Formation age
The time since a meteorite or its components first formed as solid rock, as opposed to when it travelled through space or landed on Earth.
A meteorite can carry several different ages, and the formation age is the oldest of the stories it tells. It marks when the rock or its minerals crystallised from a melt, condensed from hot gas, or last cooled enough for its radioactive clocks to start running. It is usually measured with long-lived systems such as uranium–lead or samarium–neodymium.
It is worth keeping separate from two other ages. The cosmic-ray exposure age tells how long the rock spent in space as a small body, and the terrestrial age how long it has been on Earth. A single stone might be 4.5 billion years old, have travelled for 20 million years and have landed 10,000 years ago.
Formation ages reveal the Solar System's timeline. Calcium-aluminium-rich inclusions in chondrites such as Allende date to about 4,567 million years ago and are used to define time zero. Basaltic eucrites from Vesta crystallised within a few million years after that. Martian meteorites are often much younger: the nakhlites, such as Nakhla, crystallised about 1.3 billion years ago, showing that Mars was volcanically active long after small asteroids had frozen solid. Published formation ages normally state the method and an uncertainty.
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