While we have sent probes billions of kilometres into interstellar space, humans have barely scratched the surface of our own planet, not even making it through the thin crust.
Information about Earth’s deep interior comes mainly from geophysics and is at a premium. We know it consists of a solid crust, a rocky mantle, a liquid outer core and solid inner core. But what precisely goes on in each layer – and between them – is a mystery. Now our research uses our planet’s magnetism to cast light on the most significant interface in the Earth’s interior: its core-mantle boundary.
Roughly 3,000km beneath our feet, Earth’s outer core, an unfathomably deep ocean of molten iron alloy, endlessly churns to produce a global magnetic field stretching out far into space. Sustaining this “geodynamo”, and the planetary force-field it has produced for the past several billions of years (protecting Earth from harmful radiation), takes a lot of energy.
This was delivered to the core as heat during the Earth’s formation. But it is only released to drive the geodynamo as it conducts outwards to cooler, solid rock floating above in the mantle. Without this massive internal heat transfer from core to mantle and ultimately through the crust to the surface, Earth would be like our nearest neighbours Mars and Venus: magnetically dead.
Enter the Blobs
Maps showing how fast seismic waves (vibrations of acoustic energy) that traverse Earth’s rocky mantle change in its lowermost part, just above the core. Especially notable are two vast regions close to the equator beneath Africa and the Pacific Ocean, where seismic waves travel more slowly than elsewhere.
What makes these “big lower-mantle basal structures”, or “Blobs” for short, special is not clear. They are made of solid rock similar to the surrounding mantle, but may be higher in temperature, or different in composition, or both.
Strong variations in temperature at the base of the mantle would be expected to affect the underlying liquid core and the magnetic field that is generated there. The solid mantle changes temperature and flows at an exceptionally slow rate (millimetres per year), so any magnetic signature from strong temperature contrasts should persist for millions of years.
From rocks to supercomputers
Our study reports new evidence that these Blobs are hotter than the surrounding lower mantle. And this has had a noticeable effect on Earth’s magnetic field over the last few hundreds of millions of years at least.
As igneous rocks, recently solidified from molten magma, cool down at Earth’s surface in the presence of its magnetic field, they acquire a permanent magnetism that is aligned with the direction of this field at that time and place.
It is already well known that this direction changes with latitude. We observed, however, that the magnetic directions recorded by rocks up to 250 million years old also seemed to depend on where the rocks had formed in longitude. The effect was particularly noticeable at low latitudes . We therefore wondered whether the Blobs might be responsible.
Andy Biggin, CC BY-SA
The clincher came from comparing these magnetic observations to simulations of the geodynamo run on a supercomputer. One set was run assuming that the rate of heat flowing from core to mantle was the same everywhere. These either showed very little tendency for the magnetic field to vary in longitude or else the field they produced collapsed into a persistently chaotic state, which is also inconsistent with observations.
By contrast, when we placed a pattern on the core’s surface that included strong variations in the amount of heat being sucked into the mantle, the magnetic fields behaved differently. Most tellingly, assuming that the rate of heat flowing into the Blobs was about half as high as into other, cooler, parts of the mantle meant that the magnetic fields produced by the simulations contained longitudinal structures reminiscent of the records from ancient rocks.
A further finding was that these fields were less prone to collapsing. Adding the Blobs therefore enabled us to reproduce the observed stable behaviour of Earth’s magnetic field over a wider range.
What seems to be happening is that the two hot Blobs are insulating the liquid metal beneath them, preventing heat loss that would otherwise cause the fluid to thermally contract and sink down into the core. Since it is the flow of core fluid that generates more magnetic field, these stagnant ponds of metal do not participate in the geodynamo process.
Furthermore, in the same way that a mobile phone can lose its signal by being placed within a metal box, these stationary areas of conductive liquid act to “screen” the magnetic field generated by the circulating liquid below. The huge Blobs therefore gave rise to characteristic longitudinally varying patterns in the shape and variability of Earth’s magnetic field. And this mapped on to what was recorded by rocks formed at low latitudes.
Most of the time, the shape of Earth’s magnetic field is quite similar to that which would be produced by a bar magnet aligned with the planet’s rotation axis. This is what makes a magnetic compass point nearly north at most places on Earth’s surface, most of the time.
Collapses into weak, multipolar states, have occurred many time over geological history but they are quite rare and the field seems to have recovered fairly quickly afterwards. In the simulations at least, Blobs seem to help make this the case.
So, while we still have a lot to learn about what the Blobs are and how they originated, it may be that in helping to keep the magnetic field stable and useful for humanity, we have much to thank them for.
Andrew Biggin receives funding from the Natural Environment Research Council.
