how we know what inside of the center of the earth if we never been there ?
In: Earth Science
To try and figure out what is at the centre of the earth, a scientist might do this:
First, write down everything we know about the Earth’s physical properties. What it’s surface is made of, how fast it spins, and so on.
Second, write down some guesses about what might be inside the earth; maybe it’s hollow, maybe it’s full of water, maybe it’s just rock all the way down.
Finally, the scientist would take those ideas and compare them to the things we already know. We can be pretty confident the earth isn’t hollow, because if it was, the earth wouldn’t have as much gravity. We don’t think it’s solid rock or water, because we know that heat emerges from the centre of the earth; if it was solid rock, where’s that heat coming from? The answer that makes the most sense when compared to everything we already know is that centre of the earth is full of molten rock and metal.
A lot of the information is gained from seismology. Just like an ultrasound scan, if an earthquake happens at one point in the Earth, the various waves coming from it reflect off layers of different density and composition and bounce back to detectors all the way round the planet. From the time different reflections get to widely spaced detectors, and how they are changed from place to place, seismologists can build up a picture of the layers.
We use the known to deduce the unknown:
i.e We can deduce the mass of the earth by observing its orbit around the sun ( 5.9 sextillion tonnes.) Then, we can observe that things on the surface of earth don’t have enough mass to fit that weight so we can deduce that the core must be more massive than the surface. An element that would fit the bill would be iron.
so on and so forth. logical deduction is the key and it can be tested.
Earthquakes produce waves which go through the Earth and can be detected on the other side the speed at which they go through the Earth means we can have a good guess at what they are travelling through. https://youtu.be/Oum1JnrI0XY
It’s a jigsaw puzzle with lots of pieces. From seismology we can measure the speed of sound in different places inside the earth. In the lab, you can then measure the speed of sound in lots of different combinations of materials, pressures, and temperatures.
Then it’s a bit of a guessing game based on our knowledge of the elements and compounds found in lava, etc. This mineral, at this temperature and pressure, has these properties. If a region contains these elements at these ratios, at this temperature and this pressure, what will be the combination of mineral compounds? If the pressure and temperature at this depth is typically these values, then the pressure and temperature this much deeper would probably be *these* values. And that would mean that *this* mineral would be more likely. (or maybe, *this* combination of elements in a molten solution, with *this* combination of other elements as crystals, etc.) Then add in what we know about the magnetic field.
I know you’ve been answered in terms of earthquakes, and I’ll reiterate that part briefly, but there is actually more to our understanding of Earth’s interior, which I’ll get to.
So, about earthquakes: Every time they occur, scientists can look at the way the shock waves (aka seismic waves) from these earthquakes move through the planet — both the speed of the waves and how the waves bounce off of denser parts inside the planet (the inner and outer cores). It’s essentially this way that we have determined the physical state of Earth’s separate layers:
**Crust**: solid rock
**Mantle**: solid rock (apart from a few melty bits near the top of the mantle which make magma and feed volcanoes). The mantle convects veeeeery slowly like a super-thick liquid despite being solid.
**Outer core**: liquid metal (mostly iron) which convects vigorously.
**Inner core**: solid metal (mostly iron) because the pressure becomes too high for the metal to be molten.
We have determined where the boundaries for these layers are based on seismic ‘discontinuities’ – sudden jumps in the speed that the seismic waves travel. Check out a [scaled diagram](https://i.imgur.com/NNaNTQ2.jpg) to illustrate how thick these layers are. On average, the crust is actually similar in thickness relative to the whole Earth as the skin of an apple is to the whole apple. Also, it’s not usually made explicit when just giving an overview of Earth’s insides, but Earth’s core is pretty damn big, even by planetary standards. If you were to tunnel down and reach the edge of the outer core, you’re still not halfway to the centre of the Earth.
So all the seismic stuff is the most fruitful approach there’s been to figuring out physical properties of Earth’s interior, but this is all backed up and complimented by a few other lines of thought:
• The Earth has a magnetic field, so must have a metallic core, at least some of which is fluid and circulating in order to generate the magnetic field. This is consistent with (1) the types of seismic waves which do and do not pass through the outer and inner cores respectively; and (2) the fact that some meteorites which fall to Earth are an iron-nickel alloy — these represent the cores of baby planets in the early solar system which got big enough to separate into bodies with a core and mantle, but then smashed apart to set free their innards.
• Even before we knew all that, early measurements of the Earth’s total mass meant that we knew their must be much denser material somewhere inside the Earth, as the rock of the crust was not dense enough to account for all the mass of the planet. Iron is a good fit for all the density, stellar physics tells us it’s relatively common in the universe, and there is obviously some natural process which can concentrate iron into pretty pure lumps of the stuff based on those metallic meteorites.
• With regards to the mantle, we do have certain pieces of mantle rock which get entrained into magma and eventually erupted as part of lavas which these still solid chunks of mantle rock within. (Many people think of the whole mantle as lava/magma, but it’s almost entirely solid rock with localised melty bits here and there). The chunks of mantle that occasionally get brought up to the surface are known as ‘mantle xenoliths’ and [they are a wonderful dark to striking green colour due to the mineral olivine.](https://i.imgur.com/Jhc2kiu.jpg).
• There are also high-pressure high-temperature experiments that some scientists run on [specialist lab equipment](https://en.wikipedia.org/wiki/Diamond_anvil_cell), this tells us what sort of rock exists in the deeper mantle. Essentially, when upper mantle materials are subjected (in stuff like diamond anvil cells) to the appropriate temps and pressures that exist closer to the core they turn into the kinds of minerals that make up the lower mantle, or at least give us big clues as to what’s down there when we observe how their squishiness or how the packing of their atoms changes.
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