Simon Wrote:
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I'll let Paul answer in more depth


> 1) How does the movement of molten iron in the
> earths core/mantle cause the earths magnetic field
> ? My understanding is that the current model is a
> kind of dynamo process where electricity is
> produced in "thermal reactions" between the outer
> core and the mantle.
Here is a short segment from a great book:

Richard Fortey. 2004. Earth. An Intimate History. NY: Alfred A Knopf

pp. 350-351
It is worth remembering that the magnetic field is also very weak— more than a hundred times weaker than the field between the poles of a toy horseshoe magnet. The development of accurate instruments to measure natural magnetism is another example of technique and theory advancing cheek by jowl.. Much still remains mysterious about the geodynamo. It is clear that it has been around for more than 3500 million years because remnants of ancient magnetism have been measured in rocks of this enormous antiquity. So it seems likely that magnetism is an innate property of the liquid outer core. The metallic core is, of core, a very good conductor of electricity, and also a fluid capable of movement; a magnetic field presumably must be generated by the interaction of these two properties, Nonetheless, its mathematical modeling has proved very difficult. Since the dynamo has to be driven by energy, much depends on the nature of the energy source—which must also have been rather constant for a long time. Thermal convection in the outer core is one possibility: a kind of deep, simmering turnover of the molten layer providing a motor of magnetism.. Then there are the magnetic reversals—those times when north and south poles “flip over.”.. Today, nobody seriously questions the reality of such reversals, since magnetizations have been precisely dated, using evidence provided by the signature of the characteristic fossils and confirmed by magnetized “stripes” to either side of the mid-ocean ridges was one of the crucial discoveries that hoisted the flag of plate tectonics over the bodies of its rivals.. It is clear that they [magnetic reversals] are by no means as regular as clockwork. There may be a million years or more of one “north” followed by a short-lived reversal—north to south—of perhaps a few tens of thousands, Certain time periods are characterized by more reversed than normal fields—and vice-versa. The Cretaceous was a long period of normal magnetism.. the short-lived “reversal events” within a longer period of opposite magnetizations are distinctive time markers in geological history—so distinctive, indeed, that they have been given names. As we have seen the Jaramillo Event is a short period of normal polarity about 900,000 years ago within a long period of reversed magnetism. It can be used as a precise geological time-line between events on the sea floor ad lava extrusions on the continents, a signal of great prehistoric utility, as might be the issue of a particular coin during the historical era. The “switchover” from one polarity to another is completed within 4000 years—a mere blip of geological time. Detailed studies on rocks that preserve a record of the “flip” show a decrease in the intensity of the field for a thousand years or so before the switch, and then short-lived, irregular swings of the magnetic vector, before the opposite polarity is established weakly at first. It is a subtle thing; no animal species felt it as it might the jolt of an earthquake, In fact, switching poles is a comparatively easy thing for a dynamo to do, and the “flip” may be controlled by relatively small changes of the fluid motion of the core..

[BOM] a moving electric current produces magnetism and vice-versa.

Why is this model not
> reproducible in the lab ?
Because the scale and temperatures are hard to replicate. Apart from that we already know the reltionship between electricity and magnetism

>
> 2) Considering whatever the answer to 1 was, what
> is the process/mechanism behind the field flipping
> polarity ?


>
> 3) Why is the earths core roughly the same
> temperature as it was 4 billion years ago (about
> the same as the surface of the sun) ? Surely it
> should be cooling unless there is atomic fusion
> happening ? Surely friction is not enough by
> itself ?
Apart from the heat generated at the formation of the earth, there has been continuous radioactive FISSION of uranium, thorium and potassium principally- also the core is wrapped by the rest of the earth's mantle and crust as insulators. I am not sure where you got the information that the core today is the same temperature as 4 billion years ago-- I would have to have a source for that. Even today, the core temperature is an estimate based on models.

Bernard
>
> 4) Considering all the processes
> generating/consuming electricity/magnetic
> fields/heat/convection of rock etc, why is the
> earths rotation not slowing down faster ?
>
> Simon
>
> ----
>
>
> Simon
>
> InternalSpace