paleomagnetism and measurements

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The magnetic field

and magnetic measurements

Einar Ragnar Sigurðsson

27th

November 2013

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Outline of presentation

1. The magnetic field of Earth:▫ The dipole moment▫ Reversals

2. The rock magnetization: Induced and remanent

3. The remanent magnetization such as TRM, DRM, VRM

4. Different methods for measuring the rock magnetization

5. The master’s thesis: Rock magnetization in Icelandic volcanic zone

Einar Ragnar Sigurðsson - 27.11.2013 Paleo-magnetism and magnetic measurements 2

The Earth’s magnetic field during reversal. Ref: Glatzmaier, G.A. & Coe, R.S. (2007)

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The magnetic field of Earth• Generated be an electric

current in the liquid outer core.

• The dipole magnetic moment can describe 80-90% of the magnetic field of Earth.

• Now, there is a magnetic south pole in northern hemisphere and magnetic north pole in the south hemisphere.

• Simplification since 10-20% is left and how the current actually is, is not well known


(Kristjansson L., 1985)

Magnetic dipole generated from current in a loop

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The rotation axis of Earth and the magnetic dipole don’t fit -> Declination• Since the dipole doesn’t

correspond to the geographical poles we get the declination and from the nature of the dipole moment we get inclination angle.

▫ B: The total magnetic field

▫ H: The horizontal component of the field

▫ Z: The vertical component of the field

▫ I: Inclination angle, the dip of the magnetic field

▫ D: Declination. The angle between the (true) geographical north and the magnetic north


• (Kearey et al, 2002)

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The IGRF11 model


(NOAA – National Geophysical Data Center, 2013)

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The unstable magnetic field• Diurnal variations

• Source of change is solarwind and thunderstorms close to equator

• Can disturb magnetic measurements but is also the key to some electromagnetic resistance measurements (MT measurements)

• Secular variations• Slow changes with time, both in

intensity and location of the poles

• Source of change is assumed to be changes in the current generating the Earth magnetic field

• Measurable through paleo-magnetic measurements

• Can led to reversals of the magnetic poles• Major time periods with the same

direction of the magnetic field:

- Brunhes: 0-0.78 Ma (normal)

- Matuyama: 0.78-2.6 Ma (reversed)

- Gauss: 2.6-3.6 Ma (normal)

- Gilbert: 3.6-6.0 Ma (reversed)

• Measurable through paleo-magnetic measurements


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The geomagneticreversal• During the geomagnetic

reversals the dipole field wanders down to lower latitudes.

• In same time it weakens a lot

• Then it strengthens again when going to higher latitudes again finishing the reversal, if there is a reversal.

• Based on number of lavas the dipole field is most often close to the true north and south.


(Kristjánson, L)

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The rock it self can be magnetized : Induced and remanent• The induced magnetization, Ji is

induced by the present external magnetic force

▫ Is in same direction as the external field

▫ Ji depends on the material property called susceptibility (k)

𝐽𝑖 = 𝑘𝐻▫ Will disappear when external

field is “turned off”

• The remanent magnetization, Jiis not dependent on the present external magnetic field. It depends on:

▫ Properties of the material - the minerals

▫ The paleo-magnetic field from the time of formation and the rock and later as well

▫ It is permanent in the rock and the present external field doesn’t have any measurable effect on it.


Vector diagram showing how an external magnetizing force H (that could be from the

Earth magnetic field) induces a magnetic field Jithat is added to the remanent magnetic field Jrand hence giving the total magnetic field J. (Kearey et al, 2002)

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The remanent magnetization

Primary• Thermo Remanent

Magnetization, TRM• In igneous rock that solidifies and

cools through the Curie temperature

• Magnetic minerals:• Magnetite (Fe3O4)• Ulvöspinel (Fe2TiO4)

• Depositional Remanent Magnetization, DRM• magnetic particles are deposited

by sedimentation and they align with the current magnetic field

Secondary• Chemical Remanent Magnetization,

CRM• Magnetic minerals recrystallize or

grow during metamorphism

• Viscous Remanent Magnetization, VRM• Remanent magnetism is acquired

over time if a rock is subjected to an external field different from the original field

• Isothermal Remanent Magnetization, IRM• The magnetization can changes in

extremely high magnetic field such as from a lightning


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Different kinds of magnetic measurements: Ground magnetic survey • Detailed survey of limited area

• Measurement of magnetic field to find out residual magnetic field

• Residual magnetic field anomalies are caused by different magnitude and/or direction of magnetization (remanent + induced) of objects.

• The purpose is to locate and find out magnetic properties of hidden objects underground


(Kearey et al, 2002)(Photo: Einar Ragnar Sigurðsson)

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Different kinds of magnetic measurements:Aeromagnetic survey – Marine magnetic survey

• Survey covering large area

• Magnetic anomalies caused by different magnitude and/or direction of magnetization (remanent + induced) of objects.

• The purpose is among other things mapping of magnetic anomaly pattern related to plate movements and magnetic reversals.


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Different kinds of magnetic measurements:Susceptibility measurement of a sediment core• The method is convenient and non-

destructive.• The core is put into a magnetic field and

the induced magnetization in the core is measured.

• The outcome of the measurement is the susceptibility of the material in the sediment core.

• High susceptibility value as a material property may indicate:

▫ Particulate pollution▫ Ancient forest fires▫ Floods▫ Bands of volcanic tephra

• See graph from a 12m core piece of sedimentin a lake in Washington State, U.S.


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Measurements of remanent rock magnetization

• Sample (lava) from the field• Preparation of the sample: Demagnetization of VRM• Measurement in a laboratory

• Calclation of paleo-declination and inclination• Paleo virtual dipole


copyright of figure: Leó Kristjánsson

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Drilling: Taking the core• Originally a chain saw.

• Made in USA

• An exchangeable front end with industrial diamond crown

• Can last for 100 lavas

• Diameter 25mm

• Approximately 1 minute to drill one core

• Common to take have 4 samples for each lava

• Unsuccessful drilling requires on average drilling 6 cores

• Use of cooling water -> difficulties when temperature is below 0°C

• 1 liter for one lava

• Centrifugal coupling


Photo: Einar Ragnar Sigurðsson

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The orientation of the core• Richard Doell method, 50 years old!

• 1” tube and Brunton compass

• The direction to the sun or other known place is taken with the compass Use of GPS if no such place

• For orientation of the specimen: The upper top end of the core is marked with a wire.

• Finally: Break the core from the lava


Photos: Leó Kristjánsson and Einar Ragnar Sigurðsson

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The core• Diameter 24.5 mm

• Length originally ca 50 mm

• Sawed to appropriate length to have the cylinder as symmetric as possible, that is a spear like cylinder with

𝐿 = 0.5𝐷 3 ≈ 21 𝑚𝑚

• Orientation of the specimen is very important: The z axis as the axis of the cylinder

The y axis is perpendicular to the z axis and was horizontal in the original rock

The x axis is perpendicular to the two other axis and sloping upwards 90° from the slope of the core


Photos: Einar Ragnar Sigurðsson

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The magnetic measurement


Photos: Einar Ragnar Sigurðsson and Leó Kristjánsson

• Measured in all possible directions and al orientations: In x, y, z direction Positive and negative direction For all four orientations Total: 3 x 2 x 4 = 24

measurements

• The equipment is perpendicular to Earth magnetic field and as well surrounded with a coil to minimize the effect of external magnetic field.

• Very old equipment from institute dr Förster in Germany Tube amplifier from 1978 and

design from 1968!

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The data processing• The output from the

magnetometer is mV for all 24 measurements goes to a computer program, see figures.

• Average for each of x, y and z

• Inclination and declination of the specimen

• 𝐴𝑚𝑝𝑙 = √(𝑥2 + 𝑦2 + 𝑧2)

• Calibrated with known coil (see the small figure) to change the measured amplitude in mV to the magnetization of the specimen in A/m (J0 in the figure)



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Demagnetization: “Washing” the specimen• The specimen is put in to a

“magnetic washing machine” where the specimen is rotated for a short time in a strong magnetic field.

• Rotated to get effect on the VRM in all possible directions



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The theory behind the magnetic field demagnetization

• The VRM is softer than TRM so it will disappear first in a strong external alternating magnetic field

• Magnetic field goes up to strengths such as: 10-15-20-25-30-35 mT and down again

• Note: 10 mT is approximately 200 times larger than magnetic field of the Earth

• The washing is repeated until the change in declination and/or inclination goes down to 1 or 2°

• Stop washing before all TRM has been washed out so there will be some magnetic field left for the final measurement


Ref: Kristjansson, L., 1985

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Some interesting points regarding the magnetic field demagnetization

• The first steps of washing can increase the dipole moment as for the GL lavas.

• The washing will not have so much effects on lavas younger than 700 thousand years.• In that case both TRM and VRM are normal or positive magnetized

• The washing will have very large effect on a 800 thousand years old lava. • That lava should have reversed TRM but most of the VRM is normal magnetized.


Ref: Kristjansson, L., 1985

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The master’s thesis• Measurements of rock

magnetization inside the volcanic zones

• Previous measurements mostly outside the volcanic zones Difficulties with hyaloclastite Difficulties with more complicated

geology than flat lavas Recognizing the magnetic reversal

important

• Rock magnetization measurements in Icelandic table mountains

• Find out changes in declination from the eruptions: In the pillow lava In the lavashield on top of the

mountain For how long time was the

eruption lasting?


(Náttúrufræðistofnun ísladns)

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History of magnetic field in Reykjavik


Declination and total magnetic field in Reykjavik (# Latitude: 64.14 degrees North, Longitude: 21.9 degrees East) from 1900 in IGRF 11.

Calculations from NOAA – National Geophysical Data Center web page.

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52500

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1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

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Declination

Total Field

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(Merrill & McElhinny, 1983)

Rock-magnetic measurements in table mountains• Some disadvantages:

Sometimes the declination can stay stable

The declination goes up and down

Not very precise – we need very long eruptions to see any change in declination

• Has been done at least once in Iceland Hlöðufell south of Langjökull

Did not show difference in declination


Change of declination in China

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References• Glatzmaier, G.A. & Coe, R.S., 2007. Magnetic polarity

reversals in the core. Treatise on Geophysics, Volume 8, CoreDynamics, Chp. 9, eds. P. Olson and G. Schubert (Elsevier), pp. 283-297.

• Kearey, P., Brooks P.,Hill, I (2002). An Introduction toGeophisycal Exploration. Malden: Blackwell publishing.

• Kristjansson L., 1985. Bergsegulmælingar – nytsöm tækni viðjarðfræðikortlagningu. Náttúrufræðingurinn, 54 (3-4), p. 119-130

• Merrill, R.T. & McElhinny, M.W., 1983. Earth's Magnetic Field: Its History, Origin and Planetary Perspective (International Geophysics Series). Academic Press Inc

• NOAA – National Geophysical Data Center, 2013. MagneticField Calculators, retrieved from http://www.ngdc.noaa.gov/geomag on 15.11.2013.


http://www.ngdc.noaa.gov/geomag on 15.11.2013

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Alternative method for demagnetization• Heating the specimen to

successively higher temperatures in zero field

• Measure remanent magnetic field at room temperature after cooling in zero field

• Disadvantages: The magnetic minerals are often

altered during the heating.

Time consuming

• Advantages Works better on specimens from

sedimentary rocks than demagnetization employing alternating field


Photo: www.kochi-core.jp/en/facilities_and_equipment/magnetism.html

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Other methods

• The spinner magnetometer The specimen is rotated at

constant rate by a motor, close to a pick-up coil or fluxgate probe.

Electronic circuits are used to amplify only a signal of the same frequency as that of the rotation

More time consuming method

better result with weaker magnetization

• uperconducting quantum interference device (SQUID, cryogenic) magneto- meters Can be used for very weakly

magnetic rocks, even limestones


Photo showing a spinner

Photo showing SQUID equipment (ref www.kochi-core.jp/en/facilities_and_equipment/magnetism.html)

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SQUID


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History of magnetic field in USA –Washington DC


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1750 1800 1850 1900 1950 2000

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Declination in Washington DC (# Latitude: 38.89 degrees North, Longitude: 76.59 degreesWest) from 1900 in IGRF 11.

Calculations from NOAA – National Geophysical Data Center web page.

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History of magnetic field in China


(Merril & McElhinny, 1983)

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History of magnetic field in London


paleomagnetism and measurements

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