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He came up with a figure of 90 million years for the age of the oceans.The problem with this estimate is, that salts are also taken out of circulation by reactions with seafloor volcanic rocks, and by forming evaporites that get buried in sediments and are thus out of circulation.For example, from literal study of the bible an Irish bishop (Bishop Ussher, 17th century) counted the days from the beginning of the biblical record, and concluded that the earth was created 4004 years B. This of course did not please early geologists, because from the study of thick sedimentary sequences alone (comparing them with modern sediments) it appeared that the earth must be at least hundreds of millions of years old, may be even billions of years (the problem was to prove it).Weathering produces dissolved salts that get carried into the oceans by rivers.
Since the exact rate at which uranium decays into lead is known, the current ratio of lead to uranium in a sample of the mineral can be used to reliably determine its age.If a series of zircon samples has lost different amounts of lead, the samples generate a discordant line.The upper intercept of the concordia and the discordia line will reflect the original age of formation, while the lower intercept will reflect the age of the event that led to open system behavior and therefore the lead loss; although there has been some disagreement regarding the meaning of the lower intercept ages.These types of minerals often produce lower precision ages than igneous and metamorphic minerals traditionally used for age dating, but are more common in the geologic record.During the alpha decay steps, the zircon crystal experiences radiation damage, associated with each alpha decay.
A lot of things that happen during geologic processes, such as the cooling and crystallizing of a magma, the erosion of landmasses, the transport of sediment by rivers, the filling up of ocean basins, all these processes can be observed in action in nature or in laboratory analogues (melting and crystallizing of rock in the lab, crystallization of salt from solution, weathering of building stones, floods of rivers, erosion of river banks, the filling in of embayments on the Mississippi delta within some decades).