What is the primary evidence that the Earth's outer cores is fluuid in nature?

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Homework Statement

What is the primary evidence that the Earth's outer core is fluid in nature? (2 marks)

The Attempt at a Solution

The Earth has a magnetic field which is created by molten iron that is moving. If there were a solid outer core, there would be no moving iron and no magnetic field.

Continental drift - all continents are still drifting and creating volcanoes - this includes the edges under the deep oceans.

Is this what one needs to say for two marks, do you think? So is the primary evidence just the Earth's magnetic field? Or can the speed that waves travel at through the outer core also be taken as primary evidence? But what is THE primary evidence? I tried to look this up, but there doesn't seem to be a simple straightfoward answer. Please help. Please do not say that this does not belong in Advanced Physics. This is a question from a third year course and I find it complicated.

 

[Dick]

Look up the difference between p and s waves in seismology.

 

[emgram769]

I have learned (currently taking an introductory Astrophysics class) that it is shown by measuring the discontinuity of seismic waves. An earthquake on one side of the Earth can be detected at various points on the other side of the Earth but on directly the opposite earthquake (as well as in a certain distance radially outward from that point), those detections are disrupted. P-type waves, which travel much slower through a liquid, are virtually the same as sound waves. From this we can deduce that there is something at least slowing the wave down in the core. That does not imply it is a liquid, however. S-waves, which deal with a shear force interaction (side to side like when you pull a card out of a deck of cards and others come out with it), are not present in the area on the opposite side of the Earth as I described, a property limited to liquids. By taking various measurements, one can basically isolate what the core looks like.

I also recently read an article saying that models aimed at replicating these effects have proven that our core is not as fully iron as we think. Based on the results of these models, there are theories that silicon or other lighter elements exist in non-negligible quantities inside the Earth's core.