A team of scientists from the University of Maryland have conducted a state-of-the-art study which sheds some light on the composition of the area between the Earth’s mantle and molten core. The study relied upon analysis of seismic waves and it was published in the esteemed online magazine Science. Amazingly, the findings could potentially change the way geologists think about the very evolution of the planet.
The internal structure of the Earth is commonly believed to break down into four layers. Firstly, there is the silicate crust. Then there is the dense, gluey mantle. Finally, there are two layers of the Earth’s core; the liquid outer core and the solid inner core, both of which are composed of an iron-nickel alloy.
The Earth’s outer crust accounts for less than 1 percent of the total volume of the planet. It is also the only part of the sphere’s composition that is mostly solid and cool in temperature. It consists of cooled rock which lies on top of the mantle. The Earth has oceanic and continental crust, which have different thicknesses.
The oceanic crust is made up of thick, dense rocks such as diabase, gabbro and basalt and is roughly five to ten kilometers in thickness. On the other hand, the continental crust is composed of lighter rocks such as granite, yet is much thicker, measuring from 30 kilometers to 50 kilometers. During the Cold War, the U.S. and Russia fought to see who could dig the furthest into the Earth’s crust.
This competition resulted in the Kola Superdeep Borehole; the deepest hole ever dug by man. Located deep within the snows of the Arctic Circle, Russia set up a scientific research station on the Kola Peninsula. Over the course of 20 years, the Soviets drilled into the Earth and made a hole 12.2 kilometers deep.
However, even after two decades of digging, the drill had still only penetrated a third of the way through the Baltic Shield continental crust. At this point the temperature surrounding the drill was much higher than the Russians had anticipated, and any further digging was ceased in 1992. The research station now lies dilapidated and abandoned.
Beneath the crust lies the mantle, which is a non-elastic, malleable solid that flows and creates convection currents. These currents cause continental drift and create volcanic hotspots. The mantle is made up of a shallow layer composed of rocks such as spinel, olivine, garnet, and pyroxenes, and a deeper layer composed of minerals at high pressure in a variety of structural forms.
The mantle accounts for 70 percent of the volume of the Earth, making it the thickest layer in the structure of the planet. It is estimated to be slightly less than 3,000 kilometers deep. Much of what has been learned about the composition of the mantle has come from scientists like the University of Maryland’s team analyzing the manner in which seismic waves behave inside it.
Moving onto the Earth’s core, the outer core is composed of an overwhelming amount of the planet’s nickel and iron. This layer is a molten liquid, as the temperature is hot enough to melt the iron and nickel, but there is not enough pressure to turn the liquid into a solid. The Earth’s magnetic field is believed to be created by eddy currents that move in the outer core.
The inner core is made up of the same material as the outer core. However, the important difference is that the pressure in this layer is forceful enough that the iron and nickel has become solid. It is theorized that the Earth’s inner core may be hotter than the sun’s surface.
Now that we have a better idea of the Earth’s composition, we may begin to wonder how it was formed. Nowadays most scientists subscribe to the theory that the Earth eventually formed in the aftermath of the Big Bang, the explosion that resulted in the birth of the universe. The theory proposes that, prior to the Big Bang, all energy and matter was contained inside a burning-hot singularity, which had infinite density.
The singularity was supposedly floating in a vacuum when it exploded, firing energy and gas particles outward into space. Then, as this gas cooled over time, some of the particles began to congeal. Over the course of a billion years, some of these particle clusters became what we now know as stars.
The theory posits that our Sun was formed from these particle clusters as well, roughly five billion years ago. Initially, the Sun was a hot cloud of gas that spun. At its center, it eventually ignited and became the Sun as we know it today. But, as the gas cloud continued to spin, the particles also came together to form a solar nebula. The Earth and all other planets formed inside this nebula.
Scientists believe that the Earth formed 4.6 billion years ago from particles collected inside the solar nebula. Once the Sun ignited into existence, all the extra particles were scattered, and this left the Solar System as we now understand it. The theory states that the Moon also originated inside this solar nebula.
In its formative days, the Earth was extremely hot and volcanic. As the planet became colder over time, the solid crust began to form. Couple this with the craters caused by falling asteroids, which began to fill with water over time and eventually became oceans, and the Earth was taking shape.
Plate tectonics then played a pivotal role in further shaping the Earth. The outer crust and upper mantle of the planet is known as the lithosphere, and it is divided into tectonic plates. There are seven or eight major plates, most of which are named after the continents they exist under, for example the African and Antarctic plates. There are also a large number of minor plates.
The plates move slowly: approximately one to two inches every year. The movement of the sea floor away from spreading undersea mountain ridges interacts with alterations to the density of the Earth’s outer crust, and the plates are shifted. Most geologic activity, including the creation of mountains and volcanoes, comes from the plates colliding or dividing.
Tectonic plates can move in different ways, and the type of movement determines the tectonic boundary created. Convergent boundaries are where the plates have crashed into each other. Divergent boundaries happen when the plates have moved away from each other. Transform boundaries are when the plates move alongside each other.
For example, geologists believe the Himalayas, the tallest mountain range on Earth, were created by a convergent boundary about 55 million years ago when the Indian and Asian tectonic plates collided with each other. They also believe that, as the plates continue to push against each other, the Himalayas and other mountain ranges created by convergent boundaries will keep getting taller over time. It will be imperceptible to humans, but the growth will most certainly be happening.
Plate tectonics also predicts the existence of convergent boundaries underwater; when one plate goes under another it forms a deep ocean trench. The Mariana Trench, located in the north Pacific Ocean, is held to be one such case. This process is known as subduction and it can also result in undersea volcanoes forming.
Divergent boundaries in the ocean are caused by the tectonic plates being moved apart by liquid hot magma rising from deep within the Earth’s mantle. On the surface, though, gargantuan troughs are caused when plates are moved apart over time. An example of this is the Great Rift Valley in Africa. Theoretically, after millions of years of continued movement, eastern Africa could be separated completely from the rest of the continent.
The most frightening are the transform boundaries, as they create strike-slip faults as the tectonic plates grind against each other. This is what causes devastating earthquakes, rather than giving rise to beautiful mountain ranges. The San Andreas Fault is one of the most prominent examples of a transform boundary, and it caused the 1906 San Francisco earthquake.
With all of this in mind, the June 2020 study results published in Science are particularly interesting. The University of Maryland team analyzed seismic wave data and it showed that there may be a giant structure on the boundary line between the Earth’s solid mantle and its molten core. The structure lies 2,900 kilometres beneath the south Pacific Ocean’s volcanic Marquesas Islands.
The structure is known as an Ultra-Low Velocity Zone or ULVZ. It is named as such because seismic waves travel through it at a slow speed, but its exact material composition is very much unknown. The university team aren’t sure if it is chemically different from the Earth’s mantle, made of silicate rock, or the Earth’s iron-nickel alloy core.
ULVZs are geological features believed to lead to the creation of volcanic islands. It is thought that this happens when hot rocky material rises from the core-mantle boundary to the Earth’s outer crust. Doyeon Kim, the lead scientist of the study and a postdoctoral fellow in Maryland University’s geology department, explained this theory to Newsweek magazine.
“There is a theory that if you have a plume of rising hot rock – which produces the hot spot volcanism at the surface that creates ocean island chains like Hawaii and Marquesas – that this rising rock will kind of suck on the melt and pull it up,” said Kim. “So, the ULVZ ends up being very large in areas where material is going up.” But there are other theories too.
Kim said, “However, there are others who think ULVZs just represent regions where [the] mantle is very enriched in iron.” Either way, Kim believes the new Marquesas structure to be about 1,000 kilometres long and 25 kilometres thick. The study also gleaned new information about a previously discovered ULVZ beneath Hawaii.
Kim told Newsweek, “What was known previous to our study is that there are three mega-ULVZs on Earth – beneath Hawaii, Iceland and Samoa. We observed loud echoes generated by mega-ULVZs whose properties are very different from the surrounding mantle: one beneath Hawaii which turned out to be much larger than previously thought and one beneath Marquesas which is one of the new discoveries we made.” But what exactly does Kim mean when he says “echoes”?
Kim’s team’s study analyzed the seismic activity from earthquakes that took place between 1990 and 2018 in the Pacific Ocean basin. They looked at a mind-boggling 7,000 seismic records each registering at least a magnitude of 6.5 on the Richter scale. They also limited their analysis to tremors which originated from at least 200 kilometers below the surface of the Earth.
The echoes caused by a specific kind of seismic wave, known as a shear wave, were focused on by the team. These waves travel on the very boundary of the Earth’s mantle and core. To yield the best results possible, the team looked at data from thousands of shear waves at the same time. Kim explained the reasoning behind looking at so much data to the Science website.
“By looking at thousands of core-mantle boundary echoes at once, instead of focusing on a few at a time, as is usually done, we have gotten a totally new perspective,” said Kim. “This is showing us that the core-mantle boundary region has lots of structures that can produce these echoes, and that was something we didn’t realize before because we only had a narrow view.”
The team had a problem to solve regarding their method, though. Shear wave echoes from a solitary seismogram can simply sound like indistinct, random noise, so they needed some way to distinguish patterns and trends in the echoes. Therefore, they used a machine-learning algorithm known as Sequencer, which was originally conceived to study data gleaned from astronomy.
Sequencer was recalibrated by the team to study seismic activity, instead of the stars. As Kim explained, it became essential to their findings. “Machine learning in earth science is growing rapidly and a method like Sequencer allows us to be able to systematically detect seismic echoes and get new insights into the structures at the base of the mantle, which have remained largely enigmatic,” he said.
The science behind the study of seismic waves is fascinating. As these waves are generated deep beneath the Earth’s surface, they slow, scatter or shift depending on a variety of factors, such as the composition, density, and temperature of the material through which they are traveling. The waves are measured on seismometers, located in different areas.
The properties of the seismic waves are then analyzed. By working out the intensity and time it took each wave to travel, scientists such as the Maryland team can create digital models of the rock through which they traveled. These digital models give great insight into the physical composition of the rocky structures beneath the Earth’s surface.
University of Maryland geologist Vedran Lekić said, “We were surprised to find such a big feature beneath the Marquesas Islands that we didn’t even know existed before. This is really exciting, because it shows how the Sequencer algorithm can help us contextualize seismogram data across the globe in a way we couldn’t before. We found echoes on about 40 percent of all seismic wave paths.”
Lekić continued, “That was surprising because we were expecting them to be more rare, and what that means is the anomalous structures at the core-mantle boundary are much more widespread than previously thought.” All in all, the team were ecstatic at what Sequencer had helped them find. Intriguingly, they believe that weaker echoes point to the structures beneath the Earth being spaced apart, rather than localized.
Kim elaborated on this theory when he spoke with Newsweek. He said, “We think that the most likely – but not the only – explanation is that these widespread ‘pervasive’ echoes come from the boundaries of a continent-sized structure called an LLSVP (Large Low-Shear Velocity Province). However, in some locations this cannot explain the signals, and they instead tell us about much smaller structures being present to produce the echoes, though we cannot precisely determine where.”
Overall, we’re still more or less in the dark when it comes to the exact composition of ULVZs. But Kim’s team’s discoveries have potentially massive implications for the understanding of the planet’s geological processes. They will also spur new theories on how plate tectonics have shaped the evolution of the Earth.
It may also be possible to use the Sequencer algorithm to create a highly detailed map of the interior structure of the Earth. The University of Maryland’s findings show that the algorithm can be successfully calibrated to locate mysterious and previously unknown structures. So, in theory, it could also be used to analyze different kinds of frequencies and waves, which would help build just such a high-resolution map.