If you could travel back in time 41,000 years to the last ice age, your compass would point south instead of north.
That’s because for a period of a few hundred years, the Earth’s magnetic field was reversed. Magnetic reversals have happened repeatedly over the planet’s history, sometimes lasting hundreds of thousands of years. We know this from the way it affects the alignment of magnetic minerals, that we can now study on the Earth’s surface.
Several ideas exist to explain why magnetic field reversals happen. One of these just became more plausible. My colleagues and I discovered that regions on top of the Earth’s core could behave like giant lava lamps, with blobs of rock periodically rising and falling deep inside our planet. This could affect its magnetic field and cause it to flip. The way we made this discovery was by studying signals from some of the world’s most destructive earthquakes.
Around 3,000km below our feet – 270 times further down than the deepest part of the ocean – is the start of the Earth’s core, a liquid sphere of mostly molten iron and nickel. At this boundary between the core and the rocky mantle above, the temperature is almost 4,000℃ degrees, like that on the surface of a star, with a pressure more than 1.3m times that at the Earth’s surface.
On the mantle side of this boundary, solid rock gradually flows over millions of years, driving the plate tectonics that cause continents to move and change shape. On the core side, fluid, magnetic iron swirls vigorously, creating and sustaining the Earth’s magnetic field that protects the planet from the radiation of space that would otherwise strip away our atmosphere.
Because it is so far underground, the main way we can study the core-mantle boundary is by looking at the seismic signals generated by earthquakes. Using information about the shape and speed of seismic waves, we can work out what the part of the planet they have travelled through to reach us is like. After a particularly large earthquake, the whole planet vibrates like a ringing bell, and measuring these oscillations in different places can tell us how the structure varies within the planet.
In this way, we know there are two large regions at the top of the core where seismic waves travel more slowly than in surrounding areas. Each region is so large that it would be 100 times taller than Mount Everest if it were on the surface of the planet. These regions, termed large-low-velocity-provinces or more often just “blobs”, have a significant impact on the dynamics of the mantle. They also influence how the core cools, which alters the flow in the outer core.
Several particularly destructive earthquakes over recent decades have enabled us to measure a special kind of seismic oscillations that travel along the core-mantle boundary, known as Stoneley modes. Our most recent research on these modes shows that the two blobs on top of the core have a lower density compared to the surrounding material. This suggests that material is actively rising towards the surface, consistent with other geophysical observations.
These regions might be less dense simply because they are hotter. But an exciting alternative possibility is that the chemical composition of these parts of the mantle cause them to behave like the blobs in a lava lamp. This would mean they heat up and periodically rise towards the surface, before cooling and splashing back down on the core.
Such behavior would change the way in which heat is extracted from the core’s surface over millions of years. And this could explain why the Earth’s magnetic field sometimes reverses. The fact that the field has changed so many times in the Earth’s history suggests that the internal structure we know today may also have changed.
We know the core is covered with a landscape of mountains and valleys like the Earth’s surface. By using more data from Earth oscillations to study this topography, we will be able to produce more detailed maps of the core that will give us a much better understanding of what is going on deep below our feet.
A molten puddle deep under Iceland may reveal where volcanoes get their lava.
On the boundary between Earth’s core and its mantle sit 10 to 20 blobs of rock that are nothing like the rest of our world’s subterranean realm. For more than 2 decades, scientists have pondered the nature of these mysterious regions, called ultralow-velocity zones (ULVZs). Now, researchers examining one nearly 3000 kilometers below Iceland finally have an answer: They may be the partially molten roots of plumes of hot rock that slowly rise through the mantle to feed volcanoes.
That would make ULVZs deep signposts that mark the base of the world’s plumes, says Ed Garnero, a seismologist at Arizona State University in Tempe who helped discover the zones in 1996, but was not part of the new study. “They would tell you probably where you have plumes and upwellings.”
To release heat from the liquid outer core, the solid rock in Earth’s mantle moves in slow, convective swirls, like a churning pot of hot syrup. Earth scientists have long suspected that upwellings in these mantle convection currents would manifest themselves as the plumes responsible for Earth’s volcanic hot spots. Now they have started to see them—at least their upper parts—with sophisticated computer models that use the waves from large earthquakes to create CT scan–like tomographic pictures of Earth’s interior.
The picture gets cloudier in the lower mantle, where the ULVZs live. The regions get their name from the way that earthquake waves travel so much more slowly through them. One way to explain that speed drop would be if they were partially molten. Another camp has held that the speed drops can be explained if ULVZs are made of a dense, different type of rock, perhaps enriched with iron, and chemically distinct from the rest of the mantle.
Previous studies had made tentative connections between ULVZs and the plumes underneath Hawaii and Samoa. But study author Barbara Romanowicz, a seismologist at the University of California, Berkeley, says the scene underneath Iceland provides a better picture. That’s because earthquake waves pass underneath the region from different directions and can be picked up by sensors on opposite sides of the world, unlike the Pacific islands.
Using earthquake waves picked up by arrays of sensors in the United States and China, her team better identified the position and shape of the ULVZ. They found its shape was a stubby cylinder—like a pillbox—800 kilometers across and 15 kilometers tall, more or less directly under the plume that feeds Iceland’s volcanoes, they report today in Science. She says her team’s results favor the partially molten scenario, since the other option, a chemically distinct rock, would likely have a more irregular shape and wouldn’t necessarily wind up sitting directly underneath a plume. “It’s a much more natural explanation,” Romanowicz says. “You can relate it directly to what’s going on in the plume. The temperatures are hotter.”
But Allen McNamara isn’t sure the new study rules out the chemically distinct rock scenario. A geodynamicist at Michigan State University in East Lansing, McNamara models the mantle’s slow-motion currents. And he finds that, along the core-mantle boundary, the currents are lateral, drawn toward the bases of plumes like an upside-down bathtub drain. These currents would bulldoze the dense, chemically distinct rock toward the plumes, and, over time, they could pack it into a roughly circular shape. “If you have anything dense in the lower mantle, it’s going to go along for a ride on the conveyor belt,” he says. Garnero adds that, as is the case with many phenomena, it doesn’t have to be either-or: The ULVZs could be both partially molten and chemically distinct.
Romanowicz says the debate will get resolved as pictures of the lower mantle improve. More powerful computers will allow her to use more of the high-frequency content of earthquake waves, the part that is best at illuminating shallow structures like ULVZs. Another boost, says Garnero, could come from ocean-bottom earthquake sensors. With most earthquake sensors sitting on land, two-thirds of Earth—the oceans—is a blank spot. “It’s like an ultrasound on the womb with the wand held on one spot,” he says.