11.4 Earth's magnetic field and paleomagnetism

Earth's magnetic field, generated deep in the planet's outer core, leaves a permanent record in rocks as they form. Paleomagnetism, the study of that preserved magnetic record, gave scientists some of the most convincing evidence for continental drift and seafloor spreading. This section covers how the field works, why it reverses, and how those reversals helped prove that tectonic plates move.

Earth's Magnetic Field

Origin of Earth's magnetic field

Earth's magnetic field comes from the geodynamo, a process driven by convection currents in the liquid iron-nickel outer core. As this molten metal circulates, it generates electrical currents that sustain a magnetic field through what's called the dynamo effect.

The field is dipolar, meaning it has a north and south magnetic pole, similar to a bar magnet. A few important details:

• The magnetic poles don't line up exactly with the geographic poles. They're offset and they slowly wander over time.
• Magnetic field lines run from pole to pole. At the magnetic poles, the lines point straight down into (or out of) the Earth's surface. At the magnetic equator, they run parallel to the surface.
• Field strength isn't uniform. It's strongest near the poles (~60,000 nT) and weakest near the equator (~30,000 nT).

Magnetic reversals and plate tectonics

Every so often, Earth's magnetic field completely flips its polarity: the north magnetic pole becomes the south, and vice versa. The most recent reversal, the Brunhes-Matuyama reversal, happened about 780,000 years ago.

These reversals get recorded in oceanic crust as it forms at mid-ocean ridges. Here's the process:

1. Magma rises at a mid-ocean ridge and begins to cool.
2. As the rock solidifies, magnetic minerals (especially magnetite) align with whatever direction Earth's magnetic field is pointing at that moment.
3. Once the rock cools past a critical temperature (the Curie point), that magnetic orientation is locked in permanently.
4. New magma pushes the older crust aside, and the process repeats.

The result is alternating bands of normal polarity (field oriented like today) and reversed polarity, arranged symmetrically on either side of the ridge. In 1963, the Vine-Matthews-Morley hypothesis explained these magnetic anomalies as direct evidence for seafloor spreading. The pattern was like a magnetic "barcode" stamped into the ocean floor.

By matching these reversal patterns to radiometric dates from volcanic rocks on land, scientists built the geomagnetic polarity timescale (GPTS). This timescale lets researchers determine the age of any patch of seafloor from its magnetic signature, which in turn reveals how fast plates have been moving.

Paleomagnetism and Plate Tectonics

Paleomagnetism in continental reconstruction

Paleomagnetism is the study of Earth's ancient magnetic field as preserved in rocks. When igneous rocks cool, sediments settle, or metamorphic rocks form, magnetic minerals inside them record the direction and intensity of the field at that time. This preserved magnetism is called remanent magnetization.

Because the magnetic field's orientation depends on where you are on Earth's surface, paleomagnetic data can reveal where a rock was when it formed. Two key applications:

• Paleolatitude: The angle at which the ancient field dips into the rock (its inclination) tells you how far from the equator the rock was at the time of formation.
• Apparent polar wander (APW) paths: By plotting where the magnetic pole appeared to be at different times (based on rocks of different ages from one continent), you get a path showing how that continent moved relative to the pole over geologic time.

This data strongly supports continental drift. When you take the APW paths for separate continents and slide the continents back together into their Pangaea configuration, the paths converge and match up. That convergence only makes sense if the continents were once joined.

Paleomagnetic data for tectonic movement

Paleomagnetic measurements give scientists two main pieces of information:

• Inclination (the dip angle of the preserved field) reveals paleolatitude. The relationship follows the dipole equation: $\tan I = 2 \tan \lambda$, where $I$ is inclination and $\lambda$ is latitude. Steep inclinations mean the rock formed near a pole; shallow inclinations mean it formed near the equator.
• Declination (the angle between magnetic north and true north in the rock) reveals rotation. If two adjacent crustal blocks show different declinations for rocks of the same age, one block has rotated relative to the other.

Seafloor magnetic anomalies add another layer of evidence. The age of oceanic crust increases with distance from the ridge axis, and the pattern of normal/reversed bands can be correlated across ocean basins worldwide. This allows scientists to calculate the relative motion between plates.

Combining paleomagnetic data with other geologic evidence (fossil distributions, rock type matching, structural geology) has allowed reconstruction of Earth's tectonic history over hundreds of millions of years. This includes the assembly and breakup of supercontinents like Rodinia and Pangaea, the opening of ocean basins like the Atlantic, and the closing of ancient ones like the Tethys Ocean.