The Earth's magnetic field, a protective shield against harmful radiation, has been influenced by giant 'Blobs' of rock for millions of years, according to new research. While we've sent probes into interstellar space, our planet's interior remains largely unexplored. Information about Earth's deep interior comes from geophysics and is limited. We know the Earth's structure consists of a solid crust, a rocky mantle, a liquid outer core, and a solid inner core. However, the details of each layer's behavior remain a mystery. Our research uses the planet's magnetism to shed light on the core-mantle boundary, a critical interface in the Earth's interior.
Approximately 3,000 kilometers beneath our feet, the Earth's outer core, a molten iron alloy ocean, generates a global magnetic field extending into space. This 'geodynamo' has sustained a planetary force field for billions of years, protecting the Earth from harmful radiation. The energy required to maintain this geodynamo was delivered to the core as heat during the Earth's formation. It is then released as it conducts outwards to cooler, solid rock in the mantle. Without this heat transfer, the Earth would be similar to Mars and Venus, lacking a magnetic field.
Enter the Blobs: Maps of seismic wave speeds in the lower mantle reveal two vast regions near the equator beneath Africa and the Pacific Ocean, where waves travel more slowly. These 'big lower-mantle basal structures' or 'Blobs' are made of solid rock similar to the surrounding mantle but may differ in temperature or composition. Strong temperature variations at the mantle's base would affect the underlying liquid core and the magnetic field. The solid mantle's slow temperature changes (millimetres per year) mean magnetic signatures from temperature contrasts persist for millions of years.
Our study found that these Blobs are hotter than the surrounding lower mantle, influencing Earth's magnetic field over the last few hundred million years. Igneous rocks, recently solidified from molten magma, acquire permanent magnetism aligned with the Earth's magnetic field at the time and place of formation. We observed that magnetic directions in rocks up to 250 million years old depended on their formation location in longitude, especially at low latitudes. This led us to wonder if the Blobs were responsible.
Comparing magnetic observations to geodynamo simulations on a supercomputer provided the answer. Simulations assuming uniform heat flow from core to mantle showed little longitudinal magnetic field variation or chaotic collapse, inconsistent with observations. However, simulations with heat variations on the core's surface, mimicking the Blobs, produced magnetic fields with longitudinal structures resembling ancient rock records. These fields were also less prone to collapse, enabling us to reproduce the observed stable Earth's magnetic field behavior.
The Blobs seem to insulate the liquid metal beneath them, preventing heat loss that would cause thermal contraction and sinking into the core. Since core fluid flow generates the magnetic field, these stagnant metal ponds do not participate in the geodynamo process. Additionally, conductive liquid areas 'screen' the magnetic field, creating longitudinally varying patterns in the Earth's magnetic field shape and variability, matching ancient rock records. The magnetic field typically resembles a bar magnet aligned with the rotation axis, making a magnetic compass point nearly north at most Earth surface locations.
While Blobs' origin and nature remain unknown, they may contribute to the stability and usefulness of Earth's magnetic field for humanity.
