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F13 quiz 1 condensed notes

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Unformatted text preview: d fossils may be dated indirectly if volcanic strata, which are datable because they are igneous, are sandwiched among the sedimentary strata. The sedimentary rock is younger than the volcanic rock beneath it, and older than the volcanic stratum overlying it. If the ages of the two volcanic strata are similar, the age of the associated sedimentary rock is bracketed quite closely. Metamorphic rocks have two valid kinds of age: (i) the time since the parent rock first formed (the "primary" age), and (ii) the time since the later event of heating and recrystallization (the age of metamorphism). For example, metamorphism could drive 40Ar out of a crystal of biotite even though it was not heated hot enough to melt. Metamorphism may have had no effect on the U and Pb in a crystal of zircon, a very inert mineral resistant to change. In this example, the K ­Ar age of biotite would register the time of metamorphism (or more accurately, when the rock cooled off after being metamorphosed), whereas the U ­Pb age of zircon would register the primary event that formed the parent rock. Isotopic ages have allowed geologists to construct a timeline of Earth history. Because rocks on Earth are recycled (i.e. eroded, metamorphosed and melted by rock cycle processes), much of our information on the earliest parts of Earth history come from meteorites that we assume consist of original material from condensation of the solar disk into the planets, asteroids and other bodies in our solar system. The oldest materials identified in meteorites are calcium ­ aluminum inclusions that have ages as young as 4.567 billion years and are assumed to be the first material that solidified in the solar nebula. The Earth started to form soon after, and we place its origin at about 4.55 billion years ago. The oldest minerals on Earth are zircons from the Jack Hills in northwestern Australia, which have ages going back to 4.4 billion years. These zircons are contained in a younger conglomerate that is “only” about 3.8 billion years old. The oldest complete rocks are gneisses found in northern Canada, which have an age of about 4.0 billion years. The first seven eighths of the history of the Earth are called the Precambrian, after which came the Phanerozoic, which means “evident life.” The oldest signs of life (fossils) are found in rocks about 3.5 billion years old. However, since life at that time consisted of single ­celled organisms without hard parts, evidence of it preserved in rocks is very hard to find, and we will probably never be able to pinpoint the moment at which life appeared on Earth. Life that i...
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