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Earth's magnetic field is generated primarily by convection of liquid iron in the outer core (geodynamo) and approximated as a dipole field tilted about 11° from the rotation axis. The field strength at the surface is ~25–65 microteslas and varies spatially and temporally. Secular variation (long-period changes) occurs on timescales of years to centuries; paleomagnetic reversals occur on timescales of thousands to millions of years.
You already know that Earth's interior is layered, with a solid inner core surrounded by a liquid outer core of iron-nickel alloy. It is this liquid outer core that generates Earth's magnetic field through a process called the geodynamo. Convective motions of electrically conducting liquid iron, driven by heat loss from the core and the crystallization of the inner core, create electric currents. Those currents, in turn, produce a magnetic field — and the field feeds back to organize the fluid flow, sustaining itself in a self-reinforcing loop. The result is a planetary-scale magnetic field that extends far into space and shields the surface from solar wind particles.
To a first approximation, Earth's magnetic field resembles the field of a magnetic dipole — essentially a giant bar magnet — positioned at the planet's center and tilted about 11° from the geographic rotation axis. This tilt is why magnetic north and geographic north do not coincide, producing a declination (the angle between true north and magnetic north) that varies by location. The dipole model also predicts how the field varies with latitude: at the magnetic poles, field lines are vertical and the field strength is strongest (~60–65 microteslas); at the magnetic equator, field lines are horizontal and the field is weakest (~25–30 microteslas). The angle that the field makes with the horizontal surface is called the inclination, and it varies systematically from 0° at the equator to ±90° at the poles. This latitude dependence is captured by the dipole equation tan(I) = 2 tan(λ), where I is inclination and λ is magnetic latitude — a relationship that becomes critical in paleomagnetism.
The dipole model captures about 90% of the observed field, but the real field is more complex. Non-dipole components — quadrupole, octupole, and higher-order terms described by spherical harmonic analysis — account for regional departures from the simple dipole pattern. These non-dipole features change over time in a phenomenon called secular variation: the field strength at any location drifts, declination angles wander, and patches of anomalous field migrate across the core-mantle boundary. Secular variation occurs on timescales of years to centuries and reflects changes in the pattern of convection in the outer core. Navigators have tracked declination changes for centuries, and repeat surveys of magnetic observatories document how the field evolves.
On much longer timescales — thousands to millions of years — the field undergoes polarity reversals, episodes during which the north and south magnetic poles swap. During a reversal, the dipole field weakens, the non-dipole components temporarily dominate, and the field eventually re-establishes with opposite polarity. Reversals happen at irregular intervals averaging roughly every 200,000 to 300,000 years, though some stable polarity intervals (called chrons) have lasted tens of millions of years. The record of these reversals, preserved in volcanic rocks and seafloor sediments, provides the foundation for magnetostratigraphy and was one of the key pieces of evidence confirming seafloor spreading and plate tectonics.