[Karen W.]

I just heard them somewhere, sometime back and wonder thats all??

[JimBob]

When rocks are heated, certain ratios of radioactive elements are reset as many of these are gaseous.  Potassium/Argon dating is based on this fact. The argon is the element into which the radioactive potassium is changed.

Magnetic clocks are rather dubious. As volcanic rocks (and some metamorphic rocks) harden, the iron in them is aligned to the magnetic field at the time they became solid. But dating from this is a sketchy process. If the present earths magnetic field is considered, there are areas where the poles are reversed. For example, in the southern hemisphere there is a north pole like area. Rocks here would be telling the opposite of the real story. Also, if one uses the assumption that there is only one north pole and one south pole, you would be wrong. At present there are two of each - north and south.

Dating from radioisotopes is rather accurate unless reset by metamorphic activity. Magnetic dating is much more doggy.

[Karen W.]

Ok So these are two different types or ways of dating? For dating rocks or just geographic areas in general?

[Bass]

JimBob's description of "volcanic clocks" is, once again, right on the mark.  Radioactive potassium decays into argon, so comparing the ratio of these radioisotopes allows us to date the age of the rocks.  volcanic eruptions reset the potassium-argon clocks.

The earth's magnetic poles reverse every so often- the north pole becomes the south pole and the south pole becomes the north pole.  Over the past 10 million years, the average time between reversals has been around 200,000 years.  The last reversal occured approximately 800,000 years ago.  JimBob pointed out that the iron in volcanic rocks becomes aligned with the pole when the rock cools (like a compass needle).  Continuous volumes of volcanic rock form at the mid-oceanic ridges, as these rocks cool they take on the orientation of the magnetic pole at that time.  If you cruise over the ocean with a magnetometer, you would expect to see narrow magnetic stripes (similar to zebra stripes) parallel to the ridge created when the polarity swings back and forth.
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Indeed, this is what scientists found- here is part of the Juan de Fuca Ridge off the northwest coast of the U.S.
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So the magnetic polarity allows geologists to also date rocks, but not highly accurate.