Do Chemical Leftovers From the Early Earth Sit Near Its Core?

Let’s take a journey into the depths of the Earth, down through the crust and mantle nearly to the core. We’ll use seismic waves to show the way, since they echo through the planet following an earthquake and reveal its internal structure like radar waves. Down near the core, there are zones where seismic waves slow to a crawl. New research from the University of Utah finds that these enigmatic and descriptively-named ultra-low velocity zones are surprisingly layered. Modeling suggests that it’s possible some of these zones are leftovers from the processes that shaped the early Earth — remnants of incomplete mixing like clumps of flour in the bottom of a bowl of batter. Michael S. Thorne, associate professor in the Department of Geology and Geophysics, said:

The study is published in Nature Geoscience and is funded by the National Science Foundation.

Into the mantle

Let’s review how the interior of the Earth is structured. We live on the crust, a thin layer of solid rock. Between the crust and the iron-nickel core at the center of the planet is the mantle. It’s not an ocean of lava — instead, it’s more like solid rock, but hot and with an ability to move that drives plate tectonics at the surface.

How can we have any idea what’s going on in the mantle and the core? Seismic waves. As they ripple through the Earth after an earthquake, scientists on the surface can measure how and when the waves arrive at monitoring stations around the world. From those measurements, they can back-calculate how the waves were reflected and deflected by structures within the Earth, including layers of different densities. That’s how we know where the boundaries are between the crust, mantle, and core — and partially how we know what they’re made of.

Ultra-low velocity zones sit at the bottom of the mantle, atop the liquid metal outer core. In these areas, seismic waves slow by as much as half, and density goes up by a third. Scientists initially thought that these zones were areas where the mantle was partially melted and might be the source of magma for so-called “hot spot” volcanic regions like Iceland. Thorne said:

So Thorne, postdoctoral scholar Surya Pachhai, and colleagues from the Australian National University, Arizona State University, and the University of Calgary set out to explore an alternate hypothesis: that the ultra-low velocity zones may be regions made of different rocks than the rest of the mantle — and that their composition may hearken back to the early Earth.

Perhaps, Thorne says, ultra-low velocity zones could be collections of iron oxide, which we see as rust at the surface but that can behave as a metal in the deep mantle. If that’s the case, pockets of iron oxide just outside the core might influence the Earth’s magnetic field that is generated just below. Pachhai said:

Reverse-engineering seismic waves

To get a clear picture, the researchers studied ultra-low velocity zones beneath the Coral Sea, between Australia and New Zealand. It’s an ideal location because of an abundance of earthquakes in the area that provide a high-resolution seismic picture of the core-mantle boundary. The hope was that high-resolution observations could reveal more about how ultra-low velocity zones are put together.

But getting a seismic image of something through nearly 1800 miles of crust and mantle isn’t easy. It’s also not always conclusive — a thick layer of low-velocity material might reflect seismic waves the same way as a thin layer of even lower-velocity material. So the team used a reverse-engineering approach. Pachhai said:

Over hundreds of thousands of model runs, the method, called “Bayesian inversion,” yields a mathematically robust model of the interior with a good understanding of the uncertainties and trade-offs of different assumptions in the model.

One particular question the researchers wanted to answer is whether there are internal structures, such as layers, within ultra-low velocity zones. The answer, according to the models, is that layers are highly likely. This is a big deal, because it shows the way to understand how these zones came to be. Pachhai said:

Looking back at the origins of the early Earth

What does it mean that there are likely layers?

More than 4 billion years ago, while dense iron was sinking to the core of the early Earth and lighter minerals were floating up into the mantle, a planetary object about the size of Mars may have slammed into the very early Earth. The collision may have thrown debris into Earth’s orbit that could have later formed the Moon. It also raised the temperature of the early Earth significantly — as you might expect from two planets smashing into each other. Pachhai said:

The “ocean” would have consisted of rock, gases, and crystals suspended in magma. The ocean would have sorted itself out as it cooled, with dense materials sinking and layering onto the bottom of the mantle. Over the following billions of years, as the mantle churned and convected, the dense layer would have been pushed into small patches, showing up as the layered ultra-low velocity zones we see today. Pachhai said:

Not the final word

The study provides some evidence of the origins of some ultra-low velocity zones, although there’s also evidence to suggest different origins for others, such as the melting of the oceans’ crust that’s sinking back into the mantle. But if at least some ultra-low velocity zones are leftovers from the early Earth, they preserve some of the history of the planet that otherwise has been lost. Pachhai said:

Provided by: University of Utah [Note: Materials may be edited for content and length.]

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