Paleomagnetic Dating
For reasons that we don't yet understand, the north and south magnetic poles of the earth will switch. A record of these magnetic flips is held within ince-molten rocks such as basalt.
Tiny magnetic particles within a lava line up with the prevailing magnetic field of the earth, and when the lava solidifies as a rock, the particles are set in their original orientation. Magnetic reversal events vary in length, and the complex history of magnetic reversals is preserved on the floors of the oceans. Study of the magnetic orientation of basalt on land can match them to the history from the sea floor, and an age of the basalt and its surrounding rocks can be determined.
Fission-track Dating
Uranium occurs naturally in the common mineral zircon. One of the isotopes, U238, is unstable and undergoes fission, a nuclear reaction where it splits and forms a more stable isotope. When this happens inside a zircon crystal, the parts of the isotope fly apart with such ferocity that they leave minute scratches on the crystal matrix. Like all isotopic decay, the decay of uranium 238 occurs at a known rate, so that the longer a zircon crystal has existed, the greater the number of fission scratches, or tracks, that will be discernible. Fission track dating consists of counting the tracks in zircon crystals.
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