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Lecture 20

EPSC201 Lecture 20 Notes.doc

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McGill University
Earth & Planetary Sciences
EPSC 201
Anthony Williams- Jones

EPSC201Lecture 20 NotesHow do we date the earth In the 1600s we dated the Earth using biblical sources They thought it was around 6000 years old In the 1800s Lord Kelvin and other physicists tried to calculate how long the Earth would take to cool assuming the Earth started out as moltenKelvin figured the Earth was around 100 million years old last lecture said 400 million years old Wikipedia says the Kelvin initially calculated the earth to be between 20 to 400 millions years old but eventually settled on 2040 million years oldLord Kelvin and other physicists did not account for radioactive forcesAs elements decay they release heat in the form of alpha particles which are absorbed by the rocks in the Earth and are heated This radioactivity heats the EarthRadioactivity started with Rutherford at McGill UniversityHe showed that elements have a constant rate of radiation decayRutherford made several estimates using uranium and lead decayDifferent rocks are different agesThe oldest rocks in Canada range from 38 to 42 billion years oldMeteorites can also be datedIn 1956 a meteorite in Kenya was dated to be 455 billion years oldIn 1956 we established the age of the earth using this meteoriteNot all rocks can be dated by these methodsThey must have significant amounts of radioactive elements in themThe first person to challenge the biblical date was a Danish bishopHe established this using the law of UniformitarismA Scottish geologist named James Hutton introduced UniformitarismHe is regarded as the father of modern geologyHe said if we could understand the present processes and the rate at which they occur we could use this to figure out the age of rocksHis logic was that geological processes have not changed over timeHe noticed that mud cracks now have the same features as mud cracks from 500 million years agoUniformitarianism is the assumption that the same natural laws and processes that operate in the universe now have always operated in the universe in the past and apply everywhere in the universe It has included the concept that the present is the key to the past and is functioning at the same rates Uniformitarianism has been a key principle of geology and virtually all fields of scienceSo this Danish bishop Nicholas Steno developed the principle of superpositionHe proposed that the top layer of rocks were the youngest and the deepest rocks were the oldestWe now refer to this as stratigraphyThis was his first principle
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