Early Ideas of the Age of the Earth
Archbishop Usher (Ireland), used genealogy in the Bible to calculate that
the Earth was created October 26, 4004 BC, 9:00 am
Earth was thus 6000 years old.
Led to the Doctrine of Catastrophism:
James Hutton, Father of Geology (Scotland) 1726-1797.
Published Theory of the Earth in 1785.
Hadrian's Wall built by Romans, after 1500 years no change. Suspected that Earth was then much older.
Quantitative Scientific MethodsLord Kelvin assumed that the Earth was originally molten and calculated a date based on cooling through conduction and radiation.
Age of Earth was calculated to be about 24-40 million years.
1899 - 1901
John Joly calculated the rate
of delivery of salt to the ocean. River water has only a
small concentration of salts. Rivers flow to the sea.
Evaporative concentration of salts.
Age of Ocean = Total salt in oceans (in grams) divided by rate of salt added (grams per year)
Age of Earth was calculated to be 90-100 million years.
Principles of Radiometric DatingNaturally-occurring radioactive materials break down into other materials at known rates. This is known as radioactive decay.
Radioactive parent elements decay to stable daughter elements.
Radioactivity was discovered in 1896 by Henri Becquerel. In 1905, Rutherford and Boltwood used the principle of radioactive decay to measure the age of rocks and minerals (using Uranium decaying to produce Helium. Uranium decay produces He, leading to a date of 500 million years. Boltwood suspected that lead was the stable end product of the decay of uranium. In 1907, Boltwood dated a sample of urnanite based on uranium/lead ratios. Published the age of a sample of urananite based on Uranium-Lead dating. Date was 1.64 billion years. Amazingly, this was all done before isotopes were known, and before the decay rates were known accurately.
The invention of the MASS SPECTROMETER after World War I (post-1918) led to the discovery of more than 200 isotopes.
Many radioactive elements can be used as geologic clocks. Each radioactive element decays at its own nearly constant rate. Once this rate is known, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent element and the amount of stable daughter elements.
Radioactive parent isotopes and their stable daughter products
In the above table, note that the number is the mass number (the
total number of protons plus neutrons).
Each radioactive isotope has its own unique half-life.
Half Lives for Radioactive Elements
Radioactive decay occurrs at a constant exponential or geometric rate.
The proportion of parent to daughter atoms tells us the number of half-lives,
which we can use to find the age in years.
Radioactive decay occurs by releasing particles and energy.
Uranium decays producing subatomic particles, energy, and lead.
As uranium-238 decays to lead, there are 13 intermediate radioactive daughter products formed (including radon, polonium, and other isotopes of uranium), and 8 alpha particles and 6 beta particles released. There are three types of subatomic particles involved:
No charge or mass.
Can penetrate concrete. Lead shield can be used.
So how does Carbon-14 dating work?
So far, oldest dated Earth rocks are 3.96 billion years.
Mass spectrograph was used after WWI (1918). Led to the discovery of over 200 isotopes.
Many radioactive elements can be used as geologic clocks. Each element decays at its own nearly constant rate. Once this decay rate is known, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent and the amount of stable daughter elements.
Pasteur, Curso Experimental Bilingue, São Paulo - Brazil
Lyceum Berlage, Amsterdam - Netherlands