I am told the Earth has an molten core. Is this a theory or has this been proven?
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asked May 12, 2015 at 19:09
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19$\begingroup$ Here's a hint: asking "is there evidence" for things that have been established scientific fact for decades could be interpreted as suggesting that people are making this stuff up as the go along. Much better to ask what the evidence is. $\endgroup$– dmckee --- ex-moderator kittenCommented May 12, 2015 at 19:20
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4$\begingroup$ Finally, I will more this question to Earth Science if you'd like, but I read it as on-topic on Physics if you would rather it stayed here. $\endgroup$– dmckee --- ex-moderator kittenCommented May 12, 2015 at 19:21
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2$\begingroup$ soest.hawaii.edu/GG/ASK/earths_core.html has a believable explanation $\endgroup$– FlorisCommented May 13, 2015 at 0:00
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10$\begingroup$ In science, if it is not proven it is not a theory. The word you are looking for is hypothesis. $\endgroup$– TaemyrCommented May 13, 2015 at 7:41
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3$\begingroup$ The inner core of the earth is solid, this was dicovered by Inge Lehmann: en.wikipedia.org/wiki/Inge_Lehmann $\endgroup$– AlfonsCommented May 13, 2015 at 15:44
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3 Answers
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There is pretty strong evidence falling into groups
Seismological
As transversal waves do not propagate through liquid. By monitoring the travel times and types of seismic waves globally after earth quakes, one can reconstruct the velocity profile for the different kinds of waves in inner of the earth. Then one can see that the transversal waves are reflected (by something that is pretty surely the molten core).
This way, one can even conclude that there is a hard inner core under the molten core (which is in the ongoing process of crystallizing out of the liquid outer core, the released energy is one important source for the convection of the outer core and thus continental drift, the other energy source being radioactive decay of primordial radioactive isotopes).
The Earth Magnetic Field
To my knowledge there is no sensible explanation for the magnetic field of the earth, other than it is created by a conductive fluid that is in convection in the inner of the earth.
There are probably other proofs (e.g. astronomical high-precision measurments of the moon).
answered May 12, 2015 at 19:21
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$\begingroup$ Nice and more complete than my quickie answer. $\endgroup$ Commented May 12, 2015 at 19:22
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Shear-waves propagate in solids, but not in liquids. Pressure waves propagate in both. Earthquakes generate both kinds and seismographs can be built which are sensitive to the two kinds separately.
Both kinds of waves have shadows on the opposite side of the Earth, but the differing size and shapes of those shadows show that the outer core is liquid but the inner core is solid.
answered May 12, 2015 at 19:18
dmckee --- ex-moderator kittendmckee --- ex-moderator kitten
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$\begingroup$ You address "molten" but not "iron"... $\endgroup$– FlorisCommented May 12, 2015 at 23:15
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2$\begingroup$ Hmm ... good point. Not sure that I have the answer. The p-wave propagation tells us about the speed of sound in that layer, which casts some light on the density. I think that Sebastian is closer than I am there: the magnetic properties of the deep Earth combined with the density may be the strongest route but I am now officially outside of my area of competence. $\endgroup$ Commented May 12, 2015 at 23:31
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$\begingroup$ Can shear waves propagate in non-Newtonian fluids? $\endgroup$ Commented May 13, 2015 at 23:49
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$\begingroup$ @user I would assume the answer is strongly frequency dependent. And perhaps amplitude dependent as well. $\endgroup$ Commented May 13, 2015 at 23:50
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$\begingroup$ Indeed, but then it might be better to say that assuming that no Newtonian fluid may exist at the core at such and such frequencies and amplitudes than just the blanket statement that no liquid. Also, i am wondering if there is any direct laboratory evidence to support that liquids cannot exist at these supposed temperatures and pressures? $\endgroup$ Commented May 13, 2015 at 23:57
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Is there evidence to support that the earth has a molten core?
Yes. We have these things called volcanoes:
;)
answered May 12, 2015 at 19:46
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16$\begingroup$ Though it is clear evidence of high temperatures underground, I'm not sure that the presence of modest quantities of molten rock in the the otherwise solid upper mantle and crust is a strong argument for large quantities of molten iron much deeper in the Earth. $\endgroup$ Commented May 12, 2015 at 22:20
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3$\begingroup$ See "Journey To The Center Of The Earth" for a semi-plausible explanation for volcanoes with a cool Earth core. I say semi-plausible because other discoveries since the book was written have confirmed the liquid core. Volcanos alone only prove that there is molten rock near the surface. $\endgroup$– StephenCommented May 13, 2015 at 4:51
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4$\begingroup$ Volcanos are a sign that partially molten material exists a tiny fraction of the way toward the center of the Earth. The mantle, which lies between the crust and the liquid outer core, is essentially solid. It is not molten. It supports shear waves. $\endgroup$ Commented May 13, 2015 at 9:51
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1$\begingroup$ I mean... the whole heat equation thing makes the idea of a cold - hot - cold interior (especially one that's like 4 billion years old) pretty absurd. $\endgroup$ Commented May 13, 2015 at 15:26
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1$\begingroup$ @MaciejPiechotka Who is using the heat equation to model ultra low viscosity, energetically driven systems like atmospheres? That would also be absurd. $\endgroup$ Commented May 13, 2015 at 18:47