Plate tectonics - the tape recorder

As molten rock solidifies, it records the magnetic field it is exposed to. This means that the oceanic crust flowing away from the mid-ocean ridge is a continuous tape recording of the earth's field. Since new crust flows both ways from the ridge, the recording is in duplicate, as mirror images. It yields two continuous slabs of rock with one edge at zero age and the other edge up to 100 million years old, depending on how far it is from the ridge crest, exactly as a tape recording goes from 'now' under the record head back to the beginning of the song at a distance told by the tape counter.
As rock cools, it contracts, so the ocean floor gets deeper away from the ridge. We can measure the heat flow out of the rock to see how fast it is cooling. Likewise, radioactive decay can be measured in the rock. Finally, oceanic sediment, away from the continents, depends not on river-born mud but on the planktonic life constantly dying in the ocean above, sifting down to the bottom along with a very small amount of windborn dust. There is no sediment on the newly formed rock, and it increases in thickness away from the ridge.
In thousands of holes bored by the Deep-Sea Drilling Project worldwide, the thickness of sediment above the bottom rock, the age of the rock, the depth the rock has receded to, the heatflow out of the rock, the magnetization of the rock, and the distance the hole is from the ridgecrest, all correspond to the measured rate at which new rock is being created at the ridgecrest. We have a continuous record, in duplicate, of the latest 100 million years of the planet, spread over more than 2/3 of the globe. Nowhere on the continents can we duplicate this beautiful simplicity.