Problems dating metamorphic rocks

Official publications say these rocks are more than a billion years old, but when the methods used to date them are carefully examined, a totally different story is discovered.

Clearly visible in the walls of the Inner Gorge are spectacular light-coloured rocks, such as the pink granites,1 which stand out starkly against the darker, metamorphic rocks2 (figures 1 and 2).

Rafting through Grand Canyon, northern Arizona, is a most exhilarating and enjoyable experience.

Deep below the rim, the crystalline basement rocks tower above the turbulent Colorado River.

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Clearly, the calculated ages are useless for dating any event.But the quest to test this belief by sampling rocks from deep within Grand Canyon has found it is not true.During several raft trips through Grand Canyon, many samples of these “Brahma” amphibolites were collected from various outcrops in the Inner Gorge.6 These included seven samples from a single amphibolite body (figure 5).7 All the samples were sent to two well-respected commercial laboratories for radioisotope testing.8 Both laboratories use standard, best-practice procedures on state-of-the-art equipment and routinely provide accurate and repeatable measurements of the required isotopes.Geologists overcome this problem by “interpreting” the result.For example, the calculated “age” could be taken as the “date” of metamorphism, or it could be the “age” of the original volcanic (or sedimentary) rock, or something in between, or something else.

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