3. Evidence for Plate Tectonics from Magnetics

William Wilcock

OCEAN/ESS 410

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

south pole

south pole

north pole

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.

Geographic north pole

Geographic south Pole

Magnetic south

Magnetic north

After Plummer

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.

Earth’s 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

θ - Colatitude (0º at south pole; 90º at equator; 180º at north pole

μ0 - magnetic permeability of a vacuum 4π 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

Field is twice as strong at the poles as at the equator. About 60,000 γ at poles and 30,000 γ at equator

From The way the Earth Works by P. J. Wyllie, Wiley 1976

Measuring the Orientation of the Earth’s Magnetic Field

D

From The way the Earth Works by P. J. Wyllie, Wiley 1976

I

D = Declination (angle from geographic north)

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.

PaleomagnetismIn 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’sEvidence• 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

The mechanism of polarity reversals is poorly understood but they happen quickly (within no more than ~1000 years)

Polarity Reversals

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 anomaliesThe magnetization of the oceanic crust leads to small variations in the intensity of the magnetic field measured at the sea surface

Marine Magnetic AnomaliesIf 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

Normally magnetized crust

Magma

dikesoceanic crust

Reversely magnetized crust

Normally magnetizedcrust

N

N

N

N

NN

Magma

Magma

Vine and Matthews’ Magnetic Tape Recorder

Vine and Matthews’

magnetic tape recorder

Eas

t Pac

ific

Ris

e

Mid-A

tlantic Ridge

Map shown in next slide

Global bathymetry, showing ocean ridge system

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.

19

Location of the Eltanin-19 profile

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.

Ocean depth, km

Magnetic anomaly, gamma

Eltanin 19 Magnetic Anomaly Profile

ESEWNW

WNWESE

measured profile of total intensity anomaliesmirror image of measured

profile to show symmetry

Symmetry of the Eltanin 19 profile

Polarity Reversals and Sedimentation RatesD

ep

th,

m

Polarity Reversals and Spreading Rate

Age of the Seafloor