magnetism - university of california, san...

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 �

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 �

3 �

What is �Rock magnetism?�

"   The study of the magnetic properties of rocks and minerals�

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�

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


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°

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)�

Different ways of representing the geomagnetic field

Lines of flux Field strength at the core mantle boundary


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


17 �

Geomagnetism and rock magnetism: �

the oldest (geo)physical science�

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

21

1600 (GUFM1)

1990 (IGRF)

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!)�

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)�

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: �

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�

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) �

Predictions from Korte and Constable (2005)


28

longer records

Lund et al. 1988 Essentials: Chapter 14

29

Reversal! Clement & Kent 1984


30 �

Applications �







31

Mason & Raff, 1961

Geodetic ship Pioneer towed a magnetometer

around - discovered stripes!


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)


33

magnetostratigraphy: a dating tool

Tauxe et al. (1985)

34

Ben-Yosef et al. 2008

35 �

Applications �







36

The major lithospheric plates


37

Movement for past 250 Myr

Jean Besse, Institut de Paris du Globe

But how do we know this?

39 Essentials: Chapter 16

40 �

Applications �"   Ancient geomagnetic field behavior �



"   Magnetic fabrics of materials (rocks, minerals, artifacts) – anisotropy �



get set of coeficients relating Magnetization to applied field


Isotropic

Oblate

Prolate

Triaxial

Sedimentary fabric

Particles aligned by flow

Tectonic strain

No preferred orientation

Anisotropy of magnetic parameters


43 �

Applications �







Titanomagnetite (slow cooled igneous)

Titanomagnetite (wind blown dust)

Secondary hematite

iron rich spherule cosmic dust

magnetite chains bacteria

sedimentary flocs


45 �

Applications �







“Fold Test,” Tel Rehov, Israel

Estimating firing temperature

by magnetic blocking temperature of archaeological materials

(copper slag, Arabah Valley, Israel)

47�

Take home message�"   Magnetic methods have broad

applications throughout Earth sciences�

magnetism - university of california, san...

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