Why do magnetic compasses spin when in the presence of magnetic rock?

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It has been said that magnetic compasses spin when in the presence of magnetic rock that contains a high concentration of magnetite. But the question is why does this happen? How is the magnetism of the magnetic rock turned by the magnetic compass into rotary mechanical motion?
 
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To cause the compass to spin, it would have to be constantly rotating it's own magnetic field - this doesn't happen and so the compass won't spin unless you move the magnetite around the compass in a circular motion.

Here's the wiki on it: http://en.wikipedia.org/wiki/Magnetite
 
This proves that the assumptions that the media makes about the behavior of compasses is exaggerated and wrong. This idea was wrong from the beginning but it was advisable to seek confirmation from a reliable source. There are a lot of fictional media circulating around, especially ones about surviving the wild that say that compasses spin when they are in the presence of magnetite and the Bermuda triangle but this is all just modern day popular myth.
 
In the presence of a magnetic field, a compass can't be guaranteed to be pointing north.

This is an important fact if you are in an area known for this type of activity. However, a compass isn't going to wildly spin just because it's by a magnetic rock.

To get it to spin, I suppose the simplest way is to have a motor type setup - this would induce the conditions required for the compass to spin.

EDIT: I suppose if you're in an area with a significant magnetic field the compass will align to it and as you move, it will track this field instead (and any surrounding fields) so it could appear to spin as it tracks the various magnetic locations - but you need to note it's not spinning because of the rocks magnetic field, it will only move as and when you move between them (imagine moving a compass around a number of magnets on your desk).
 
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