Paleomagnetism and measurements
Transcript of Paleomagnetism and measurements
Slide no:
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
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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
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(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
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• (Kearey et al, 2002)
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The IGRF11 model
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(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.
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(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.
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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
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(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
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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
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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
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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
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Photos: Einar Ragnar Sigurðsson
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The magnetic measurement
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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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Photos: Einar Ragnar Sigurðsson
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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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Photos: Einar Ragnar Sigurðsson
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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
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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.
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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?
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(Náttúrufræðistofnun ísladns)
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History of magnetic field in Reykjavik
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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.
50000
50500
51000
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52000
52500
-40
-35
-30
-25
-20
-15
-10
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0
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
Tota
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net
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ield
[n
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Dec
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Year
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
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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.
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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
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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
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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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-12
-10
-8
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-4
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0
1750 1800 1850 1900 1950 2000
Dec
linat
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[d
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Year
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
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(Merril & McElhinny, 1983)
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History of magnetic field in London
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