In Biology / Senior High School | 2025-03-05

The original source of most rock on Earth is magma, a hot, molten material deep under Earth's surface. When magma cools, it hardens and becomes rock. As this occurs, the atoms in the magma join and crystallize as different kinds of minerals, each with a characteristic structure and composition. Zircon is a mineral that consists mainly of zirconium silicate molecules. Some of the molecules in a newly formed zircon crystal have uranium atoms substituted for zirconium atoms. Uranium is a radioactive element with a half-life of 4.5 billion years. Its daughter element, thorium, is another radioactive element. Thorium decays at a predictable rate into another radioactive element, a n d so on until the atom becomes lead, a stable element. Over time, uranium atoms disappear from a zircon crystal, and lead atoms accumulate in it. The ratio of uranium atoms to lead atoms in a zircon crystal can be used to calculate its age. Using this technique, we know that the oldest known terrestrial rock, a tiny zircon crystal from the Jack Hills in Western Australia, is 4,404 billion years old.

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