Paleomagnetism and a Mechanism
Paleomagnetism and Apparent Polar Wandering
Paleomagnetism showed that the apparent polar wandering paths indicated continents had moved across the Earth’s surface over time. However, these apparent polar wandering paths (APWP) alone do not explain how that motion occurred. A second feature of the paleomagnetic record did point toward a possible mechanism: the Earth’s paleomagnetic field is generated by the flow of liquid metals in the outer core, and changes in this flow have caused the magnetic field to reverse polarity many times.
Mechanisms for Reversals: Normal vs Reverse Polarity
How could reversals happen? If a planet like Earth is spinning rapidly and has a solid inner core, the inner core’s slightly faster rotation can organize the outer core flow into spirals that generate a magnetic field. There are two equally stable options for the direction of these spiral patterns. They can spiral from the planet’s South pole toward the North Pole, which produces the current orientation and is termed normal polarity. The same spinning configuration could just as easily produce spirals that flow from the planet’s North pole toward its South Pole, resulting in a magnetic field with the opposite orientation, termed reverse polarity. Both patterns are stable, and each is as likely to form as the other. We call the current orientation normal polarity because it is the one we are used to.
What causes the Earth’s magnetic field to flip back and forth? We still do not know for certain. One plausible possibility is that metals in the outer core begin to crystallize along the bottom of the core–mantle boundary as Earth slowly loses heat. The crystals formed would be denser than the surrounding liquid metal, which could alter the flow and lead to a reversal.
Paleomagnetic Signatures in Rock: Direction, Inclination, and Polarity
In the previous segment we discussed how compass direction and inclination of a rock’s paleomagnetic signature allow reconstruction of the parent polarity monitoring paths for each rock layer. The paleomagnetic signature itself has a polarity that can be measured. Although the exact disruption mechanism of outer-core flow is unknown, we know that disruptions have occurred, because rocks carry paleomagnetic signatures recording successive layers (such as lava flows) that show reversals. The pattern of reversals was first recognized in land records and was known during Wegener’s lifetime. It wasn’t until after Wegener’s death that we obtained a robust record of seafloor paleomagnetic signatures.
Seafloor Magnetism and the Zebra Stripe Pattern
During World War II, magnetometers were used to detect enemy submarines. When instruments were pulled above the seafloor, abrupt transitions between higher and lower magnetic intensities appeared, revealing a zebra-like stripe pattern of high and low magnetic intensities. The Earth’s present magnetic field is roughly uniform in intensity, so these anomalies must be recording paleomagnetism of the seafloor rocks as they formed under the Earth’s magnetic field.
If a seafloor rock has normal polarity, its magnetization adds to the current field and increases the measured field. If it has reverse polarity, its magnetization opposes the current field and lowers the measured intensity. The pattern of normal and reverse polarity is symmetrical about the axis of the mid-ocean ridges, which can be explained by magma rising at ocean ridges, cooling, and forming new seafloor rock. As the magma rises and cools, the newly formed crystals align with the Earth’s magnetic field at the time of formation. Thus, the seafloor rocks record the polarity of the field when they formed. If the Earth’s field later reverses, rising magma will form seafloor rock that records the new polarity. In short, reversals are recorded during rock formation, much like a tape recorder captures the surrounding environment.
Seafloor Age, Spreading, and Wegener’s Missing Mechanism
This means that the pattern of magnetic intensity measured over the seafloor can indirectly be used to determine-seafloor age, implying that the seafloor is relatively young and did not form all at once as previously thought. This realization—that seafloor spreading creates new ocean crust—was Wegener’s missing mechanism for continental motion. The continents did not plow across preexisting seafloor; instead, they moved as parts of larger lithospheric plates riding on a hotter, weak, flowing asthenosphere.
Plate Tectonics: Evidence from Earthquakes and Subduction Zones
Earthquake studies provided the missing pieces. The behavior of seismic waves showed that the asthenosphere beneath Earth’s lithosphere is hot, partly molten, and sufficiently weak to reduce friction, enabling the overlying lithosphere to move. Continent motion is not the result of one rigid solid moving across another, as Wegener proposed, but of a rigid lithosphere moving over a heat-softened, convecting mantle. We also already knew of Benioff (subduction) zones.
In some places on the seafloor, earthquakes become progressively deeper with distance from the ridges. Before plate tectonics, this pattern was hard to explain. Once it was understood that new seafloor is created at ocean ridges, it followed that older seafloor must be destroyed elsewhere, hence subduction zones where older oceanic lithosphere sinks back into the mantle. This completes the mechanism that Wegener’s ideas began to outline: there is a dynamic plate tectonics system with seafloor spreading at ridges and subduction at trenches.
The Current Field, Reversals, and Public Perception
Some studies suggest the Earth’s current magnetic field is weakening, which could indicate the start of a reversal event. The geologic idea popularized by the Hollywood film The Core—that the outer core flow stops and the magnetic field collapses, allowing harmful solar radiation to reach the surface—is scientifically dubious. In reality, a weakening field does not guarantee a reversal, and even if a reversal begins, it could take up to years to complete. It is unlikely anyone living today would witness a reversal in real time.
A common misconception is that a magnetic reversal would spell disaster for civilization or wipe out life. The Earth has undergone many reversals in its history, and a weakening or reversal would not necessarily produce dramatic catastrophes like melted bridges or mass extinctions. The record over the last years shows that reversals do not coincide with mass extinction events (for example, the dinosaur extinction occurred during a period of stable field). Humans lived through reversal events long before electric technology. Campfire rocks, heated around ancient fires and then cooled, reset their magnetization to reflect the Earth’s magnetic field at the time they cooled, providing evidence that reversals occurred long before us.
During a reversal, increased cosmic radiation could disrupt power grids, satellite communications, and ozone, but the species would persist. The paleomagnetic reversal pattern seen in seafloor rocks led to the recognition of seafloor spreading as Wegener’s missing mechanism, which, together with seismic studies, explains how lithosphere plates move on a convecting mantle and reveals subduction of old seafloor. In the next segment, we will explore plate tectonics and plate boundaries in more detail.