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OCEAN/ESS 410
5. Evidence for Plate
Tectonics from Magnetics
William Wilcock
Lecture/Lab Learning Goals
• Understand the basic characteristics of the Earth’s
magnetic field and how one measures its orientation
• Know the different kinds of rock magnetization and
their use in paleomagnetism
• Be able to explain the historical concept of polar
wander and its explanation in terms of continental
drift
• Be able to explain patterns of marine magnetic
anomalies in terms of plate spreading and magnetic
field reversals
• Know how to interpret marine magnetic anomalies LAB
Magnetic north
north pole
Geographic north pole
Earth’s Magnetic Field
The Earth is surrounded by a
magnetic field that is strongest
near the poles. The magnetic
poles are displaced ~ 11.5°
from the geographic poles
about which the Earth rotates.
Magnetic south
south pole
Geographic south Pole
south pole
After Plummer
Geodynamo Theory: The
magnetic field is generated in
the liquid metal region of the
outer core. The outer core is
extremely hot and flows at a
rate of several km/yr in large
convection currents. Convecting
metal (Fe) creates electrical
currents, which in turn create
the magnetic field.
Earth’s Magnetic Field
•The Earth’s magnetic field close to a dipole. The
radial (vertical) and tangential (north-south)
components a dipole field are given by
2 m0 M cos q
Br =
4p r 3
m M sin q
Bq = 0
4p r 3
θ - Colatitude (0º at south pole; 90º at
equator; 180º at north pole
μ0 - magnetic permeability of a vacuum 4 x
10-7 N A-2
r - distance to the center of the earth (6.4 x
106 m at the Earth’s surface)
M - is the dipole moment which for the earth
is 7.95 x 1022 A m2
B - is the magnetic field. It units are Teslas
1 T = 1 kg A-1 s-2. 1 nT = 10-9 T = 1 Gamma
Earth’s Magnetic Field
From The way the Earth Works by P. J. Wyllie, Wiley 1976
Field is twice as strong at the poles as at the equator.
About 60,000 at poles and 30,000 at equator
Measuring the Orientation of the
Earth’s Magnetic Field
D
I
D = Declination (angle from geographic north)
From The way the Earth Works by P. J. Wyllie, Wiley 1976
I = Inclination (dip angle)
Measurements of the Earths
Magnetic Field in the Oceans
Measurements of the Earth’s magnetic field in the oceans
were developed in the 2nd World War as a way to detect
submarines (and later mines)
Measurements of the
magnetic field were first
made with a fluxgate
magnetometer. Such
instruments are still in use
today
Rock Magnetization
• Most minerals either repel or concentrate the Earth’s
magnetic field lines but do not themselves become
magnetized.
• A few ferromagnetic minerals retain magnetization.
In the oceanic crust the most important is magnetite
(Fe3O4). Others include ilmenite (FeTiO3), hematite
(Fe2O3), and pyrrhotite (FeS).
• Forms of rock magnetism
– Thermo remnant magnetism - rock becomes
magnetized when it cools below the Currie
temperature (580°C) in a magnetic field
– Detrital remnant magnetism - sediments settle in a
magnetic field
– Chemical remnant magnetism - Hematite
precipitates from a fluid circulating through a rock.
Paleomagnetism
In the 1950’s scientists learned how to measure the
remnant magnetism of rock samples. If one can be sure
that the rock has not been rotated by tectonic processes
then:
•The Declination of the remnant magnetism gives the
apparent direction of the North Pole at the time the rock
formed.
•The Inclination gives the latitude of the rock when it
formed
Geochronology
In the 1950’s scientists also developed reliable techniques
of dating rocks using radioactive isotopes
The potassium isotope 40K decays to 40Ar with a half-life of
1.3x109 years. As argon is a gas any traces of that element
will escape from rocks when they are molten. Therefore,
any argon found in solid rocks must have been produced
since that molten state ended and the rock solidified. The
ratio of 40K to 40Ar can be analyzed and a numerical date
since the last molten state can be assigned.
By combining paleomagnetic data from lava flows with the
lava ages, scientists were able to look at changes in the
apparent position of the Earth’s magnetic pole with time.
“Polar Wander”
(a) Position of the North pole relative to Europe and Asia
(b) Position of the North pole relative to Eurasia and North
America
Opening of the Atlantic
http://museum.gov.ns.ca/fossils/geol/globe.htm
Polar Wander and Continental Drift
K - 100 Myr; Tru - 200 Myr; Cu - 300 Myr; € - 500 Myr
(a) Polar wander for North America and Eurasia
(b) Polar wander corrected for the opening of the Atlantic
Evidence for Continental Drift - pre1960’s
Evidence
• Fit of the Atlantic Coastlines and Geology
• Paleontology (Fossils)
• Paleoclimate
• Paleomagnetism
Why wasn’t this evidence accepted?
• Physical impossibility of drift (the mantle is solid - it
transmits seismic waves)
• Difficulties of magnetic measurements - scatter,
reversals
• Conservatism
Polarity Reversals
The mechanism of
polarity reversals is
poorly understood but
they happen quickly
(within no more than
~1000 years)
Using volcanic rocks to develop a
polarity timescale
Most geoscientists were initially skeptical of magnetic reversals but
interest increased once it was realized that they provided a means to
date events
Polarity timescale from magnetized lava flows
The first timescales were obtained in the early 1960’s
History of Polarity Reversals
Cretaceous Quiet Zone
Jurassic Quiet Zone (a
period of very rapid reversals?)
Marine magnetic anomalies
The magnetization of the oceanic crust leads to small
variations in the intensity of the magnetic field measured
at the sea surface
Marine Magnetic Anomalies
If we remove the background Earth’s magnetic field from the
total magnetic intensity, we obtain the magnetic anomaly
Relationship Between Magnetic
Anomalies and the Polarity of the Crust
Magnetic Stripes
Raff and Mason, 1961
Vine and Matthews’ Magnetic Tape Recorder
Normally
magnetized
crust
N
dikes
oceanic crust
Reversely
magnetized
crust
Magma
N
N
Magma
N
N
N
Magma
Normally
magnetized
crust
Vine and
Matthews’
magnetic tape
recorder
Global bathymetry, showing ocean ridge system
Map shown
in next slide
Location of the Eltanin-19 profile
19
Ship track across the East Pacific Rise which obtained the magnetic
anomaly profile shown in the next slide. The measurements were made in
the 1960’s by the Columbia University research vessel Eltanin.
Eltanin 19 Magnetic Anomaly Profile
Magnetic anomaly, gamma
Ocean depth, km
The vertical scale for total intensity anomaly 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.
Symmetry of the Eltanin 19 profile
ESE
WNW
WNW
ESE
mirror image of measured
profile to show symmetry
measured profile of
total intensity anomalies
Polarity Reversals and Spreading Rate
Depth, m
Polarity Reversals and Sedimentation Rates
Age of the Seafloor