Conclusive Proof About Earth's Core?

A child of the 1970s, I was never taught the earth has a liquid outer core and a solid inner core. I was simply taught there is a liquid (molten iron) core - period. However, my son corrected me. Looking into how they know this, I learned it was conjectured in the 1930s or 1940s and proven in the 1970s. However, all the articles (such as this one) I found when Googling all make the same wide-sweeping claim that reaches an illogical conclusion as proof that the inner core must be solid. In short, they say that scientists study the characteristics of shock waves from earthquakes at 180 degrees opposite the epicenter. These waves reveal “images” of the core of the earth. At first, they were using the properties of S-waves, but they found these waves did not arrive at the diametrically opposed location. Since they apparently knew S-waves do not traverse liquid, there must be liquid at the core. But, some time later, they utilized P-waves that will penetrate liquid and this proves there is a solid inner core. HUH??? This leap of faith and logic fails to conclusively prove anything. So, how do we know for certain that the earth has a solid inner core?

Related to this, I should also ask: I recently read (somewhere in my Google search) that the molten lava from volcanoes does not come from the core of the earth, but rather it is from the mantle. I thought the mantle was solid. Are there pockets of lava running throughout the mantle around the globe? How are we to picture the make-up of the earth’s mantle? And, where does the heat come from to keep the lava in a molten phase?

I would tend to reserve skepticism for bad journalism rather than bad science, but for sure that article is as clear as mud. I’m not an expert, but I think it’s probably that primary waves travel through both liquid and solid, but behave differently when passing through each medium. And that the difference in path is a more subtle effect that the simple absence of secondary waves, so it took the more sensitive techniques of the 1970s to confirm conclusively.

You realize that article is a surface level summary right? Wikipedia goes one step further and mentions that P-waves are reflected from the liquid/solid boundary:

The answer to your question is we know the Earth has a solid inner core through extensive analysis of different types of shock waves detected at the Earth’s surface after having moved through the inside of the Earth in various ways. The precise details of these analyses is beyond the easily explainable to most non-seismologists, so you’ll just have to take it on faith in science, like all other not-easily-replicated-in-a-kitchen scientific findings.

Also there is the fact at some depth the pressure will be so high that the iron has to be solid no matter how hot it is.

As for your last question, the heat comes from radioactive decay. (The heat from gravitational collapse has long since dispersed.)

Molten lava from volcanoes comes from lava in the crust, or from the very surface layer of the mantle where the pressure is not yet so great to prevent rock from turning into liquid. The heat comes from a few sources, mainly radioactive decay, but also leftover heat from the formation of the earth caused by the initial collision of matter that formed the Earth, and finally from gravitational pressure.

Can we at least get agreement on this issue? :confused:

Much of the heat initially came from the formation of the earth, but this heat has long dissipated. You could say that with other things keeping the earth warm, this heat is still around, but that’s a bit existential.

As far as gravitational heating, the earth still does provide some of that. The core regions are not as dense as they can be, and are becoming denser over time. The liquid core is slowly solidifying on the solid core. Those create heat.

But that heat is relatively insignificant compared to the heat from radioactive materials (uranium and thorium).

Mea culpa… my knowledge seems to be a bit outdated. I’ll take Darren’s cite as writ.

Well, you can’t say which heat has left the core… But the amount of internal heat that has left the Earth over its entire history is much greater than the amount that would have been initially deposited by gravitational collapse. I think that makes it a close enough approximation to say that “the initial heat from gravitational collapse has dissipated”, though it also wouldn’t really be incorrect to say that simply most of it has dissipated.