Paleogeography Lab Report

1 Archean Rocks • The Beartooth Mountains on the Wyoming and Montana border consists of Archaean-age gneisses – some of the oldest rocks in the US.
2 Precambrian • The Precambrian lasted for more than 4 billion years! – This large time span is difficult for humans to comprehend • Suppose that a 24-hour clock represented all 4.6 billion years of geologic time then the Precambrian would be slightly more than 21 hours long, constituting about 88% of all geologic time
3 Precambrian
4 Precambrian • The term Precambrian is informal but widely used when referring to both time and rocks • The Precambrian includes – time from Earth’s origin 4.6 billion years ago to the beginning of the Phanerozoic Eon, 542 million years ago • It encompasses all rocks below the Cambrian system • No rocks are known for the first 600 million years of geologic time – The oldest known rocks on Earth are 4.0 billion years old
5 Rocks Difficult to Interpret • The earliest record of geologic time preserved in rocks is difficult to interpret because many Precambrian rocks have been • • • • • altered by metamorphism complexly deformed buried deep beneath younger rocks fossils are rare, and the few fossils present are not useful in biostratigraphy • Subdivisions of the Precambrian have been difficult to establish • Two eons for the Precambrian are the Archean and Proterozoic which are based on absolute ages
6 Eons of the Precambrian • Eoarchean refers to all time from Earth’s origin to the Paleoarchean, 3.6 billion years ago • Earth’s oldest body of rocks, the Acasta Gneiss in Canada is about 4.0 billion years old • We have no geologic record for much of the Archaen • Precambrian eons have no stratotypes – unlike the Cambrian Period, for example
7 What Happened During the Eoarchean? • Although no rocks of Eoarchean age are present on Earth, – except for meteorites, • We do know some events that took place then – Earth accreted from planetesimals and differentiated into core and mantle • and at least some crust was present – – – – Earth was bombarded by meteorites Volcanic activity was widespread An atmosphere formed, quite different from today’s Oceans began to accumulate
8 Hot, Barren, Waterless Early Earth • about 4.6 billion years ago • Shortly after accretion, Earth was – – – – a rapidly rotating, hot, barren, waterless planet bombarded by meteorites and comets with no continents, intense cosmic radiation and widespread volcanism
9 Oldest Rocks • Continental crust was present by 4.0 billion years ago – Sedimentary rocks in Australia contain detrital zircons (ZrSiO4) dated at 4.4 billion years old – so source rocks at least that old existed • The Eoarchean Earth probably rotated in as little as 10 hours – and the Earth was closer to the Moon • By 4.4 billion years ago, the Earth cooled sufficiently for surface waters to accumulate 10 Eoarchean Crust • Early crust formed as upwelling mantle currents of mafic magma, and numerous subduction zones developed to form the first island arcs
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