angular change

commences, and the shortening ^{continues until the}

deviation is reduced ^to about one half of its initial value.The vector, ^{on further} reducing its deviation becomes longer ^and approaches ^the final vector whose length ^{is the same as that of the initial one.}

A qualitative ^but plausible explanation ^{of the}

chronic change ^{of the} type ^of the isosceles triangle

~

is ^as follows. The initial magnetic ^{vector OA} (Fig. 4), owing ^{to tlie absence of an} applied magne-

FIG. 4.

Schematic representation shoving progress of the chronic change.

tic field in ^that direction, decays with time and

~

becomes OC in an interval of _say t. On the other

hand, ^{in the direction of the} geomagnetism ^and

also in the ^same interval t, owing ^{to the} présence

geomagnetic ^{vector decreases} monotonously ^with

of the applied field, ^there grows with time ^{a new}

~

vector CB whose length ^is nearly equal ^{to the}

~

length ^{of the} reduction that has occurred in OA.

~ ~

Thé result is, ^{the initial vector OA becomes} OB,

~

the net vector difference being ^{AB. lt is seen}

~ ~ ~

that the three vectors AB, BC, and CA form an

isoceeles triante ^ABC and also that this triangle

261

expands ^with increasing lapse ^{of time and con-}

tinues to do so until at last the final triangle ^OAA’

is completed. ^{Several small} arrows are drawn ^on this diagram, ^{to show the} direction in which the

triangular ^area expands ^{with time.}

We here recall that the relaxation time T of the magnetization ^a magnetic domain possesses ^Wàà first introduced by Néel [2], ^who represented ^{it ag ail in-} creasing ^{function of both the} volume of the domain V and of the coercive force Hr,. ^Four simplicity’s sake, ^Néel has agâuined these domains as consisting

of a single substance, ^but having ^{différent V and}

H,, whereas in our test spécimens, ^{instead of a} single phase, there have been found ^â fiumber of

ferromagnetic phases ^with différent Curie points.

As has alrèady ^been reported [11], ^one of thé wri-

ters, ⁱⁿ carrying ^out thermo-magnetic ^and x-ray analyses ^{of these rock} specimens, ^{has detected}

cubic Ti-ferrites of varying composition. ^{The Curie} points ^of these ferrites are found to be distributed

nearly continuously from that of pure magnetite (575 OC) ^{down to room} temperature ^{or even lower.}

It has also been reported [11] that the lower the lowest Curie point ^{of the} sample, the faster the relative rate of the time-change ^{of the} magnetism.

It is noteworthythat ^Néel’s equation ^{for T shows}

a shortening ^{of T at} high temperatures, ^{and that}

this shortening ^{is much} greater ^at temperatures

near the Curie point than that due to the decreases in V and Hi only.

Connecting this with our observed results, ^the following conclusion can be drawn ; in the tri an-

gular inodél of time-change, ^the part of the ini- tial vector which first alters its orientation towards the geomagnetic ^{field is due to the} domains whose Curie points ^are lowest. Next comes the part ^of the vector due to those domains whose Curie point

is next lowést, ^{and so} on, in the order of increasing

Curie points.

REFERENCES

[1] ^THELLIER (E.), Ann. Inst. Phys., Glove, 1938, 16;

157 ; Thesis, Paris, 1948.

[2] ^NÉEL (L.), ^Ann. Géophys., 1949, 5, 99.

[3] ^NÉEL (L.), ^J. Physique Rad., 1950, 11, 49 ; 1951, 12,

339.

[4] ^NAGATA (T.), ^Bull. Eearthquake ^Res. Inst., 1943, 21, 1.

[5] ^BARBER (J. G.), C. R. Acad. Sc., 1950, 230, 1040.

[5] ^LLIBOUTRY (L.), ^{C. R. Acad.} Sc., 1950, 230, 1042.

[7] ROQUET (J.), ^Ann. Géophys., 1955, 11, 461.

[8] ^ROCHE (A.), ^{C. R. Acad.} Sc., 1957, 244, 2952.

[9] ^KAWAI (N.), ^KUME (S.), ^J. Geomag. Geoele., 1953, 5, 66.

[10] ^GRAHAM (J. W.), ^J. Geophys. Res., 1949, 54, ^131.

[11] ^KAWAI (N.), ^Proc. Japan Acad., 1956, 31, 364.

DISCUSSION

M. Thellier (Remarque).

^{Nous avons} discuté,

le 1111 Kawaï et moi-même, ^{et nous le ferons à nou-}

veau, des aspects purement paléomagnétiques ^des

fàits présentés, qui ^{sont loin des} préoccupations

de la Conférence actuelle. Je voüdrais seulement faire quelques ^remarques générales :

Je soulignerai ^d’abord que le traînage magné-

tique ^{dans les} roches, qui ^était entièrement méconnu, ^{hors de} France, jusqu’à ^{ces dernières}

annégs (il ^{n’en est} pratiquement pas question ^dans l’ouvrage ^Rock Magnetism ^de 1953), ^a pris ^une importance considérable au point que certaines

roches, ^{telles celles} considérées par le Pr Kawaï,

sont considérées comme n’ayant pratiquement que

ce type d’aimantation.

En ce qui ^{concerne les} mesures, ^les paléomagné- ticiens, ^en général, ^ne considèrent que l’effet ^du traînage ^sur la direction de l’aimantation. ^Nous préférons travailler sur la valeur des aimantations.

Exactement, ^{nous mesurons} toujours trois compo-

santes rectangulaires ^des moments, ^Oe qui ^nous

donne évidemment le traînage ^en direction, ^mais

aussi en valeur.

Dernière remarque : lorsqu’il s’agit ^{d’établir les}

lois relatives à un type d’aimantation nous tra- vaillons non sur des aimantations naturelles, ^résul-

tant en général ^de plusieurs types d’aimantations,

mais sur des aimantations définies provoquées ^au

labôratoire. Dans le cas de l’aimantation de traî- nage, ^nous partons ^de corps désaimantés ^{sur lès-}

quels ^nous pouvons suivre réquisition ^ptogres-

sive, ^ou portant ^uné aimantation rémanente iso- therme sur lesquels ^{nous suivons la} désaimantation spontanée. ^Avec des aimantations natùrêl1es, ^télles

que celles considérées par le Pr Kawaï, ^les phéno-

mènes d’acquisition d’aimantation visqueuse ^{et de}

désaimantation s’emmêlent et l’existence possible

d’une aimantation plus dure initiale (aimantation

par dépôt, par cristallisation...) complique ^encore l’interprétation. ^{Dans le cas} présent, l’existence d’une telle aimantation dure entraîne celle d’une déviation nioyenne irréductible .entre ^{la direction}

dé l’aimantation observée et celle du champ ^ter-

reste actuel, ^cet angle ^{étant doutant} plus grand

que ^la composante ^{dure est} plus grande ; ^{la conclu-}

sion sur là limite de l’évolution du véétéur âiman-

tatibn perd ^{de sa valeur de ce fait.}