Earth’s Magnetic Field and Paleomagnetism (Key Concepts)
Earth’s Magnetic Field: global, dynamic, and generated by the geodynamo in the outer core; extends into space and protects Earth from solar wind. It is asymmetric far from Earth due to solar wind influence.
Dynamo mechanism: swirling liquid iron in the outer core, driven by Earth’s rotation and convection, sustains the magnetic field; dynamic and can change over time.
Earth behaves like a giant magnet; compass needles align with the field. The north end of a compass points toward the Earth’s north magnetic pole, implying the Earth’s north pole is the south end of the global bar-magnet.
Magnetic records in rocks: many minerals are magnetic and, when rocks form, align with Earth’s magnetic field. A magnetometer measures this recorded field.
If a rock’s measured field doesn’t match Earth’s current field, either the field has changed or the rock has moved, or both, over geologic time.
Plate tectonics context: Earth’s lithosphere comprises rigid plates that move; rocks record the magnetic-field position relative to the field as they form.
Poles and declination: Geographic poles are Earth’s spin axis; magnetic poles are where a compass points. They move over time; averaged over thousands of years, magnetic north tends to align with geographic north.
Magnetic declination: the angle between magnetic north (MN) and true/geographic north (TN). Depends on location; zero along a line where MN and TN coincide. East declination is positive; west is negative. It changes with time and location; described by models such as the World Magnetic Model (WMM).
Magnetic inclination: tilt of the field lines; horizontal at the equator, vertical at the magnetic poles. Measured with an inclinometer; inclination indicates latitude.
Inclinometer basics: 3D-rotating compass needle shows inclination; Northern Hemisphere: N is angled downward; Southern Hemisphere: N is angled upward.
Plate tectonics + paleomagnetism: past rock magnetizations reveal paleopoles and the Apparent Polar Wander Path (APWP), which tracks how rocks moved relative to the pole, not how the pole moved.
Paleomagnetism case study (India): rocks dated from 60 Ma to present show India migrated from the Southern Hemisphere toward the Northern Hemisphere; current latitude is farther north. Paleopoles recorded in rock layers document past pole positions and latitudes.
Key terms: paleopole, APWP, apparent polar wander, continental drift, inclination, declination, magnetometer, geodynamo.
Example figures: 60 Ma, 40 Ma, 20 Ma, Recent inclination show India’s northward drift and changing paleolatitude over time.
Takeaway: Magnetic records in rocks, together with declination and inclination data, provide evidence for plate tectonics and continental drift, through the tracking of past pole positions and rock movements.
Important formulas and concepts to recall:
- Declination angle:
- Relation to pole movement: paleomagnetism records past latitude via inclination; present rocks record present pole position; changes reflect rock movement and/or pole motion over time.
- Inclination tells latitude: horizontal field at the equator, vertical field at poles; inclination angle increases toward the poles.
- Paleopole concept: past magnetic pole location inferred from rocks formed at a given time and place; APWP links paleopoles from a single site over time to show rock motion.