E10 Gravitational and Magnetic Field

Topic

Earth's gravitational and magnetic fields are two fundamental global observables that reveal both the planet's internal structure and its active dynamics.

A gravity anomaly measures the mass excess or deficit beneath a given measurement station: a negative value under a mountain range indicates that the light crustal root extending into the mantle compensates for—or even outweighs—the additional mass contributed by the surface topography itself; conversely, a positive value appears in volcanic arcs and subduction zones, where dense mass—specifically a sinking cold slab—approaches the surface.

Two complementary isostatic mechanisms explain how the crust maintains equilibrium while floating on the mantle: one proposes that the entire crust has uniform density and that a deeper root compensates for a taller mountain; the other proposes that crustal density varies laterally, such that a lighter crustal column rises higher without needing to extend an additional root into the mantle.

The geomagnetic field is generated by a geodynamo—electric currents circulating within the liquid iron-nickel outer core. Magnetohydrodynamic mechanics convert the kinetic energy from convection in this liquid core into the magnetic energy that sustains the geomagnetic field; the resulting field is currently predominantly dipolar, tilted at an angle of nearly eleven degrees relative to Earth's axis of rotation.

When the magnetic north and south poles completely swap places, the reversal is recorded in the rocks themselves as they cool below a critical temperature that locks in the magnetic field's orientation at that moment; this rock-recorded paleomagnetism provided the evidence that ultimately confirmed both seafloor spreading and the comprehensive theory of plate tectonics. The inclination recorded by that paleomagnetism also makes it possible to calculate the latitude at which the rock formed in the past, using a well-established formula that directly relates that magnetic inclination to geographic latitude; this closes the link between the gravitational and magnetic fields—with which this description began—and the complete history of the planet that the two fields, together, allow us to reconstruct.

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