chronology of geomagnetic field reversals magnetic anomaly “number” ocean floor age, millions of...

Chronology of geomagnetic field reversals

magnetic anomaly “number”

Ocean floor age, millions of years (Ma), determined largely from deep sea drilling

Age, Ma

moho

Seafloor spreading is a tape recorder of the geomagnetic field!

crust

upper mantle

The recording head of the tape recorder

The tape drive

The recorded reversal chronology

Ocean Ridge system

Eas

t Pac

ific

Ris

e

Mid-A

tlantic Ridge

Modern view of ocean bathymetry derived from satellite altimetry. see EXPLORING THE OCEAN BASINS WITH SATELLITE ALTIMETER DATA Global Bathymetric Prediction for Ocean Modelling and Marine Geophysics

Global bathymetry

Map shown in next slide

Ship tracks across the East Pacific Rise which obtained the magnetic anomalies shown in the next slide. The measurements were made in the 1960’s by the Columbia University research vessel Eltanin.

21

20

19

The Eltanin 19 profile is among the most influential geophysical profiles ever published. It provided the “smoking gun” evidence for seafloor spreading, evidence that turned a majority of skeptics into a majority of believers. The profile was published together with three others in 1966 by Pitman and Heirtzler in “Magnetic Anomalies over the Pacific-Antarctic Ridge” (Science, 154, 1164-1171). The figures above comes from that paper. The track lines (ELT 19-21) of the research vessel Eltanin are shown together with the correlated magnetic anomalies (numbered dashed lines) and the 2000 fathom bathymetric contour. The crosses are earthquake epicenters. The inferred active spreading center would be between anomalies 1 and 1’. The voyage occurred in 1965.

The famous Eltanin 19 profile

The four profiles show total intensity anomalies and bathymetry (ocean depth in km) along the four tracks shown on the previous map. Note that track 20 crosses the ridge system twice.

The vertical scale for total intensity anomaly, F, is shown in “gammas”. This is the same as nanoTeslas or nT. The horizontal lines are at zero anomaly; the scale is thus minus 500 to plus 500 nT.

Eltanin profiles of magnetic anomalies

The incredible symmetry of the Eltanin 19 profile

ESEWNW

total intensity anomaly calculated from model

WNWESE

measured profile of total intensity anomalies

mirror image of measured profile to show symmetry

+500

-500

+500

-500

0

0

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”)

paral

lel 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

Map of magnetic anomaly numbers

Deep Sea Drilling sites

magnetic anomaly number

Age (Ma) from geomagnetic reversal chronology extrapolated in South Atlantic assuming constant rate of spreading

pale

onto

logi

cal a

ge,

Ma

Seafloor ages from deep sea drilling versus geomagnetic reversal chronology

Deep sea drilling in the South Atlantic Ocean

Chronology of geomagnetic field reversals

magnetic anomaly “number”

Ocean floor age, millions of years (Ma), determined largely from deep sea drilling

Geologic time scale

http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html

4600 My 600 My

Age range of modern ocean floor

Transform faults

7000

3500

1500

-250

0

-150

0

-500

0 500

-550

0

-450

0

-350

0

-650

0

South Atlantic Ocean

7000

3500

1500

-250

0

-150

0

-500

0 500

-550

0

-450

0

-350

0

-650

0

South Atlantic Ocean

inactive fracture zone

active transform faultactive ridge crest

inactive fracture zone

active transform fault

active ridge crest

inactive fracture zone

Transform faults and fracture zones

Map pattern of magnetic anomaly number

Deep sea drilling calibration of chronology of geomagnetic field reversals back to 180 Ma.

Map pattern of ocean floor age

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