– Ian Titan, San Diego

ANSWER: Rocks greater than 3.5 billion years old have been found on every continent. The oldest known rocks (4.03 billion years old) are in the Acasta Gneiss formation, near Great Slave Lake in Canada's Northwest Territories. The lower limit of Earth's age is therefore 4.03 billion years, since the planet must be at least as old as its oldest rock formation.

The ages of rock formations cannot be used to establish an upper limit for the age of the Earth because complex processes are constantly reshaping Earth's surface. Sediments from the weathering of rock are deposited and eventually form sedimentary rock. The Earth's crust is also being broken down and regenerated through plate tectonics.

Crystals of zircon 4.4 billion years old have been discovered in younger rocks in Western Australia. From the chemistry of the zircon crystals, researchers concluded that they originally formed in magma that was in contact with water as it cooled. This suggests Earth was already formed and had oceans 4.4 billion years ago.

Extraterrestrial rocks provide information about the age of the solar system and place an upper limit on the age of the Earth. Most meteorites are primitive rocks from the early solar system that escaped accretion into planets. The oldest known meteorites are 4.57 billion years old. Approximately the same age are the oldest rocks returned to Earth from the moon, which is not being shaped by plate tectonics. Therefore, 4.57 billion years is the current upper limit on the age of the Earth.

Early estimates of the age of the Earth were based on calculations of how fast a body of Earth's size could be expected to cool after its formation. In 1846, the physicist Lord Kelvin concluded that the Earth was just a few hundred million years old. At the time, radioactivity had not been discovered. Radioactivity provides a source of heat that Kelvin did not factor into his calculations.

Radioactivity also provides a method to determine the age of rocks – radiometric dating. Radioactive isotopes that occur naturally in rocks decay into more stable end products. The amounts of the radioactive isotope and end products can be measured and used to calculate the rock's age. Many long-lived radioactive isotopes can be used for radiometric dating, and there is relatively good agreement between the ages calculated using the decay of the different isotopes.

Sherry Seethaler is a UCSD science writer and educator. Send scientific questions to her at Quest, The San Diego Union-Tribune, P.O. Box 120191, San Diego, CA 92112-0191. Or e-mail sseethaler@ucsd.edu. Please include your name, city of residence and phone number.