An effective tool for deep mantle
We live on the Earth's crust, which is a thin layer of hard rock; at the center of the Earth is the iron nickel core; between the crust and the core is the mantle, an area that is not like an "ocean" of lava as many people think, but a hard, hot, and rocky rock with the ability to move, capable of driving the tectonics of the plates on the surface.
How can we, who are on the surface, know about the mantle and core? The answer lies in seismic waves. A seismic wave is a type of wave that reverberates on Earth after an earthquake has occurred. Usually, scientists calculate how seismic waves are reflected and deflected by the Earth's internal structure by measuring how and when they arrive at monitoring stations around the world. In this way, they can reveal the internal structure of the Earth and speculate on some of the information related to the crust, mantle and core.
Mysterious ultra-low speed band
Near the Earth's core, there is a section of ultra-high density. This part of the structure is located at the bottom of the mantle, above the liquid metal outer nucleus, and may be hundreds of kilometers wide. In this area, the speed of seismic waves has slowed by 50%, and the density has increased by 1/3, which is called the ultra-low speed zone by geologists.
Regions made up of different rocks are called ultra-low velocity zones, and they congregate at earth's nuclear-mantle boundary. | Image credit: Edward Garnero/ASU
Since the ultra-low speed zone is located nearly 2900 km below the surface (deep in Africa and the central Pacific Ocean, respectively), this makes it very difficult to analyze this part of the rock structure. Their origins remain a mystery to this day.
Initially, scientists thought that the mantle in these areas had partial melting, possibly as a source of magma in so-called "hot" volcanic areas such as Iceland. However, most of the areas known as the ultra-low speed zone are not located beneath the hot volcanoes, so this claim does not explain the full picture of the story.
Now, a new study, by combining seismic data with computer models, was surprised to find that the structure of these mysterious ultra-low-speed zone regions is not uniform, but has a layered structure of different materials formed by years of accumulation. Their new findings, published in the journal Nature-Earth Science, are significant for geologists because they provide new ideas for understanding the origin stories of these ultra-low-speed zones.
Use models to match seismic waves
In the new study, the researchers want to explore another hypothesis of origin: the rocks that make up the ultra-low velocity zone may be different from the rocks in other parts of the mantle, and these compositions may be traced back to early Earth. More specifically, perhaps the ultra-low speed band is a collection of iron oxides, and on the surface we see rust, but it may behave like a metal deep in the mantle. If this is the case, the iron oxide outside the Earth's core may affect the Earth's magnetic field below the Earth's core.
They studied the ultra-low speed zone under the Coral Sea between Australia and New Zealand. This is an ideal place to study this problem, because earthquakes are frequent here and a large number of seismic waves are generated, so high-resolution seismic images can be provided for the nuclear-mantle boundary. However, the accuracy of the images provided by seismic wave features observed from thousands of meters deep is limited. And sometimes, a thicker layer of low-speed material may emit the same seismic waves as a thinner layer of low-speed material.
So the team decided to use a reverse engineering method, where they created a theoretical model of the Earth and then calculated through simulation what the seismic waves would look like as they passed if that was what the Earth really looked like. They simulated the models under a number of conditions and then compared the results of these simulations with those actually observed under the Coral Sea to assess how well each model matched the actual situation.
After hundreds of thousands of model runs, this approach produced a robust model of the Earth's interior. The model shows that there is a high probability that there is a possibility of a layered internal structure in these ultra-low speed bands.
Back to the early Earth
What exactly can the existence of layered structures mean?
The story goes back more than 4 billion years, around the time when the rocky crust of the early Earth first formed. Below the surface, heavier elements (such as iron) sink into the core of the early Earth; lighter elements (such as silicon) float to the mantle. At this point, a Martian-sized planetary object (Theia) may have crashed into the newborn Earth. The collision may have thrown a large amount of debris into Earth's orbit, which, according to the hypothesis, may have even contributed to the formation of the moon later.
For Earth, the key shift is that the impact of the two planets has greatly increased the temperature of the entire Earth and formed a huge "ocean" of molten material on the Earth's surface. The various rocks, gases and crystals formed during the collision are scattered in this "ocean". But they won't last forever.
During the cooling process, the "ocean" "plans" itself, with dense material sinking to the bottom of the mantle, and then lighter matter eventually forming a dense layer of iron and other elements at the core-mantle boundary. Over the next few billion years, as the mantle churned and convected, the dense mantle layer split into smaller clumps, scattered over deeper mantles, revealing the layered ultra-low velocity bands we see today.
Not a final decision
The new study has changed the way scientists think about the origin of the ultra-low speed band, providing evidence for the origin of some ultra-low speed bands. But the researchers note that these results may not explain the origin of all ultra-low-speed zones, as there are other phenomena that could explain the origins of ultra-low-speed zones, such as the melting of the oceanic crust sinking into the mantle.
However, the new simulations suggest that the Theia impact hypothesis can reliably explain how these dense, layered regions formed, and that if at least some of the ultra-low velocity zones are remnants of early Earth-planetary impacts, they preserve some of Earth's history. From this perspective, the new findings provide new clues to understanding the initial thermal and chemical states of the mantle and their long-term evolution.
#创作团队:
Author: Light rain
Typography: Wenwen
#参考来源:
https://attheu.utah.edu/uncategorized/chemical-leftovers/
https://www.popsci.com/science/earths-mantle-regions-explained/
https://www.livescience.com/ulvz-giant-impact-hypothesis.html
#图片来源:
Cover: Tim Bertelink, CC 4.0
First image: Argonne National Labs