The Age of The EarthThe Age of The Earth

Moving towards the billion years Moving towards the billion years old concept.old concept.

TechniquesTechniques

• 1860: Average Sedimentation Rate Age: 3 million years: Highly faulty method as it does not account for past erosion and large variability in sedimentation rates.

• 1897: Lord Kelvin assumes earth is initially molten and would take 20-40 million years to cool.

• 1899: Joly uses ocean salinity and delivery rate from streams to calculate 90-100 million years.

• So by 1900, a reliable age still eludes us

Radioactive MethodsRadioactive Methods

• 1896: Becquerel discovers radiation.• 1905: Rutherford and Boltwood uses

Uranium decay to Helium to measure 500 million years for the oldest rocks.

• But, how do you know where the oldest rocks are?

• 1907 Boltwood believes that Lead (not Helium) is the stable end point for Uranium decay and now gets 1.64 Billion years as the Age.