Measuring Ancient Field Strength
While paleomagnetic directions are recorded directly by the orientation of magnetic minerals, field intensity is encoded in the magnitude of magnetization — a much more difficult quantity to extract reliably. The Thellier double-heating method, developed in 1959, remains the gold standard: by systematically replacing the rock's natural magnetization with a laboratory-induced one at progressively higher temperatures, the ratio of ancient to laboratory field strength can be determined with high precision.
The Arai Plot
The Thellier experiment produces data plotted on an Arai diagram: NRM remaining versus TRM acquired at each temperature step. An ideal specimen yields a perfectly straight line whose slope directly gives the paleointensity ratio. In practice, thermal alteration (mineral changes during heating) causes the line to curve at high temperatures, and pTRM checks — repeating earlier steps to detect alteration — are essential quality controls. Only specimens passing strict linearity and alteration criteria yield reliable results.
Virtual Dipole Moments
To compare paleointensity measurements from different latitudes, the local field strength is converted to a Virtual Dipole Moment (VDM) using the dipole formula. This removes the latitude dependence (the field is twice as strong at the poles as at the equator) and provides a global measure of geodynamo strength. The present-day dipole moment is about 8×10²² A·m²; values during the Mesozoic Dipole Low dropped below 4×10²² A·m².
The Geodynamo Through Time
Paleointensity records spanning billions of years reveal a dynamic geodynamo. The field was relatively weak and volatile before the inner core nucleated (estimated at 1-1.5 Ga), strengthened dramatically afterward, experienced the prolonged Mesozoic Dipole Low during a period of infrequent reversals, and has fluctuated on million-year timescales ever since. Understanding these long-term intensity variations constrains models of core evolution, inner core growth, and the thermal history of the planet.