Today I read a story in the WP relating to the rotation of earths “solid” inner iron core. I looked it up from a different source that confirmed it is solid as opposed to liquid. How can it be so hot and not be molten? Thank you!
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The vast majority of time when someone mentions the melting or boiling points of a substance, they are actually talking about the melting and boiling points at standard pressure. Standard pressure is defined as 1 atmosphere of pressure, or about 15 pounds per square inch.
For example, when someone says that water boils at 100°C, what they are really saying is that the boiling point of water *at standard pressure* is 100°C. They are just dropping the *at standard pressure* part because it’s easier to say and the *at standard pressure* is already implied.
The reason we have to make the distinction of *at standard pressure*, is because the melting point and boiling point of substances isn’t constant, it changes depending on the pressure it is under. For example, the more pressure you put water under, the harder it is to get it to boil, meaning that if water is under high pressure, you have to raise its temperature much higher than 100°C to get it to boil. Likewise, if water is under very low pressure, it will boil well before 100°C. In fact, you can get water to boil at room temperature if you put it in a vacuum chamber. This is also why pressure cookers are a thing, if you put food and water in a pressure cooker and bring that water to a boil under very high pressures, that means the water is actually much hotter than 100°C and will cook your food faster.
I’ve been using water as an example, but this is true of all substances. The amount of pressure will change the boiling and melting points, and iron is no exception. The core of the earth is made mostly of iron, and while the core of the earth is indeed very hot, much hotter than the melting point of iron *at standard pressure*, it’s also under tremendous pressure from all the iron and rock above it. There’s so much pressure in fact that the melting point of the iron is raised so high that even the extreme temperatures of the core of the earth can’t actually melt the iron. However, if you start to leave the core of the earth and go up, the pressure starts to drop (because there’s less stuff above pushing down) and the melting point of the iron will start to decrease and it eventually gets low enough that the temperature of the core IS hot enough to melt iron. This is why the inner core is solid while the outer core is liquid.
The same reason pressure cookers allow you to heat water beyond its normal boiling point. If you put something under pressure you change the points at which in undergoes phase change (solid to liquid, liquid to vapor).
The earths core is extremely hot but it’s also under immense pressure. The pressure increases the melting point of the iron core allowing it to remain solid at higher temperatures
The state of matter is not only dependent on temperature but also on pressure. For most day to day applications we only concern ourselves with the temperature since we all live roughly in a constant pressure of 1 bar (approximate average atmospheric pressure at sea level).
But at the center of the Earth the pressure is immense, since literally the entire weight of the Earth is concentrating into a relatively small area. With pressure so high, the temperatures that it would have to be for the core to be liquid are much higher than they are, so it’s solid. The outer layers though between the core and crust are liquid since the pressure is lower.
The core temperature is probably around 4000-5000°C. This is around the temperature of the photosphere of the sun.
The reason it’s still solid is the extreme pressure. This high pressure locks the iron and nickel into a crystalline form despite the high energy of individual iron atoms. It prevents the iron atoms from moving around freely as in a liquid.
Thanks again for the thoughtful replies. I found it interesting that scientists still debate the behavior of the core of our cozy home and thus its possible relevance to life on Earth.
Thank you all. It seems obvious the way you all explain it.
It is because the pressure keeps it from actually melting. The materia estate of a substance depends, mainly, of its temperature and its pressure (as inverse of volume). It would depend of other things like density but we could keep this aside for metals and for simplicity.
The high temperature would force molecules or atoms to vibrate more and push neighbours aside, that would break the bonds between neighbours making it easy to flow, thus becoming liquid.
The thing is that the metal core is surrounded by the other layers of molten metal, crust, water and atmosphere. All this mass surrounding the core leads to a pressure in all directions that overcomes the natural tendency of a hot metal under a lower pressure (1 atmosphere, like at sea level). This means that the molecules and atoms will keep acting as a solid metal cause they have not room to expand and flow.
If you were able to instantly drill a hole down to rhe core it would flow upwards and spill around like… a volcano.
I hope this helps.
Because despite being really really hot, it’s also really really compressed.
The phase (solid, liquid, or gas) of something is determined by 3 things:
1. The physical properties of the substance
2. The temperature
3. The pressure.
If the pressure is high enough, most things can stay solid, up to pretty ridiculous temperatures.
And it might not be surprising to learn that the core of a planet is under some pretty high pressures.
This also explains why we have 2 cores. The solid inner core, and a liquid outer core. Even though they’re made of the same stuff, because the outer core is under less pressure, it can become a liquid.
Well the core is molten. But not solid. We wouldn’t have volcanic activity if it was solid.
The pressure and force exerted on an object can change its melting point. The gravitational force and weight on top is immense, thus its a solid.
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