The terrestrial age is the time that has elapsed since a meteorite fell to Earth. It can be calculated from the abundances of some relatively short-lived radioactive isotopes that were formed, while the meteorite was in space, as a result of bombardment by cosmic rays. After the meteorite lands, no more of these radioactive isotopes are made and the ones already in the rock continue to decay at a steady (known) rate, thus serving as a kind of clock. Terrestrial ages are usually determined from the isotopes carbon-14, beryllium-10, and chlorine-36.
Most meteorites weather quickly in the oxidizing environment of Earth. However, other meteorites fell at more fortuitous locations and were preserved, e.g., in the ice fields of Antarctica and in the hot deserts of Africa. Some of them have been preserved for 40,000 years or more. The oldest of all are fossil meteorites, preserved in sediments or in other geologic strata conducive to preservation, of which the record-holder is the meteorite of Osterplana, Sweden, that was found in 1987 imbedded in some limestone. This limestone, which dates from Ordovician times, showed that the embedded meteorite had fallen 480 million years ago. This just beats out the Brunflo meteorite, also found in Swedish limestone in 1980, which has a terrestrial age of 450 million years. As with true fossils, most of the original meteoritic minerals in fossil meteorites has been replaced by terrestrial minerals leaving only the outer structure of the meteorite preserved. Although both meteorites have been classified as chondrites, only their chondritic structure remains as evidence of their extraterrestrial origins. The oldest intact meteorite is the Lake Murray iron. A single mass with a thick iron-shale was found in a gully in Oklahoma, in 1933. The meteorite was imbedded in some Antler Sandstone dating from the Lower Cretaceous, suggesting that Lake Murray landed in a near-shore, shallow sea, while these beds were being deposited about 110 million years ago. In addition, although the exterior of this meteorite has been heavily corroded, the inner nickel-iron core has remained unaltered, establishing Lake Murray as the oldest meteorite on Earth.