Magnetic Field 3

Earth’s magnetic field

Intensity (30,000 nT – equator to 60,000 nT – poles)

Inclination (0-equator to 90-poles)

Declination (most pronounced at the poles)

Induced magnetic anomalies

Difference in the magnetic anomalies depending on the magnitude latitude

Assumes a profile running ina south-north direction

Induced magnetic anomalies

Difference in the magnetic anomalies depending on the magnitude latitude

Interpretation of induced magnetic anomalies

amb

ambind

ind

amb

indamb

FFF

FFF

fieldmagneticinducedF

regiontheinfieldmagneticambients'EarthF

fieldmagnetictotalF

FFF

Interpretation of induced magnetic anomalies

ionmagnetizatremanenttoduefieldmagneticF

ionmagnetizatinducedtoduefieldmagneticF

regiontheinfieldmagneticambients'EarthF

FFFF

materialtheofionmagnetizatinducedJ

materialtheofionmagnetizattotalJ

JJJ

rem

ind

amb

remindamb

ind

remind

The total field anomaly is a function of the induced and remanent fields

Types of remanent magnetization

When the materials cool below the Curie temperature, strong magnetization occurs, parallel to theEarth’s ambient field! Crystals cannot rotate, the collection of magnetic moments is oriented in the direction of the ambient magnetic field

As sediments settle out in water, small mineral grains rotate so that their magnetic domains preferentially orient with the ambient field.

As ions are precipitated from solution, their magnetic domains align with the ambient field.

Paleomagnetic Interpretation

• Interpretation of the ages of the rocks (geochronology, paleomagnetic stratigraphy)

• Position of crustal blocks when the blocks formed (paleolatitude)

Paleomagnetic Interpretation

• Basic information for this studies – observation of the periodical reversal of the Earth’s magnetic field

-The magnetic field decreases to about 10% or less of its normal value and a reversal occurs then. (This is not completely proven.)-Time for a reversal to take place ~ 5,000 years (Cox and Dalrymple, 1973)

Polarity reversal studies – based on:

• Remanent magnetization of isotopically dated young (< 5 million year old) igneous rocks

• Observation of marine magnetic anomalies accompanied by potassium/argon dating

• Magnetic stratigraphy studies of sedimentary sections with unusually complete fossil records

Geomagnetic polarity reversal

• Observations:

Campbell, 2003

(notice the different character of the anomalies measured at different depth)

Geomagnetic polarity reversal

• Scale: (for the last 4x106 years, Mankinen&Dalrymple, 1979)

Superchron – long-lasting chronsEpochs (chron) – Brunhes, Matuyama, Gauss, GilbertEvents (subchron) – changes within the epochs (e.g. Kaena, Nunivak)Excursions (short-lived divergences of the magnetic axis away from the rotation axis of the Earth)

Parkinson, ????; Campbell, 2003

Geomagnetic polarity reversal rate

Increase of the reversal rate during the past 100 million years.

We are overdue for another major reversal. The last reversal was about 710 thousand years ago.Now the Earth’s dipole moment is rapidly declining.

Campbell, 2003

Geochronology based on paired magnetic anomalies across mid-ocean ridges

Lillie

Black arrows – normal polarityWhite arrows – reversed polarity

Geochronology based on paired magnetic anomalies across mid-ocean ridges

Lillie

Chrons only for the last 5 my

Geochronology based on paired magnetic anomalies across mid-ocean ridges

Campbell, 2003

Field reversal for Reykjanes Ridge south of Iceland

Geochronology based on paired magnetic anomalies across mid-ocean ridges

Lillie

Slow spreading

vs.

Fast spreading