How did geologists determine that the Earth is 4 and a half billion years old when the oldest known rocks on Earth are only 3.8 billion years old?
Radiometric dating of meteorites--"free" samples of space rock--is one line of evidence for the age of the Earth and our solar system. Another line of evidence for the age of the Earth comes from closer to home-individual crystals of the mineral zircon found in ancient metamorphic rocks from Australia.
Zircon is a mineral that contains small amounts of radioactive uranium and thorium, so it can be radiometrically dated, and is formed by igneous processes that created the granitic crust of the continents. The Australia zircons give a radiometric date of about 4.2 billion years, indicating that the Earth’s granitic crust had developed by that time, very early in Earth's 4.6 billion-year history.
Illustration and more information about the Australian zircons available here.
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