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Let's go down through the Earth's crust and mantle 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.
There are zones where the waves are slow. The University of Utah found that the ultra-low velocity zones are more than one. Some of the zones are thought to be leftovers from the processes that shaped the early Earth, like clumps of flour in the bottom of a bowl of batter.
"Ultra-low velocity zones are the most extreme features we know about in the deep mantle," says Michael S. Thorne, associate professor in the Department of Geology and Geophysics. These are some of the most extreme features on the planet.
The study is funded by the National Science Foundation.
The mantle has been entered.
Let's look at how the Earth is structured. We live on a thin layer of rock. The mantle is between the iron-nickel core and the crust. It's not an ocean of lava, it's more like a solid rock that's hot and can move.
How can we know what's happening in the core and mantle? There are waves. Scientists on the surface can measure how the waves arrive at monitoring stations around the world after an earthquake. They can back-calculate how the waves were reflected by structures within the Earth, including layers of different densities. We know where the boundaries are between the crust, mantle and core through this method.
There are ultra-low velocity zones at the bottom of the mantle. In these areas, the waves are slower and the density increases.
Scientists thought that the zones were areas where the mantle was partially melted, and that the source of magma was in the "hot spot" volcanic regions like Iceland.
Most of the things we call ultra-low velocity zones don't appear to be located beneath hot spot volcanoes, so that cannot be the whole story.
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.
It is possible that ultra-low velocity zones are collections of iron oxide, which we see as rust at the surface but which can behave as a metal in the deep mantle. The Earth's magnetic field might be influenced by pockets of iron oxide just outside the core.
The thermal and chemical status of the Earth's lowermost mantle is an essential part of mantle convection that drives plate tectonics, according to Pachhai.
Seismic waves are reverse-engineering.
The researchers studied ultra-low velocity zones beneath the Coral Sea. It's an ideal location because of the abundance of earthquakes in the area, which 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.
The simulation shows the thermal, chemical, and thermochemical evolution of the Earth's interior over time. The core-mantle boundary is at the bottom of each field. The light blue zones can be seen in the top and middle fields. Credit: Surya Pachhai.
It's not easy to get a picture of something through the 1800 miles of crust and mantle. A thick layer of low-velocity material might reflect seismic waves the same way as a thin layer of even lower-velocity material.
The team used a reverse-engineering approach.
"We can create a model of the Earth that includes ultra-low wave speed reductions, and then run a computer simulation that tells us what the seismic waves would look like if that is what the Earth actually looked like," Pachhai says. We will compare the predicted recordings with the recordings we have.
The method called "Bayesian inversion" yields a robust model of the interior with a good understanding of the uncertainties and trade-offs of different assumptions.
The researchers wanted to know if there are internal structures within ultra-low velocity zones. According to the models, layers are highly likely. This is a big deal because it shows how these zones came to be.
This is the first study to use a Bayesian approach at this level of detail to investigate ultra-low velocity zones, and it is also the first study to show strong layers within an ultra-low velocity zone.
Looking back at the beginning of the planet.
What does it mean that there are more than one layer?
A planetary object about the size of Mars may have slammed into the infant planet four billion years ago, while dense iron was sinking to the core. The debris from the collision could have formed the Moon. It raised the temperature of the Earth significantly, as you might expect from two planets smashing into each other.
A large body of molten material, known as a magma ocean, formed. The "ocean" would have been made of rock, gases and crystals.
The ocean would have sorted itself out as it cooled, with dense materials on top of the mantle.
The dense layer would have been pushed into small patches as the mantle churned and convected.
"So the most surprising finding is that the ultra-low velocity zones are not homogeneity but contain strong heterogeneity," Pachhai says. The origin and dynamics of ultra-low velocity zones have been changed by this finding. We found that this type of ultra-low velocity zone can be explained by the creation of chemical Heterogeneities at the very beginning of the Earth's history and that they are still not well mixed after 4.5 billion years of mantle convection.
Not the final word.
There is some evidence of the origins of some ultra-low velocity zones, but there is also evidence to suggest different origins for others. Some of the history of the planet that has been lost if some ultra-low velocity zones are leftovers from the early Earth.
Pachhai says that the discovery provides a tool to understand the initial thermal and chemical status of Earth's mantle.
The internal structure of ultralow-velocity zones is consistent with the origin of the ocean. The DOI is 10.1038/s41561-021-00871-5.
The journal information is Nature Geoscience.
The chemical leftovers from early Earth were retrieved from the core on December 30, 2021.
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