Magnetism - University of California, San...
Transcript of Magnetism - University of California, San...
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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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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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What is �Rock magnetism?�
" The study of the magnetic properties of rocks and minerals�
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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�
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!
First order model: geocentric axial dipole (GAD)
2005 reference field model
Essentials: Chapter 2
Geomagnetic applications seek to define B, D and I as a function of space and time
Essentials: Chapter 2
Geocentric Axial Dipole (GAD); D=?
The intensity of the field is twice as intense in the polar than in the equator
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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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)�
Different ways of representing the geomagnetic field
Lines of flux Field strength at the core mantle boundary
Essentials: Chapter 2
Map of geomagnetic intensity of the IGRF for 1995 in µT!
Map of geomagnetic inclinations of the IGRF for
1995 !
Map of geomagnetic declinations of the IGRF for 1995!
VGPs and VADMs
Essentials: Chapter 2
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Geomagnetism and rock magnetism: �
the oldest (geo)physical science�
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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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1600 (GUFM1)
1990 (IGRF)
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" 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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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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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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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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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) �
Predictions from Korte and Constable (2005)
Essentials: Chapter 14
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longer records
Lund et al. 1988 Essentials: Chapter 14
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Reversal! Clement & Kent 1984
Essentials: Chapter 14
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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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Mason & Raff, 1961
Geodetic ship Pioneer towed a magnetometer
around - discovered stripes!
Essentials: Chapter 15
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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magnetostratigraphy: a dating tool
Tauxe et al. (1985)
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Ben-Yosef et al. 2008
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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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The major lithospheric plates
Essentials: Chapter 16
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Movement for past 250 Myr
Jean Besse, Institut de Paris du Globe
But how do we know this?
39 Essentials: Chapter 16
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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) �
get set of coeficients relating Magnetization to applied field
Essentials: Chapter 13
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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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) �
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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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) �
“Fold Test,” Tel Rehov, Israel
Estimating firing temperature
by magnetic blocking temperature of archaeological materials
(copper slag, Arabah Valley, Israel)
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Take home message�" Magnetic methods have broad
applications throughout Earth sciences�