TRM in low magnetic fields: a minimum field that can be recorded by large multidomain grains

Thermally acquired remanent magnetization is important for the estimation of the past magnetic field present at the time of cooling. Rocks that cool slowly commonly contain magnetic grains of millimeter scale. This study investigated 1-mm-sized magnetic minerals of iron, iron–nickel, magnetite, and hematite and concluded that the thermoremanent magnetization (TRM) acquired by these grains did not accurately record the ambient magnetic fields less than 1 μT. Instead, the TRM of these grains fluctuated around a constant value. Consequently, the magnetic grain ability to record the ambient field accurately is reduced. Above the critical field, TRM acquisition is governed by an empirical law and is proportional to saturation magnetization (M_s). The efficiency of TRM is inversely proportional to the mineral's saturation magnetization M_s and is related to the number of domains in the magnetic grains. The absolute field for which we have an onset of TRM sensitivity is inversely proportional to the size of the magnetic grain. These results have implications for previous reports of random directions in meteorites during alternating field demagnetization, or thermal demagnetization of TRM. Extraterrestrial magnetic fields in our solar system are weaker than the geomagnetic field by several orders of magnitude. Extraterrestrial rocks commonly contain large iron-based magnetic minerals as a common part of their composition, and therefore ignoring this behavior of multidomain grains can result in erroneous paleofield estimates.     

The case for a martian origin of the shergottites, II. Trapped and indigenous gas components in EETA 79001 glass

Analysis of nitrogen and light noble gases in a large sample of glass (lithology C) from the antarctic shergottite EETA 79001 yields a minimumδ¹⁵N > +300‰ for the isotopic composition of nitrogen trapped in the glass. The new data fall on the mixing line through the martian atmospheric composition defined byδ¹⁵N vs.⁴⁰Ar/¹⁴N for two smaller samples analyzed previously. The results from all three samples are consistent with a two-component nitrogen system in which 84 ppb of trapped martian atmospheric N is mixed in variable proportions with another, more thermally labile N component during stepped heating. This second component, which appears to be indigenous to the glass rather than adsorbed from air and is present in amounts that vary by more than a factor of 3 from sample to sample, may represent volatiles from the martian interior. Data from crystalline phases of several SNC meteorites indicate that the indigenous gas may haveδ¹⁵N < −35‰ and³⁶Ar/¹⁴N 3 × 10⁻⁶, similar to the enstatite chondrites.Neon compositions in EETA 79001 glass samples suggest an earth-like value of 10.1 ± 0.7 for the unknown²⁰Ne/²²Ne ratio in the martian atmosphere. The nitrogen-argon correlation systematics yield trapped⁴⁰Ar/³⁶Ar= 2260 ± 200, within error of the Viking value. There is evidence that³⁶Ar/³⁸Ar in the martian atmosphere is4.1 ± 0.2, strikingly different from terrestrial or typical chondritic ratios near 5.3. Attribution of this low value to excess³⁸Ar generated over martian history by galactic cosmic-ray (GCR) spallation of surface materials would be difficult for a number of reasons, among them the excessive GCR fluences required and the absence of a corresponding²¹Ne excess.     

Magnetism of the Moon - a lunar core dynamo or impact magnetization?

Surface and satellite observations of lunar crustal magnetization and the remanent magnetization of the lunar samples returned by the Apollo missions of 1969–72 provide evidence for past magnetic fields on the Moon. During the more than twenty years in which research has been carried out there has been controversy concerning the origin of the field, with two possible processes of major interest. Dynamo generation of the field in a molten, electrically-conducting core is consistent with most of the observations, but encounters theoretical difficulties associated with the deduced magnitude of the ancient field and lack of positive evidence for a lunar core. The most likely alternative process is the generation of a transient magnetic field during meteorite impacts followed by thermoremanent or shock magnetization of debris and adjacent crust. This paper reviews the evidence and compares the observations with characteristics of lunar magnetism expected as a result of each of the two possible processes. It is concluded that the evidence very strongly favours the past existence of a dynamo-generated lunar magnetic field, with impact magnetization playing a minor role.     

Observing,Modeling, and Interpreting Magnetic Fields of the Solid Earth

Many Earth system processes generate magnetic fields, either primary magnetic fields or in response to other magnetic fields. The largest of these magnetic fields is due to the dynamo in the Earth’s core, and can be approximated by a geocentric axial dipole that has decayed by nearly 10% during the last 150 years. This is an order of magnitude faster than its natural decay time, a reflection of the growth of patches of reverse flux at the core–mantle boundary. The velocity of the North magnetic pole reached some 40 km/yr in 2001. This velocity is the highest recorded so far in the last two centuries. The second largest magnetic field in the solid Earth is caused by induced and remanent magnetization within the crust. Controlled in part by the thermo-mechanical properties of the crust, these fields contain signatures of tectonic processes currently active, and those active in the distant past. Recent work has included an estimate of the surface heat flux under the Antarctic ice cap. In order to understand the recent changes in the Earth’s magnetic field, new high-quality measurements are needed to continue those being made by Ørsted (launched in 1999), CHAMP and the Ørsted-2 experiment onboard SAC-C (both launched in 2000). The present paper is motivated by the advent of space surveys of the geomagnetic field, and illustrates how our way of observing, modeling, and interpreting the Earth’s magnetic field has changed in recent years due to the new magnetic satellite measurements.     

Meteorite paleomagnetism - From magnetic domains to planetary fields and core dynamos

Meteorites represent the earliest records of the evolution of the solar system, providing information on the conditions, processes and chronology for formation of first solids, planetesimals and differentiated bodies. Evidence on the nature of magnetic fields in the early solar system has been derived from chondritic meteorites. Chondrules, which are millimeter sized silicate spherules formed by rapid melting and cooling, have been shown to retain remanent magnetization records dating from the time of chondrule formation and accretion of planetesimals. Studies on different meteorite classes, including ordinary and carbonaceous chondrites, have however provided contrasting results with wide ranges for protoplanetary disk magnetic fields. Developments on instrumentation and techniques for rock magnetic and paleointensity analyses are allowing increased precision. Micromagnetic and an array of geochemical, petrographic and electronic microscopy analyses provide unprecedented resolution, characterizing rock magnetic properties at magnetic domain scales. We review studies on chondrules from the Allende meteorite that reveal relationships among hysteresis parameters and physical properties. Coercivity, remanent and saturation remanence parameters correlate with chondrule size and density; in turn related to internal chondrule structure, mineralogy and morphology. Compound, fragmented and rimmed chondrules show distinct hysteresis properties, related to mineral composition and microstructures. The remanent magnetization record and paleointensity estimates derived from the Allende and other chondrites support remanent acquisition under influence of internal magnetic fields within parent planetesimals. Results support that rapid differentiation following formation of calcium-aluminum inclusions and chondrules gave rise to differentiated planetesimals with iron cores, capable of generating and sustaining dynamo action for million year periods. The Allende chondrite may have derived from a partly differentiated planetesimal which sustained an internal magnetic field.     

The case for a martian origin of the shergottites: nitrogen and noble gases in EETA 79001

Nitrogen and noble gases were measured in samples of a glass inclusion and the surrounding basaltic matrix from the antarctic shergottite EETA 79001. A nitrogen component trapped in the glass, but not present in the matrix, has a δ¹⁵N value at least as high as +190‰. Ratios of⁴⁰Ar/¹⁴N and¹⁵N/¹⁴N in the glass are consistent with dilution of a martian atmospheric component (δ¹⁵N = 620 ± 160‰,⁴⁰Ar/¹⁴N= 0.33 ± 0.03) by either terrestrial atmosphere adsorbed on the samples or by indigenous nitrogen from the minerals of the rock. Trapped noble gases in the glass reproduce, within error, the elemental and isotopic compositions measured in Mars' atmosphere by Viking, and are in general agreement with previous measurements except for much lower abundances of neutron-generated krypton and xenon isotopes. The most reasonable explanation at the present time for the noble gas pattern and the isotopically heavy nitrogen is that a sample of martian atmosphere has been trapped in the EETA 79001 glass, and that this meteorite, and thus the shergottites and probably the nakhlites and chassignites as well, originated on Mars.Nitrogen in the non-glassy matrix of EETA 79001 amounts to less than 0.5 ppm and has a spallation-corrected δ¹⁵N value in the range 0 to ?20‰; it may reflect indigenous nitrogen in the basalt or a mixture of indigenous and adsorbed terrestrial nitrogen. Spallogenic noble gases yield single-stage exposure ages between 400,000 and 900,000 years, depending on irradiation geometry. Trapped argon may have an unusually low³⁶Ar/³⁸Ar ratio. Trapped krypton, except for a small excess at⁸⁰Kr, is smoothly mass-fractionated with respect to either terrestrial or chondritic Kr. The trapped xenon composition is consistent with addition of neutron-capture, radiogenic and fissiogenic isotopes to a base composition resembling terrestrial atmospheric Xe. The elemental⁸⁴Kr/¹³²Xe ratio of 25 is close to the terrestrial value and very different from the chondritic ratio.     

Age of the source of the Jarrafa gravity and magnetic anomalies offshore Libya and its geodynamic implications

The interpretation of the Jarrafa magnetic and gravity highs, NW Libyan offshore, suggests that it may be caused by a body of high-density and high magnetization. Analysis of their power spectra indicates two groups of sources at: (1) 2.7 km depth, probably related to the igneous rocks, some of which were penetrated in the JA-1 borehole, (2) 5 km depth, corresponding to the top of the causative body and (3) 10 km depth, probably referring to the local basement depth. The boundary analysis derived from applied horizontal gradient to both gravity and magnetic data reveals lineaments many of which can be related to geological structures (grabens, horsts and faults).The poor correlation between pseudogravity fields for induced magnetization and observed gravity fields strongly suggests that the causative structure has a remanent magnetization (D = −16°, I = 23°) of Early Cretaceous age, fitting with the opening of the Neo Tethys 3 Ocean.Three-dimensional interpretation techniques indicate that the magnetic source of the Jarrafa magnetic anomaly has a magnetization intensity of 0.46 A/m, which is required to simulate the amplitude of the observed magnetic anomaly. The magnetic model shows that it has a base level at 15 km.The history of the area combined with the analysis and interpretation of the gravity and magnetic data suggests that: (1) the source of the Jarrafa anomaly is a mafic igneous rock and it may have formed during an Early Cretaceous extensional phase and (2) the Jarrafa basin was left-laterally sheared along the WNW Hercynian North Graben Fault Zone, during its reactivation in the Early Cretaceous.     

Magnetic properties of the Olivenza meteorite—possible implications for its evolution and an early solar system magnetic field

The magnetic properties of samples of the Olivenza chondrite (LL5) obtained from four collections have been investigated. The natural remanent magnetization (NRM) consists of a very stable primary component, which is randomly scattered in direction on a scale of 1 mm³ or less within the samples, and a secondary magnetization widely varying in intensity, and probably also in direction. The origin of the secondary NRM is not clear, and may be of terrestrial origin. It is concluded that the NRM is carried by the ordered nickel-iron mineral, tetrataenite. The origin of the primary NRM could be a magnetic field associated with the solar nebula, out of which the metal grains condensed and acquired a thermo-remanent magnetization (TRM), or Olivenza could be a fine-grained breccia, the constituent fragments possessing randomly directed magnetization. The implications for the origin and evolution of Olivenza and its parent body if the former magnetizing process has occurred are discussed.     

Declination and inclination errors in experimentally deposited specularite-bearing sand

Naturally disaggregated specularite-bearing sandstone from the Triassic Moenkopi Formation, artificially deposited in controlled magnetic fields of ～5 × 10^?2 mT, acquires a stable remanent magnetization that has systematic errors in inclination and declination. Inclinations about 12° shallower than the applied fields are produced by deposition on a horizontal surface in still water. Deposition from flowing water on a surface inclined 6–10° results in inclination errors of as much as 20°. Water flowing obliquely to the applied field results in declination errors of about 10°, with declinations systematically rotated toward the upstream direction of current flow. These experimental results indicate that specularite-bearing sediment responds to the earth's field in a manner similar to magnetite-bearing sediment, and support observational evidence for a primary magnetization of depositional origin in specularite in red beds of the Moenkopi Formation.     

LINEAR TRANSFORMATIONS OF GRAVITY AND MAGNETIC FIELDS*

The spectral representation of gravity and magnetic fields shows that the mathematical expressions describing these fields are the result of convolution of factors which depend on the geometry of the causative body, the physical properties of the body and the type of field being observed. If a field is known, it is possible to remove or alter these factors to map other fields or physical parameters which are linearly related to the observed field. The transformations possible are: continuation, reduction to the pole, converting between gravity and magnetic fields, converting between components of measurement, calculation of derivatives, and mapping magnetization and density distribution, relief on interfaces, and vertical thicknesses of layers.     

Magnetic properties of a freshly fallen LL ordinary chondrite: the Bensour meteorite

A comprehensive rock magnetic, magnetic anisotropy and paleomagnetic study has been undertaken in the brecciated LL6 Bensour ordinary chondrite, a few months only after its fall on Earth. Microscopic observations and electronic microprobe analyses indicate the presence of Ni-rich taenite, tetrataenite and rare Co-rich kamacite. Tetrataenite is the main carrier of remanence. Magnetization and anisotropy measurements were performed on mutually oriented 125 mm³ sub-samples. A very strong coherent susceptibility and remanence anisotropy is evidenced and interpreted as due to the large impact responsible for the post-metamorphic compaction of this brecciated material and disruption of the parent body. We show that the acquisition of remanent magnetization postdates metamorphism on the parent body and predates the entering of the meteorite in Earth’s atmosphere. Three components of magnetization could be isolated. A soft coherent component is closely related to the anisotropy of the meteorite and is interpreted as a shock remanent magnetization acquired during the same large impact on the parent body. Two harder components show random directions at a few mm scale. This randomness is attributed either to the formation mechanism of tetrataenite or to post-metamorphic brecciation. All components are likely acquired in very low (≈μT) to null ambient magnetic field, as demonstrated by comparison with demagnetization behavior of isothermal remanent magnetization. Two other LL6 meteorites, Kilabo and St-Mesmin, have also been studied for comparison with Bensour.     

Remanent Magnetization in Portland-Cement-Based Materials

The purpose of this research is to demonstrate the possibility of applying one geophysical method to studying untraditional systems as is the case with Portland-cement-based materials. The research demonstrates how conventional paleomagnetic methodology can be employed in studying the mode of magnetic recording in present-day industrial materials. Portland-cement admixtures such as fly ashes and furnace slags should be discriminated, because those particles interact in soils and sediments in nature. Moreover, a better undertanding of magnetic remanent acquisition in model materials can serve to improve the interpretation of magnetic remanent acquisition in natural rocks formed a long time ago. The magnetic constituents of Portland-cement paste and mortar acquire a magnetic remanence due to their alignment with the earth's magnetic field at the casting place. This magnetization can be measured using ordinary paleomagnetic techniques. The alignment of the individual magnetic particles accounts for the intensity of the magnetic remanence, which can be increased by adding water and by vibration before setting and hardening. Blast furnace slag admixtures also add to the enhancement of the intensity of remanence. The magnetization of Portland-cement-based materials shows a near linear relationship with the water /cement (w/c) ratios employed in the experimental work; the w/c ratios range between 0.2 – 0.6 in pastes and 0.3 – 0.6 in mortar. Stable remanent magnetization was obtained during the first seven days of setting and hardening, a period necessary for magnetic particles to become locked parallel to the earth's magnetic field. The stability of magnetic remanence predicts the usefulness of the methodology in studying the properties of Portland cement and particularly in the control of iron-bearing admixtures.     

Magnetic properties of natural Mn-oxides

Summary The paper is devoted to the study of the magnetic properties, of the volume magnetic susceptibility in a weak magnetic field, of the specific magnetic susceptibility in a strong magnetic field, of natural remanent magnetization and its stability in A.C. fields, and of the temperature dependence of magnetic susceptibility of natural Mn-oxides. Samples of pyrolusite, polianite, manganite, hausmannite, psilomelan and wad from different localities all over the world were studied; the measured values are statistically treated so that the given values are valid in general. Hausmannite displayed typical values of the magnetic susceptibility, different from the values of the other Mn-oxides. In general, many natural Mn-oxides contain heterogeneous admixtures of Fe-oxides, which are responsible for their ferromagnetic properties. A high degree of magnetic hardness, predetermining the minerals for palaeomagnetic research, was proved with many of the natural Mn-oxides.     

Imprinting chemical remanent magnetization in claystones at 95 °C

Thermal effects related to burial and hydrothermal alteration leads to chemical remanent magnetization (CRM). We present an experimental study of CRM production by heating claystones at 95 °C. A vertical magnetic field of 2 mT was applied to the claystones during heating and the evolution of the remanence during heating in air is monitored intermittently for up to four months. Solid fragments (9 to 26 g) of claystones are included in a Teflon holder that is placed in the oven under a controlled atmosphere. Newly formed grains acquire a CRM and a thermoviscous magnetization (TVRM), both being parallel to the applied magnetic field. CRM is related to the amount of newly formed grains that pass the critical volume during the reaction. To measure the acquired remanence, the claystones are first cooled in a zero magnetic field and then measured using a 2G SQUID magnetometer.In the frame of the research programme on the feasibility of radioactive waste disposal in a deep geological formation, we investigate the magnetic transformation of Mont Terri Lower Dogger claystones (Switzerland) due to thermal imprinting at 95 °C. We simulate the dehydration that occurs in the walls of galleries after excavation when interstitial water evaporates and rehydration when the galleries are refilled allowing water to move towards dehydrated zones. During dehydration, the remanence gains one order of magnitude at the beginning of the experiment and then it follows a linear rate of 0.23 ± 0.07 mA m^− 1/day between 3 and 14 days. The magnetic susceptibility increases by a few percent. The increase of the remanence and of the magnetic susceptibility stops after 15 days. Mass monitoring indicates that interstitial water evaporates when remanence and magnetic susceptibility stabilizes. During rehydration, the remanence increases again whilst magnetic susceptibility drops by a few percent. After 20 days, the remanence during rehydration follows a rate of 0.42 ± 0.15 mA m^− 1/day. By contrast, when rehydration takes place later, after 66 days, the rate is much lower (0.09 ± 0.04 mA m^− 1/day). Low temperature investigation of magnetic properties indicates an initial magnetic assemblage of magnetite and pyrrhotite. Newly formed magnetite and hematite carry the remanence. We propose that magnetite is formed at the expense of pyrite. Hematite results from the progressive oxidation of newly formed magnetite. Our results suggest the possibility that any claystones that pass the oil window can be remagnetized due to the unique action of temperature.     

Meteorite magnetism and the early solar system magnetic field

The ferromagnetism of irons, stony-irons, E-, H-, L- and LL-chondrites and achondrites is due to a metallic phase comprising mostly Fe and Ni and small amounts of Co and P. The ferromagnetic constituent in non-metamorphosed C-chondrites is magnetite, but some metamorphosed C-chondrites contain FeNi metallic grains too.

Among the stony meteorites, the content of metals as determined by their saturation magnetization (I_S) sharply decreases in the order E → H → L → LL → achondrites, whereas the I_S value for magnetite and additional metals in C-chondrites ranges from the I_S value of achondrites to that of L-chondrites.

With an increase of Ni-content in the metallic phase in chondrites of the order E → H → L → LL → C, the relative amount of Ni-poor kamacite magnetization, I_S(), in the total I_S decreases in the same order, from I_S()/I_S 1 for E-chondrites to I_S()/I_S 0 for C-chondrites. Thus, E-, H-, L-, LL- and C-chondrites and achondrites are well separated in a diagram of I_S()/I_S versus I, which could be called a magnetic classification diagram for stony meteorites.

As the surface skin layer of all meteorites is anomalously magnetized, it must be removed and the natural remanent magnetization (NRM) of the unaltered interior only must be examined for the paleomagnetic study. The NMR of C-chondrites is highly stable and that of achondrites is reasonably stable against AF-demagnetization, whereas the NMR of E-chondrites and ordinary chondrites as well as stony-iron meteorites is not very stable in most cases. Although the NRM of iron meteorites is reasonably stable, it is not attributable to the extraterrestrial magnetic field.

The paleointensity for Allende C₃-chondrite is estimated to be about 1.0 Oe assuming that its NRM is of TRM origin. The paleointensity for other reasonably reliable C-chondrites (Orgueil, Mighei, Leoville and Karoonda) is also around 1 Oe.

The paleointensity for two achondrites has been determined to be about 0.1 Oe. The NRM of other achondrites also suggests that their paleointensity is roughly 0.1 Oe.

The NRM of ordinary chondrites is less stable than that of C-chondrites and achondrites so that the estimated paleointensity for ordinary chondrites is less reliable. The paleointensity for comparatively reliable ordinary chondrites ranges from 0.1 to 0.4 Oe.

The paleointensity values of 1 Oe for C-chondrites and 0.1 Oe for achondrites may represent the early solar nebula magnetic field about 4.5 × 10⁹ years ago. A possibility that the paleomagnetic field for achondrites was a magnetic field attributable to a dynamo within a metallic core of their parent planet may also not be rejected.     

A note on magnetic modelling with remanence

The general problem of magnetic modelling involves accounting for the effect of both remanent magnetization and the application of an external magnetic field. However, as far as the disturbing field of a magnetic body in a magnetic environment is concerned, there is an equivalence between the effects of these two causations that allows the remanence to be represented in terms of an equivalent primary magnetic H field. Moreover, due to the linearity of the magnetic field in terms of its causations, the general modelling problem involving an applied magnetic field in the presence of remanence can be simply and more efficiently dealt with in terms of an equivalent primary field acting in the absence of any remanent magnetization.     

On the interpretation of lunar magnetism

It is proved that if a spherical shell is magnetized in the direction of and proportional to a magnetic field of origin internal to the shell and the magnetizing field later disappears, no magnetic field exists external to the shell. Similarly if a spherical shell is magnetized parallel to and proportional to a magnetic field of external origin and this magnetizing field later disappears, the magnetic field internal to the shell is zero. These theorems are true only if these ideal conditions are met, but are applicable to the interpretation of the natural remanent magnetization of the lunar crust. It is shown that the present absence of a magnetic dipole field of the Moon supports the hypothesis that the magnetizing field was of internal origin but does not distinguish whether this was due to a dynamo in the lunar core or to a primaeval magnetization of its interior. Local magnetic fields around the Moon are interpreted as arising from the departure from sphericity of the shell and large craters.     

Magnetic features of the magmatic intrusion that occurred in the 2007 eruption at Stromboli Island (Italy)

Significant changes in the local magnetic field marked the resumption of eruptive activity at Stromboli volcano on February 27, 2007. After differential magnetic fields were obtained by filtering out external noise using adaptive filters and seasonal thermal noise using temperature data, we identified step-like changes of 1–4 nT coincident with the opening of eruptive fissures in the upper part of the Sciara del Fuoco. The magnetic variations detected at two stations are closely related to the propagation of a shallow NE–SW magmatic intrusion extending beyond the summit craters area. These observations are consistent with those calculated using piezomagnetic models in which stress-induced changes in rock magnetization are produced by the magmatic intrusion. No significant magnetic changes were observed when the first fractures opened along the NE crater rim. Indeed, the stress-induced magnetization caused by this magmatic activity is expected to be too low because of the structural weakness and/or thermal state of the summit area. The continuous long-term decay characterizing the post-eruptive magnetic pattern can be related to a time-dependent relaxation process. A Maxwell rheology was assumed and the temporal evolution of the piezomagnetic field was evaluated. This allowed us to estimate the rheological properties of the medium; in particular, an average viscosity ranging between 10¹⁶ and 10¹⁷ Pa⋅s was a relaxation time τ of about 38 days.     

Thermo-magnetic measurements as a possible tool in the prediction of volcanic activity in the volcanoes of the Rabaul Caldera, Papua New Guinea

Volcano-magnetic effects provide a possible way of monitoring temperature or stress changes within a volcano. A systematic study of several volcanoes in and close to the Rabaul caldera in east New Britain, Papua New Guinea, has been made to identify those in which thermal changes would be expected to cause changes in magnetic field. Two out of nine were chosen as suitable for a programme of magnetic monitoring which commenced in February 1974 and is continued at approximately six monthly intervals. The main criterion used for selection is that the volcano must have a magnetic structure causing a strong anomaly in magnetic field with its source located where its temperature would respond to changing conditions in the volcano. In the first three years of monitoring, no thermomagnetic effects have been observed, but the ongoing programme may be a useful tool in the prediction of volcanic eruption. Estimation of the magnitude of possible field changes depends on determining a satisfactory model of the source of the magnetic anomaly and how the remanent magnetization of the source depends on temperature. The former is a standard problem in geomagnetism and may be solved by computer modelling methods. Measurements of the temperature dependence of thermo-remanent magnetization (TRM) for rocks collected in the Rabaul caldera show the latter to be a much more difficult problem to solve in detail. This is because the temperature range in which TRM changes most rapidly with temperature varies greatly from specimen to specimen, and depends both on the magnetic minerals present and the cooling history of the rock. Rocks from within the anomaly sources were not accessible so doubt remains as to how similar their magnetic properties are to those of rocks collected from the surface. Within this limitation, it is shown that for at least one of the volcanoes studied, changes of total magnetic field of several hundred nanoteslas (1 nT = 1γ) are possible prior to a future eruption.