Magnetic field intensity,F obs distance along ship track intensiy anomaly, F 0 0 Smoothly varying global field plus small, short wavelength effects of.

mag

net

ic f

ield

inte

nsit

y,F

obs

distance along ship track

inte

nsi

y an

omal

y,

F

0

0

Smoothly varying global field plus small, short wavelength effects of crustal magnitizations

distance along ship track

subtract global field to yield total intensity anomaly

Mag

netiz

ed re

ctang

ular p

risms

(“stri

pes”

) para

llel to

sprea

ding a

xis

direction of cross section (perpendicular to axis of spreading)

thickness of magnetized prisms

Strike of spreading ridge

Typical model for seafloor spreading type of anomaly: very long rectangular prism (2-D model)

long

itudi

nal o

r stri

ke di

recti

on, a

ssum

ed in

finite

Vertical, downwards

perpendicular to strike

Typical model for seafloor spreading type of anomaly: very long rectangular prism (2-D model)

long

itudi

nal o

r stri

ke di

recti

on, a

ssum

ed in

finite

thickness, t

width, w

Vertical direction of cross section, z axis

horizontal direction of cross section, x axis

vertical downwards magnetization equivalent to two parallel strips of magnetic poles as shown: top strip of negative poles and bottom strip of positive poles

Vertical, downwards

perpendicular to strike

strips extend to “infinity”

strips extend to “infinity”

+++ + + + + + + + + + + + +

- - - - - - - - - - - - - - - -

J

Magnetic field lines for vertically downwards magnetization in cross-sectional view

+++ + + + + + + + + + + + +

- - - - - - - - - - - - - - - -J

Magnetic field lines for vertically upwards magnetization

+++ + + + + + + + + + + + +

- - - - - - - - - - - - - - - -

J

Magnetic field due to magnetized prism taken along the surface above the prism (directions only)

Earth’s field, HeVertically downwards magnetization parallel to vertical earth’s field

ocean surface

+++ + + + + + + + + + + + +

- - - - - - - - - - - - - - - -

J

Magnetic field due to magnetized prism taken along the surface above the prism (directions only)

Earth’s field, He Magnetized prism field adds to Earth’s field, F positive

+++ + + + + + + + + + + + +

- - - - - - - - - - - - - - - -

J

Magnetic field due to magnetized prism taken along the surface above the prism (directions only)

Earth’s field, He Magnetized prism field adds to Earth’s field, F positive

+++ + + + + + + + + + + + +

- - - - - - - - - - - - - - - -

J

Magnetic field due to magnetized prism taken along the surface above the prism (directions only)

Earth’s field, He Magnetized prism field perpendicular to He, F = 0

+++ + + + + + + + + + + + +

- - - - - - - - - - - - - - - -

J

Magnetic field due to magnetized prism taken along the surface above the prism (directions only)

Earth’s field, He Magnetized prism field subtracts from He, F negative

F/2J

Magnetic total intensity anomaly from single vertically magnetized prism

distance, x, along cross section perpendicular to the trend of the rectangular prism

width = 10 km

F/2J

Magnetic total intensity anomaly from single vertically magnetized prism

width = 80 km

distance, x, along cross section perpendicular to the trend of the rectangular prism

F/2J

Five rectangular prisms: one central one plus two pairs symmetrically on either side (seafloor spreading model in the region near the spreading axis); magnetic anomaly of each prism is plotted separately

Earth’s field, He

Combination of rectangular prisms

Wo Wd

t

axis of spreading

F/2J

Multiple prisms vertically magnetized in alternate directions: combined effect

5 6

10.9 120.4

anomaly no. 1813 25 31 3421 M16M10N

M4M0 M21 M25

154.3 180

Age of ocean floor

From Muller, et al., 1997

Pangea

Tethys OceanPacific Ocean

Creation of Atlantic & Indian oceans and associated “passive margins”

closing of Tethys Ocean: collisional tectonics of Mediterranean-central and southeast Asia

Interaction of East Pacific Rise with western North America

Continued subduction beneath western South America as East Pacific Rise comes closer

5 6

10.9 120.4

anomaly no. 1813 25 31 3421 M16M10N

M4M0 M21 M25

154.3 180

Age of ocean floor

From Muller, et al., 1997