How did scientists find out what layers exist under the Earth’s crust? How did they determine that Earth has a solid core?
In: Earth Science
We don’t know nearly as much about the composition of the earth as we say we do.
That being said, the going idea is that they use seismigraphs and time the return ping for an idea of the crusts depth.
Outside of that, and everything under the outer crust of the earth, it is all assumption. We assume it has a molten iron core. That explains our electromagnetic field. What we do is basically backwards math. We can measure some things, we extrapolate and correlate it into things we couldn’t possibly prove at this time.
Most of this is figured out through seismic surveys. You make a loud sound somewhere and then measure how the sound waves travel through the Earth. This is exactly how ultrasonic surveys work in humans. The sound waves will experience different levels of refraction or even reflection as they hit layers where the density changes. We can measure this and figure out the density of all the rock. It turns out that the core of the Earth is very solid compared to the material above it. So similar to how sound bounces between the walls in a room but is muffled outside the room any loud sounds on Earth is bounced back from the core but is very muffled on the other side of the Earth.
The first good evidence came when the Cavendish Experiment of 1797 measured the density of the earth at 5.4 times greater than water (the modern figure is 5.5). Now given that rocks generally have a density under 3, that meant the inside of the earth had to be different—much denser—than the crust we can see.
You asked about the core specifically, so I’ll just concentrate on that. It was actually hypothesised that Earth had an iron core long before we could make seismic measurements that deep because:
• Measurements of the Earth’s mass indicated that the Earth was on average quite a bit denser than the rocks we find at the surface and even the (slightly denser) rocks brought up from much deeper that we occasionally find in volcanic rock. There must be a region of something much denser inside the Earth just based on this.
• We have also known for a long time that the Earth has a magnetic field, and so something metallic is a good candidate for all that extra density down there. A formal publication on Earth’s magnetism was first made in 1600 proposing lodestone as the magnetic source, though this was before we had the mass measurements of the Earth and lodestone is still not dense enough, nor does it produce the right type of magnetism. It was not until 1919 that a self-exciting dynamo was proposed as an explanation for the Earth’s magnetic field. This forms the basis for our current geodynamo theory.
• The study of meteorites as rocks from space (rather than just superstitious stories or false assumptions of volcanic products) began in the early 1800s. It became known that some meteorites had a rock-like composition, while others were much denser, composed largely of iron. In 1897 E. Wiechert, (who subsequently became a renowned German seismologist), suggested that the interior of the Earth might consist of a dense metallic core, cloaked in a rocky outer cover. He called this cloak the “Mantel,” which later became anglicized to mantle. Metallic meteorites do in fact represent the cores of long gone planetoids, which managed to differentiate the heavier elements to their centre of mass before being smashed apart by collisions in the early Solar System.
Meanwhile, the Milne seismograph had been invented in 1880, and subsequent refinements to seismic measurements meant we were able to put constraints on the density and composition of Earth’s interior further and further into the planet. By 1906, the first seismologic detection of the Earth’s fluid (outer) core was made by R. D. Oldham, who showed that P-waves have a significant slowing when travelling through the core. Oldham also predicted a P-wave shadow zone beyond 103° from the origin, [shown here between 103° and 142°.](https://m.imgur.com/xfe2vqB)
Around this time it was also found that no S-waves arrived at the other side of the Earth beyond the 103° mark, ie. they do not pass through the core at all, so that the S-wave shadow zone stretches between both the 103° points from either side of the origin. S-waves rely on shear strength of the medium in order to propagate and fluids have zero rigidity, so zero shear strength. This is how it was deduced that the core is fluid, which then led to that 1919 proposal for a self-exciting dynamo via the movement of conductive molten iron in the core.
It was not until 1936 when Inge Lehmann, a Danish seismologist, reported weak P-wave arrivals within the aforementioned P-wave shadow zone (103° – 142°) which she interpreted as an inner core with higher seismic velocity, possibly solid. The limitations and difficulty of interpreting weak seismic signals, and quite possibly the fact that Lehmann was a woman meant that this remained controversial for some time, but it is 100% true.
Nowadays, we can use seismic tomography to build up more detailed pictures of the Earth’s interior. This is the generation of many 2-D seismic slices through the Earth and then the stacking of them to produce a 3-D image, the same principle used for medical CAT scans. This is shedding light on the fact that the mantle is not particularly homogenous (it seems like the inner and outer cores are). The mantle has large (continent sized) structures of hotter rock within it, thought to be associated with the generation of mantle plumes. [This is the sort of visualisation that can be generated from seismic tomography data.](https://i.imgur.com/bqJv3In.gif)
We have just finished analysing the first bunch of data from the InSight seismometer which has been on Mars for the last couple of years, so we now have data from another planet for the first time, and it looks like Mars also has a large iron core which is molten (but not convecting enough to produce a magnetic field today).
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