Tag Archives: inner core

Scientists have new evidence that Earth’s inner core may be rotating

A new study of Earth’s inner core used seismic data from repeating earthquakes, called doublets, to find that refracted waves, blue, rather than reflected waves, purple, change over time – providing the best evidence yet that Earth’s inner core is rotating. Credit: Michael Vincent.

Geologists have been debating for decades whether the planet’s inner core is rotating or not. New evidence obtained by Chinese researchers seems to hint towards the former, according to seismic data.

The motion of molten iron alloys in the Earth’s outer core acts as a planetary dynamo, generating a massive magnetic field called the magnetosphere. It extends for several tens of thousands of kilometers into space, well above the atmosphere, sheltering the planet from the charged particles of the solar wind and cosmic rays that would otherwise strip away the upper atmosphere, including the ozone layer that protects life from harmful ultraviolet radiation.

However, there is still much we don’t know about how the planet’s core interacts with complex physics to generate magnetic fields. For instance, the north and south poles have “wandered” and flipped periodically over Earth’s geological history, and these behaviors aren’t completely understood.

For decades, the motion of the inner core has been the realm of theoreticians. But in 1996, Xiaodong Song, now a geology professor at Peking University in China, detected seismic waves passing through the inner core that suggested differential rotation of the inner core relative to Earth’s surface.

These initial findings were rather quickly dismissed, with other studies pointing towards the reflection of seismic waves off the ununiform inner core boundary, which can act like canyons or mountains.

For their new study, Song and colleagues — including researchers at the University of Illinois at Urbana-Champaign — reviewed seismic data from a range of geographical locations across the world. The data also included repeating earthquakes, known as doublets, that occur in the same spot over time.

These doublets proved essential because they offer the separating factor enabling scientists to distinguish changes due to variation in relief from changes due to movement and rotation of the planet’s core.

According to the findings, some of the earthquake-generated seismic waves penetrated the iron layer right below the inner core boundary and changed over time. This shouldn’t have happened if the inner core were stationary, the researchers wrote in the journal Earth and Planetary Science Letters.

“Importantly, we are seeing that these refracted waves change before the reflected waves bounce off the inner core boundary, implying that the changes are coming from inside the inner core,” Song said.

“This work confirms that the temporal changes come mostly, if not all, from the body of the inner core, and the idea that inner core surface changes are the sole source of the signal changes can now be ruled out,” he added.

In a previous study while he was a professor at Columbia University, Song and colleagues estimated that the inner core rotates in the same direction as the Earth and slightly faster, completing its once-a-day rotation about two-thirds of a second faster than the entire Earth.

While that might not seem like a lot, it’s still some 100,000 times faster than the drift of continents — and over time it adds up. Over the past 100 years that extra speed has gained the core a quarter-turn on the planet as a whole, the scientists found. 

Earth’s inner core is solid, seismic waves reveal

A new study has sent ripples throughout the seismological world — a team of researchers from the Australian National University have found a new way to confirm that the Earth’s core is solid.

A simplified schematic of the Earth’s structure.

If you think about it, the very fact that we know so much about the Earth’s interior is stunning. The deepest hole we’ve dug is “only” around 12 km deep, whereas the radius of our planet is 6378 km — so how could we know so much about the Earth’s structure?

Seismic waves

Scientists use many bits and pieces of information to study this structure, but the most important clues come from seismic waves. Whenever an earthquake takes place, it sends out pressure waves in all directions — much like the acoustic waves we use to speak and hear sounds. These waves propagate through the subsurface, reflecting and refracting as they move from one environment to the other. Eventually, they reach the surface, where they are picked up by seismographs — devices that measure the movement of the earth.

Using seismographs, researchers can infer a surprisingly large amount of information. For instance, by measuring the arrival time of seismic waves at different seismograph stations, the position of the earthquakes’ epicentres can be triangulated — this is how we know where earthquakes happen.

But that’s just the start of it.

A typical seismogram.

More than just data about the earthquake itself, we can derive data about the rocks through which the seismic wave has traveled. For instance, generally speaking, there are three types of seismic waves:

  • P or Primary waves, which are longitudinal waves (these are the fastest and the first to arrive);
  • S or Secondary waves, which are shear waves; and
  • Surface waves, which are slower than both P and S waves, but have larger amplitude and more complex movements.

A simple simulation of an S wave structure.

P waves travel through any type of medium, whereas S waves only move through solid environments — not liquid. This was a huge indication that the Earth’s outer core is a liquid, as S waves don’t appear to pass through it. It’s not a liquid in a conventional sense (think of it more like viscous, molten lava, or thick honey), but it’s definitely not a solid.

A depiction of different seismic waves propagating through the Earth.

This also revealed that the inner core is solid, as S waves appear to be able to pass through it. However, the small size of the inner core makes detecting shear waves very difficult, and this theory could still use additional verification. This is where the new study comes in.

J waves

A J wave is nothing more than an S wave that passes through the inner core — seismologists love giving different names to waves depending on where they pass through, but don’t let it confuse you, it’s still a shear, S wave.

J waves are rather elusive because they have small amplitudes, and the Earth’s inner core has a relatively small volume, which means that many seismic waves simply go around it. The small, feeble amplitude is so problematic that detecting J-waves has sometimes been referred to as the “Holy Grail” of global seismology.

However, Associate Professor Hrvoje Tkalčić and PhD Scholar Than-Son Phạm believe they’ve found a new way to identify J waves. What they did is pretty creative: instead of looking directly at the wave as it comes, they simply ignore it for the most part and look at the wave signal hours after the largest rumbles have passed. By studying the similarities between these signals at two receivers after a major earthquake, they’re able to observe a correlation of patterns, and this correlation is the key to the J-wave identification. A similar approach was used by other scientists to study the thickness of Antarctic ice.

“Using a global network of stations, we take every single receiver pair and every single large earthquake – that’s many combinations – and we measure the similarity between the seismograms,” lead author Hrvoje Tkalčić explained.

“That’s called cross correlation, or the measure of similarity. From those similarities we construct a global correlogram – a sort of fingerprint of the Earth.”

“We’re throwing away the first three hours of the seismogram and what we’re looking at is between three and 10 hours after a large earthquake happens. We want to get rid of the big signals,” Dr. Tkalčic added.

Using this approach, they confirmed that the inner core seems indeed solid, though it seems to exhibit some differences from current models. In fact, it’s a bit like two very familiar materials: gold and platinum.

“We found the inner core is indeed solid, but we also found that it’s softer than previously thought,” Associate Professor Tkalčić said.

“It turns out – if our results are correct – the inner core shares some similar elastic properties with gold and platinum. The inner core is like a time capsule, if we understand it we’ll understand how the planet was formed, and how it evolves.”

We should still wait for additional observations to confirm this study but, for now, it seems like the information about the nature of the inner core is becoming more and more solid.

This research was published in Science.

Though you may be familiar with the phrase “molten core,” the reality is that Earth’s inner core is actually solid, and it’s the outer core that surrounds this enormous ball of heavy metals which remains liquid. Image: geek.com

When Earth’s solid inner core formed: 1 to 1.5 billion years ago

Our planet’s magnetic field is the ultimate shield that guards life from the elements of space, particularly radiation. It’s enough to look at Mars, which also had a magnetic field but only for 500 million years, to see what could happen were it absent: what was likely once a “blue planet”, filled with vasts oceans of liquid water, maybe even vegetation and other life forms, is now a barred red rock.

This invisible, protective shield likely existed shortly after the planet formed 4.5 billion years ago, when it was still a big blob of molten rock. It was only after the super hot iron liquid core lost enough heat to freeze (more properly said, it solidified) did the field become strong enough to allow life to foster. Previous studies estimated this happened sometime between 500 million and 2 billion years ago. A more refined analysis by University of Liverpool places the timeline between 1 billion and 1.5 billion years ago.

Though you may be familiar with the phrase “molten core,” the reality is that Earth’s inner core is actually solid, and it’s the outer core that surrounds this enormous ball of heavy metals which remains liquid. Image: geek.com

Though you may be familiar with the phrase “molten core,” the reality is that Earth’s inner core is actually solid, and it’s the outer core that surrounds this enormous ball of heavy metals which remains liquid. Image: geek.com

Once the planet cooled down, it formed an outer crust while the inside was still liquid – like a glazed liquor candy. As the planet cooled further, heat was transferred from the core until it froze partially: the inner core became solid despite being smocking hot (at great pressure, matter like iron can still stay solid despite thousands of degrees Celsius), while the outer core stayed liquid. This process is called nucleation. The molten iron in the outer core is in constant churning movement, driven by convection as the outer core loses heat to the static mantle. This process is what keeps the planetary magnetic field floating above, and once the inner core solidified the field intensified, Liverpool researchers claim.

planet-core-2

Image: Wikipedia

To find out when the nucleation first began, Andy Biggin, a paleomagnetism researcher at the University of Liverpool, analyzed a database filled with the orientation and intensity of magnetic fields from ancient rocks. For instance, basalt contains magnetic minerals and as the rock is solidifying, these minerals align themselves in the direction of the magnetic field. By studying really ancient rocks, you can find out how the magnetic field was positioned at the time that these solidified. It’s very handy, and provides one of the strongest evidence in favor of the theory of plate tectonics.

What Biggin and colleagues found that the steep increase in Earth’s magnetic field intensity occurred between  1.5 billion and 1 billion years ago. Moreover, the researchers were able to calculate that the solid inner core increases in diameter with 1 millimeter every year. “This finding could change our understanding of the Earth’s interior and its history,” Biggin said.

While the Mars’ story might startle some, the good news is that Earth’s magnetic field is destined to be just as strong for another billion years, the researchers report in Nature. We have other problems to solve before that.

Answering our readers: The Earth’s core

I’ve been receiving questions from you people for years now – and really, this made me happy, because it says that people want to learn and understand more. But I’ve been answering them individually, and now, I’m thinking it would be better to post the answers on the website, so more people can read them. So, starting today, we have a new category, in which we’ll be doing just this; also, I’d like to take advantage of this to encourage you to send any scientific questions you might have, regardless of how silly you might feel they are – we’ll do our best to answer them. I’ll post a shorter version of the question.

Question: Why is the core so hot, and is it connected in any way to fossil fuels?

No, it’s not connected in any way to fossil fuels; fossil fuels are located in the crust at (generally speaking) several kilometers blow surface, and the core is at 5150 kilometers beneath surface. Let’s detail.

For starters, we’ll be discussing about Earth’s core. The Earth’s core is made of two parts: a solid inner core and a liquid outer core. The outer core has a temperature of 4400 °C – 6100 °C, while the inner core has an average of 5505 °C – which is about as big as the temperature on the surface of the Sun – and there are a number of reasons why this temperature is so high.

Generally speaking, the temperature rises as you’re going towards the inner core, growing with pressure. Earth has a significant amount of heat left over from when it fist condensed as a planet; sure, we’re walking around here find and dandy, but the depths of the Earth haven’t had enough time to cool down. This amounts for about 5-10 percent of the heat. Gravitational heat matters about just as much. What is gravitational heat? Well imagine our planet, in its initial stages, as a huge mass of lava (or honey, if that works better for you). If you put, say, a rock in a jar of honey, it will go down. The same thing happened with the Earth: the denser elements went down towards the core, and this movement continues today, causing friction which generates heat.

Then you have latent heat, which arises as the Earth cools from inside out; liquid metal solidifies, releasing heat in the process. But by far, the most important source of heat our planet has comes from radioactivity (mostly in the mantle). Radioactive decay causes up to 90 percent of all the heat inside our planet! There’s a good chance sometime in the future our planet will cool enough to fully solidify, and will become ‘dead’, much like the Moon. However, before this happens, the Sun will grow into a red supernova so large that it will also engulf Earth – so any heat in the mantle will hardly matter.