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How Old is the Earth?
by Steven Newton, Public Information Project DirectorThe age of the Earth is 4.54 billion years (Ga or Gyr), a number which geologists have determined by several independent methods. Because of the recycling of crustal rocks by plate tectonics, no direct material from the earliest Earth still exists. However, geologists are able to analyze meteorites and lunar rocks returned by the Apollo missions.
Rocks in the 4-5 Ga range may be dated by measuring ratios of isotopes of uranium/lead, rubidium/strontium, potassium/argon, argon/argon, and neodymium/samarium. One way to judge the reliability of a radiometric measurement is to compare the results of different measurement techniques for the same sample.
Note that these methods do not include carbon dating. Creationists frequently describe carbon being used to date rocks; in fact, it cannot be used in that way, as rocks do not accumulate carbon in the manner that living material does. Also, the relatively brief half-life of 14C (5730 years) means that after ~50,000 years, so little 14C remains that the machine used to measure isotopes, a mass spectrometer, cannot reliably detect this amount within its normal background error.
Meteorites are pieces of the primordial solar system. They formed at the same time as Earth and the other planets. Meteorites test within a narrow time range between 4.48-4.56 Ga.
The Apollo missions returned 382 kg of lunar materials. Although much of this consisted of relatively young volcanic flows, older crustal highland rocks ranged from 4.3-4.5 Ga. In 2005, researchers used a refined technique involving tungsten isotopes and hafnium to find a lunar formation date of 4.527 Ga.
Science Courseware has an excellent radiometric dating exercise called “Virtual Dating” that walks you through each step of the process of dating rocks. These exercises are especially helpful because of their explanation of corrections in the measurements.