How do we know the crust and mantle exist?

How did we learn about the mantle and crust?
16 June 2020

Interview with 

Marijan Herak, University of Zagreb


Crack in earth


Despite the fact that the Earth is 4500 million years old, and we’ve lived on it for thousands of years, it’s only in the last century or so that we’ve had any more than a vague notion of what actually lies inside our planet. We now know that there are several layers inside the planet, the outermost one being a relatively thin crust, and below that is the mantle; this is a rocky layer that extends 3000km down to the outer edge of the core. The reason we know this is thanks to earthquakes. These send waves of vibrations through the planet. Some of these waves shake from side to side, while others compress the thing in front of them. And, critically, these waves change their paths in predictable ways when they meet the boundaries between different layers. The first person to realise that this could reveal what lies inside the Earth was Croatian mathematician Andrija Mohorovičić. At University of Zagreb, where he worked, they have many of his original notebooks and records in which he proved where the crust must stop and the mantle begins. Geologist Marijan Herak showed Chris Smith around...

Marijan - We are in the Mohorovičić Memorial Rooms in the building of the Department of Geophysics faculty of science in Zagreb. In this room, we tried to collect everything we have from Professor Mohorovičić, who is one of the founding fathers of seismology. Universally Mohorovičić is famous for his discovery of the crust-mantle boundary, which is today known as the Mohorovičić discontinuity. When Mohorovičić published the discovery in a paper in 1910, this proved that the earth is best described as being made of shells. So the uppermost of those shells is today known as the crust. And he conclusively proved that the crust exists, that it has the depth of about 50 kilometres, the average is about 33 kilometres, and that the waves and the properties of the earth abruptly change at such discontinuities.

Chris - Why was that so groundbreaking at the time?

Marijan - Because by proving that the properties of the earth do not change continuously, he added an important piece of information into the common knowledge. He established ways to see into the depths of the earth, by observations at the surface of the earth. And this was one of the goals of seismologists, he postulated it to continue where the geologists stops. And he realised that by modern instruments, a scientist is given a kind of binoculars with which he can look into the greatest depths of the earth.

Chris - How did he do it?

Marijan - He was a bit lucky that soon after he installed the modern seismographs in Zagreb, an earthquake occurred not far from here in the Valley of the Kupa river. He recorded it beautifully and was able to collect the seismograms from all over Europe.

Chris - Other people were also doing similar measurements in different parts of Europe, and so he was able to bring the same records of the same event together.

Marijan - Yes, seismology was in its infancy. The seismographs had just gotten good enough to record faithfully the movements of the earth during earthquakes. Of course, this was more complicated than today. There were no emails or scanners or anything like that. He had to obtain copies, or in some instances, the originals were sent to him by post.

Chris - Because you've got an example of one on the table here in front of us. And these are long pieces of paper or card, which have a scratched sort of signature, which the stylus of the seismograph has scratched into the surface as it's moved in response to the vibration. So he would have been receiving things like that from across Europe.

Marijan - Yes. Things like that or photographic copies. So he analysed those records and realised that some things were unexpected. There were four waves observed instead of two.

Chris - Just explain these different waves for a second. So when you have an earthquake, what are the two sorts of waves that should be coming out of that earthquake then?

Marijan - So when the earthquake occurs, two waves radiate from the source. One of those are the longitudinal waves and those waves, which are like sound waves, the particles oscillate in the direction of the wave spreading. The other type is the transversal wave which has particles oscillating perpendicularly to the direction of the spreading of the wave.

Chris - So if I had a slinky spring and I pulled some of the coils towards me and let them go, and they would ping away, the wave would travel along that spring in the direction the spring was stretched out. Whereas if I sort of wiggled the spring from side to side, that would be like a transverse wave. Those would be the two sorts of waves that you should get from an earthquake. And you're saying he actually saw not just those two, but when he made his recordings, it was clear there were four waves.

Marijan - In some stations, yes. This was funny in some stations, you'll see what you expect, two waves, then you'll see four. And then again, you see two.

Chris - Were they the same waves except they were delayed or something, were they arriving at different times?

Marijan - He knew that only two waves can exist. So he had to come to the conclusion that this interval of distances, and those waves were not four different ways, but two pairs of waves of which one came directly from the source, and the other pair were initially going down and then they were refracted to unknown depths. And then refracted back to the earth, after some distance traveled.

Chris - So one of the waves goes straight through the earth, the other goes deep into the earth and hits this boundary that we now know exists between the upper part of the earth, the crust and the mantle, which is deeper. And that bends the waves back up towards the surface, and that was his breakthrough.

Marijan - Yes, in a nutshell.

Chris - That must have been an incredible amount of maths for him to work that out.

Marijan - Yes. It was really difficult for him to understand. But once he understood it, then all he had to do was find the model to invert the observations on the surface for the properties of the earth. The only thing to do is to find the model of the velocities and the depths of the discontinuity and to match the observations. And he did it and we called such problems inverse problems. So like your doctor when you go to have a CT or X-rays, by observing things on the surface of the body, you conclude about the composition of the interior of the body. The math, of course, without computers, without anything, and I must say, without any help, he did it alone. And we have stacks of papers in his handwriting. He worked with logarithms to seven decimals and he made it so difficult for himself because he wanted to have it very realistic. So some of the assumptions we even do not use today, he used them. This was a breakthrough.


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