4.4 Paleomagnetism and its applications

Paleomagnetism and Rock Magnetism

Principles of Paleomagnetism

Paleomagnetism studies Earth's ancient magnetic field as preserved in rocks. When rocks form or cool below their Curie temperature, magnetic minerals lock in a record of the field's direction and intensity at that moment. This "fossil magnetism" gives us a direct window into what Earth's magnetic field looked like millions or even billions of years ago.

The permanent magnetization rocks acquire this way is called natural remanent magnetization (NRM), and it comes in several flavors depending on how the rock formed:

• Thermal remanent magnetization (TRM): Acquired when magnetic minerals in igneous rocks cool below their Curie temperature in the ambient field. This also applies to sedimentary rocks baked by nearby igneous intrusions. TRM tends to be the most stable and reliable form of NRM.
• Chemical remanent magnetization (CRM): Acquired when new magnetic minerals form or grow through chemical processes (diagenesis, metamorphism) in the presence of an ambient field. Because the minerals crystallize at lower temperatures, CRM can sometimes record a field direction different from the original formation age.
• Detrital remanent magnetization (DRM): Acquired when magnetic mineral grains physically align with Earth's field as they settle through water and become locked into sedimentary layers. DRM is common in fine-grained sediments like red beds and deep-sea clays.

Natural Remanent Magnetization (NRM)

NRM is the foundation of all paleomagnetic work. Its usefulness depends entirely on how faithfully it preserves the original field signal.

The stability of NRM is controlled by several factors:

• Magnetic mineral composition: Magnetite ($\text{Fe}_3\text{O}_4$) and hematite ($\alpha\text{-Fe}_2\text{O}_3$) are the most common carriers. Magnetite has a Curie temperature of ~580°C and tends to carry strong, stable remanence. Hematite has a higher Curie temperature (~675°C) but weaker magnetization.
• Grain size: Single-domain and pseudo-single-domain grains hold the most stable remanence. Large multi-domain grains are more susceptible to remagnetization.
• Secondary overprints: Later thermal or chemical events can partially or completely reset the original NRM. Paleomagnetic lab techniques (progressive thermal or alternating-field demagnetization) are used to strip away these overprints and isolate the primary magnetization.

Rocks with particularly stable NRM include basalts, volcanic tuffs, and well-preserved sedimentary rocks such as red beds and certain limestones.

Apparent Polar Wander and Plate Tectonics

Apparent Polar Wander (APW)

If you measure paleomagnetic directions from rocks of different ages on a single continent, the calculated pole positions appear to move through time. This phenomenon is called apparent polar wander (APW).

The key insight: the magnetic poles haven't actually migrated across the globe. Instead, the continent itself has moved relative to Earth's spin axis. The "wandering" is apparent because we're viewing the pole from a reference frame fixed to a moving plate.

APW paths are constructed by plotting paleopole positions (ancient magnetic pole locations) from progressively older rocks on a single continent. Each continent produces its own APW path. For example, the North American APW path and the European APW path diverge going back in time, reflecting the opening of the Atlantic Ocean.

Plate Tectonics and Paleomagnetism

APW paths provided some of the most compelling early evidence for plate tectonics:

• When APW paths from two continents that were once joined (say, North America and Europe before the Atlantic opened) are reconstructed, they converge into a single path for the period when those continents were together. This convergence is strong evidence for past supercontinents like Pangaea and Rodinia.
• Differences between APW paths from separate continents reveal when and how those continents split apart and drifted to their current positions.
• Paleomagnetic data have been instrumental in reconstructing the breakup of Pangaea (~200 Ma), tracing the separation of Africa and South America, the northward drift of India, and the assembly of earlier supercontinents.

Paleomagnetic Data for Reconstruction and Geodynamo Studies

Continental Reconstruction

Paleomagnetic measurements give two pieces of information that are directly useful for placing continents on the ancient globe:

1. Inclination tells you the paleolatitude. Earth's magnetic field geometry means that inclination varies systematically with latitude. The relationship is given by the dipole formula: $\tan(I) = 2\tan(\lambda)$, where $I$ is the magnetic inclination and $\lambda$ is the latitude. Steep inclinations indicate high latitudes; shallow inclinations indicate positions near the equator.
2. Declination tells you the rotation of the continent relative to magnetic north. If a continent has rotated clockwise since the rock formed, the recorded declination will be offset from the expected north direction.

By comparing paleolatitudes and declinations from rocks of the same age on different continents, you can reconstruct their relative positions. This approach has been used to map out the configurations of Gondwana, Laurasia, and the full Pangaea supercontinent.

One important limitation: paleomagnetic data constrain paleolatitude and rotation but not paleolongitude. Determining east-west positions requires additional geological or hotspot-track evidence.

Geodynamo Studies

Beyond plate reconstructions, paleomagnetic records also reveal how Earth's magnetic field generator (the geodynamo) has behaved over deep time.

• Field reversals: Paleomagnetic data show that Earth's magnetic field periodically flips polarity, with magnetic north and south switching places. These reversals are not periodic; the intervals between them range from tens of thousands to tens of millions of years. The most recent major reversal is the Brunhes-Matuyama reversal (~780 ka), and shorter events like the Jaramillo subchron (~1.07–1.00 Ma) are also well documented.
• Paleointensity: The strength of the ancient field can be estimated from the intensity of NRM (using techniques like the Thellier method for TRM-bearing rocks). These records show that field intensity drops significantly during reversals and varies by factors of 2–5 over millions of years.
• Secular variation: Shorter-term fluctuations in field direction and intensity, recorded in rapidly deposited sediments or lava flow sequences, provide constraints on the dynamics of convection in Earth's outer core.

Together, these observations help geophysicists test and refine numerical models of the geodynamo.

Magnetic Stratigraphy for Dating and Correlation

Dating Sedimentary Sequences

Magnetostratigraphy exploits the fact that polarity reversals are recorded globally and (essentially) simultaneously, making them powerful time markers.

Here's how it works in practice:

1. Collect oriented samples at closely spaced intervals through a sedimentary section.
2. Measure and demagnetize each sample in the lab to isolate the primary NRM and determine whether it records normal or reversed polarity.
3. Construct a magnetic polarity stratigraphy for the section: a column of normal and reversed polarity zones.
4. Compare this pattern to the geomagnetic polarity timescale (GPTS), which is the calibrated global record of polarity reversals dated using radiometric methods (primarily $^{40}\text{Ar}/^{39}\text{Ar}$ dating of lava flows and ocean-floor magnetic anomalies).
5. Match the observed polarity pattern to the GPTS to assign ages to the section.

Pattern matching works best when you have at least one independent age tie-point (a radiometric date or a well-dated biostratigraphic datum) to anchor the correlation.

Correlating Geological Events

Because polarity reversals are global, magnetostratigraphy allows you to correlate sedimentary sequences across different basins and even different continents with high precision.

This capability is especially valuable for:

• Dating sequences that lack fossils or volcanic ash layers. Many continental sedimentary sections and deep-sea cores have limited biostratigraphic control, and magnetostratigraphy can fill that gap.
• Establishing the timing of major geological events. Magnetic stratigraphy has been used to precisely date the Cretaceous-Paleogene (K-Pg) boundary (~66 Ma), constrain the duration of the Eocene-Oligocene climate transition (~34 Ma), and bracket the Messinian salinity crisis (~5.96–5.33 Ma) when the Mediterranean Sea largely evaporated.
• Building integrated timescales. Magnetostratigraphy is routinely combined with biostratigraphy, cyclostratigraphy (orbital tuning), and radiometric dating to produce the most accurate geological timescales available.

The resolution of magnetostratigraphy depends on the reversal frequency during the time interval of interest. During periods of frequent reversals (like much of the Cenozoic), resolution can be better than 100 kyr. During long intervals of stable polarity (like the Cretaceous Normal Superchron, ~84–124 Ma), magnetostratigraphy provides no age resolution at all.