magnetism - university of california, san...
TRANSCRIPT
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1 �
Chapter references are to Essentials of Paleomagnetism, UC Press, 2010 �
http://magician.ucsd.edu/Essentials �
Magnetism: �Applications in �
Earth and Environmental Sciences and �
Analytical Archaeology��
UCSD Fall 2011, SIO 249 - SIO 190 �
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2 �
Course structure�
3 major parts: �
• Introduction (history of research, basic concepts and tools for geomagnetists and archaeomegentists)�
• Physics and laboratory procedures�
• Applications �
" Follow syllabus / website: http://magician.ucsd.edu/~ltauxe/sio190 �
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3 �
What is �Rock magnetism?�
" The study of the magnetic properties of rocks and minerals�
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4 �
What is �Paleomagnetism?�Archaeomagnetism?�
" Paleo- application of magnetic measurements of rocks, minerals to solve geological problems, including problems related to the field itself�
" Archaeo- application of magnetic measurements of archaeological artifacts to solve archaeological and/or geological problems is called archaeomagnetism�
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The present magnetic field
Numerical simulation of the geomagnetic field !(Gary Glazsmaier, Nature 377:203-209,1995)!(a) Core fluid motion!(b) Magnetic lines of flux. Blue is out and orange is in
(a) Core fluid motion! (b) Field generated!
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First order model: geocentric axial dipole (GAD)
2005 reference field model
Essentials: Chapter 2
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Geomagnetic applications seek to define B, D and I as a function of space and time
Essentials: Chapter 2
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Geocentric Axial Dipole (GAD); D=?
The intensity of the field is twice as intense in the polar than in the equator
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Geocentric Axial Dipole (GAD)!!
The earth dipole is located in the center of the Earth and !
the dipole is parallel to the earth’s axis of rotation!
! therefore!
! The geomagnetic field can serve as an
absolute frame of reference!!
tan I = 2 tan λ (angle of geographic latitude)! D = 0°
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10 �
Getting a reference field model�
" Make a bunch of measurements of the geomagnetic field vector all over the world�
" Find the best-fit coefficients to a special equation (the geomagnetic potential equation)�
" These coefficients are the international geomagnetic reference field (IGRF)�
" Can calculate the geomagnetic field vector ANYWHERE outside the source (the core)�
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Different ways of representing the geomagnetic field
Lines of flux Field strength at the core mantle boundary
Essentials: Chapter 2
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Map of geomagnetic intensity of the IGRF for 1995 in µT!
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Map of geomagnetic inclinations of the IGRF for
1995 !
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Map of geomagnetic declinations of the IGRF for 1995!
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VGPs and VADMs
Essentials: Chapter 2
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17 �
Geomagnetism and rock magnetism: �
the oldest (geo)physical science�
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20
A. Jackson et al. (2000) compiled data from ship’s logs dating from 1600. Extended
IGRF-like field models back to 1600. Called the GUFM model
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21
1600 (GUFM1)
1990 (IGRF)
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22 �
" Prior to about 1600, measurements are few and far between �
" To study the field in times before that, we rely on “accidental” records�
" archeological materials (field of archeomagnetism)�
" geological materials (paleomagnetism)�
" Field models now combine sedimentary, archaeomagnetic and paleomagnetic data (e.g., Korte and Constable, 2005 goes back 7000 years!)�
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23 �
How do rocks get and stay magnetized? �
" Magnetic grains have directions of magnetization that are “easier” than others (lower energy) and it takes energy (anisotropy energy) to switch from one to another.�
" Thermal energy can cause magnetic moments to jump the barriers.�
" Over time, the magnetization can come into equilibrium�
" The ratio of anisotropy energy to thermal energy controls the rate at which equilibrium with applied field achieved (relaxation time) �
" Secret is to change relaxation time from short (in equilibrium) to long (blocked)�
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24 �
Types of magnetic remanence�
" Changing temperature from high to low (e.g., lava flows): thermal remanence (TRM)�
" Changing volume from small to large (e.g., chemical weathering): chemical remanence (CRM)�
" Changing from free rotation in fluid to fixed (e.g., in sediment): depositional remanence (DRM)�
Neel equation: �
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25 �
Basic concepts so far: �
• Rock magnetism�
• Paleomagnetism�
• Archaeomagnetism�
• GAD �
• B, D, I �
• tan I = 2 tan λ �
• VGP�
• VADM�
• Gilbert (16th century England)�
• GUFM�
• TRM, CRM, DRM�
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26 �
Applications �
" Ancient geomagnetic field behavior �
" Dating (geological and archaeological time scales)�
" Tectonic reconstructions (plate tectonic and structural geology)�
" Magnetic fabrics of materials (rocks, minerals, artifacts)�
" Magnetic mineralogy, magnetic grain sizes, domain states �
" Case-oriented problem solving (key: creativity) �
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Predictions from Korte and Constable (2005)
Essentials: Chapter 14
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28
longer records
Lund et al. 1988 Essentials: Chapter 14
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29
Reversal! Clement & Kent 1984
Essentials: Chapter 14
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30 �
Applications �
" Ancient geomagnetic field behavior �
" Dating (geological and archaeological time scales)�
" Tectonic reconstructions (plate tectonic and structural geology)�
" Magnetic fabrics of materials (rocks, minerals, artifacts)�
" Magnetic mineralogy, magnetic grain sizes, domain states �
" Case-oriented problem solving (key: creativity) �
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31
Mason & Raff, 1961
Geodetic ship Pioneer towed a magnetometer
around - discovered stripes!
Essentials: Chapter 15
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32 Pitman et al., (1966)
Seafloor spreading (Hess, 1962) plus reversals of the magnetic
field (Cox et al., (1963) explained the stripes
(Vine and Matthews, 1963)
Essentials: Chapter 15
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33
magnetostratigraphy: a dating tool
Tauxe et al. (1985)
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34
Ben-Yosef et al. 2008
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35 �
Applications �
" Ancient geomagnetic field behavior �
" Dating (geological and archaeological time scales)�
" Tectonic reconstructions (plate tectonic and structural geology)�
" Magnetic fabrics of materials (rocks, minerals, artifacts)�
" Magnetic mineralogy, magnetic grain sizes, domain states �
" Case-oriented problem solving (key: creativity) �
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36
The major lithospheric plates
Essentials: Chapter 16
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37
Movement for past 250 Myr
Jean Besse, Institut de Paris du Globe
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But how do we know this?
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39 Essentials: Chapter 16
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40 �
Applications �" Ancient geomagnetic field behavior �
" Dating (geological and archaeological time scales)�
" Tectonic reconstructions (plate tectonic and structural geology)�
" Magnetic fabrics of materials (rocks, minerals, artifacts) – anisotropy �
" Magnetic mineralogy, magnetic grain sizes, domain states �
" Case-oriented problem solving (key: creativity) �
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get set of coeficients relating Magnetization to applied field
Essentials: Chapter 13
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Isotropic
Oblate
Prolate
Triaxial
Sedimentary fabric
Particles aligned by flow
Tectonic strain
No preferred orientation
Anisotropy of magnetic parameters
Essentials: Chapter 13
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43 �
Applications �
" Ancient geomagnetic field behavior �
" Dating (geological and archaeological time scales)�
" Tectonic reconstructions (plate tectonic and structural geology)�
" Magnetic fabrics of materials (rocks, minerals, artifacts)�
" Magnetic mineralogy, magnetic grain sizes, domain states �
" Case-oriented problem solving (key: creativity) �
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Titanomagnetite (slow cooled igneous)
Titanomagnetite (wind blown dust)
Secondary hematite
iron rich spherule cosmic dust
magnetite chains bacteria
sedimentary flocs
Essentials: Chapter 8
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45 �
Applications �
" Ancient geomagnetic field behavior �
" Dating (geological and archaeological time scales)�
" Tectonic reconstructions (plate tectonic and structural geology)�
" Magnetic fabrics of materials (rocks, minerals, artifacts)�
" Magnetic mineralogy, magnetic grain sizes, domain states �
" Case-oriented problem solving (key: creativity) �
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“Fold Test,” Tel Rehov, Israel
Estimating firing temperature
by magnetic blocking temperature of archaeological materials
(copper slag, Arabah Valley, Israel)
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47�
Take home message�" Magnetic methods have broad
applications throughout Earth sciences�