1. Magnetic Field 3

2. 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)

3. Induced magnetic anomalies Difference in the magnetic anomalies depending on the magnitude latitude Assumes a profile running in a south-north direction

4. Induced magnetic anomalies Difference in the magnetic anomalies depending on the magnitude latitude

5. Interpretation of induced magnetic anomalies

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

7. Types of remanent magnetization When the materials cool below the Curie temperature, strong magnetization occurs, parallel to the Earth’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.

8. Paleomagnetic Interpretation Interpretation of the ages of the rocks (geochronology, paleomagnetic stratigraphy) Position of crustal blocks when the blocks formed (paleolatitude)

9. 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)

10. 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

11. Geomagnetic polarity reversal Observations: Campbell, 2003 (notice the different character of the anomalies measured at different depth)

12. Geomagnetic polarity reversal Scale: (for the last 4x10 6 years, Mankinen&Dalrymple, 1979) Superchron – long-lasting chrons Epochs (chron) – Brunhes, Matuyama, Gauss, Gilbert Events (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

13. 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

14. Geochronology based on paired magnetic anomalies across mid-ocean ridges Lillie Black arrows – normal polarity White arrows – reversed polarity

15. Geochronology based on paired magnetic anomalies across mid-ocean ridges Lillie Chrons only for the last 5 my

16. Geochronology based on paired magnetic anomalies across mid-ocean ridges Campbell, 2003 Field reversal for Reykjanes Ridge south of Iceland

17. Geochronology based on paired magnetic anomalies across mid-ocean ridges Lillie Slow spreading vs. Fast spreading