In the event you might journey again to 41,000 years in the past within the final ice age, your compass could be pointing south as a substitute of north. It’s because for a interval of some hundred years, the Earth’s magnetic area was reversed. These reversals have occurred repeatedly all through the planet’s historical past, typically lasting tons of of 1000’s of years. We all know it from the way in which it impacts the formation of magnetic minerals, that we will now research on the Earth’s floor.
A number of concepts exist to elucidate why magnetic area reversal happens. Certainly one of these has simply change into extra believable. My colleagues and I discovered that areas on the prime of the Earth’s heart can behave like big lava lamps, with blob of rock rising and falling inside our planet every now and then. This will have an effect on its magnetic area and trigger it to overturn. The way in which we made this discovery was by finding out the indicators of a number of the most devastating earthquakes on this planet.
About 3,000 km under our ft – 270 instances under the deepest a part of the ocean – is the start of Earth’s core, a liquid area of principally molten iron and nickel. At this boundary between the higher core and the rocky mantle, the temperature is about 4,000 °C, just like the floor of a star whose stress is greater than 1.3 m bar at Earth’s floor.
On the mantle facet of this boundary, strong rock flows slowly over thousands and thousands of years, triggering plate tectonics that trigger continents to maneuver and alter form. On the principle facet, fluid, magnetic iron spins vigorously to create and keep Earth’s magnetic area that shields the planet from the radiation of area that may in any other case strip our environment.
Since it’s nonetheless underground, the principle solution to research the core-mantle boundary is to search for seismic indicators produced by an earthquake. Utilizing details about the dimensions and velocity of seismic waves, we will discover out by way of which a part of the planet they reached us.
Particularly after a big earthquake, the complete planet vibrates like a ringing bell, and measuring these oscillations at totally different places can inform us how the construction modifications inside the planet.
On this approach, we all know that there are two giant areas on the prime of the core the place seismic waves journey extra slowly than the encircling areas. Every sphere is so giant that if it had been on the floor of the planet it might be 100 instances increased than Mount Everest. These areas, referred to as large-low-velocity-provinces or extra typically merely “blobs”, have a major influence on the dynamics of the mantle. Additionally they have an effect on how the core cools, which diverts the flux to the outer core.
A number of notably devastating earthquakes in current a long time have enabled us to measure a selected kind of seismic oscillations that journey alongside the core-mantle boundary, generally known as the Stoney Mode. Our most up-to-date analysis on these modes means that the 2 droplets on the prime of the core have a decrease density than the encircling materials. This implies that the fabric is actively shifting towards the floor, in step with different geophysical observations.
These areas could also be much less dense, as they’re hotter. However an thrilling different risk is that the chemical composition of those components of the mantle causes them to behave just like the blob in a lava lamp. This might imply that they warmth up and periodically rise towards the floor, earlier than cooling down and splashing again onto the core.
Such habits would change the way in which warmth is faraway from the core’s floor over thousands and thousands of years. And this will clarify why Earth’s magnetic area typically reverses. The truth that the sector has modified many instances over Earth’s historical past means that the interior construction we all know in the present day could have modified as effectively.
We all know that the core is roofed by a panorama of mountains and valleys, just like the floor of the Earth. Through the use of extra information from Earth’s oscillations to review this topography, we will produce extra detailed maps of the core that can give us a greater understanding of what’s taking place beneath our ft.
Paula Koelmeijer, Postdoctoral Fellow in World Seismology, College of Oxford
This text was initially revealed on The Dialog. Learn the unique article.