TRAVELLER A gem as old as the hills
ORIGIN Earth鈥檚 innards
DESTINATION Asia
DURATION 4 billion years
The zircon crystal comes into being deep inside a young Earth that is covered in ocean and pockmarked by volcanoes spewing white-hot lava. Hundreds of kilometres down, zirconium ions drift in magma droplets through the slowly creeping molten rock. A quirk of mantle currents gathers them beneath a new continent. The magma cools as it rises.
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Crystals begin to grow, countless atoms slotting into place in perfectly regular mineral lattices. The zirconium ions form their own crystal, and theirs is quite special: a clear, millimetre-sized structure so hard that it will last for all of eternity 鈥� at least as far as Earth is concerned. It is a haven, too, for uranium atoms. As soon as they are encased in the crystal, they begin to decay into lead, starting an atomic clock that will tick for billions of years. Together the gem and its clock will travel the world.
There is no time for rest. The magma is stirring, rising towards a volcanic vent. The zircon flies out of the crust in a luminous whirlwind of hot gas and rock and is dumped in a thick layer of volcanic ash. Later, more ash layers pile above it, and eventually the volcano goes extinct.
Over millions of years, wind and rain slowly exhume the crystal and leave it exposed on a lifeless grey wasteland of rock and rubble, under an atmosphere of carbon dioxide and methane. It is a dangerous place. Meteorites pound the planet and the thump of one landing nearby rearranges part of the zircon lattice.
A few thousand years pass. Storms slowly inch the crystal across the landscape until finally it tumbles into a river and is dragged out to sea. It鈥檚 moving much faster now, pushed by currents across the sea floor. The long journey smooths its edges. Sand starts to pile on top of it, and for the second time it ends up deep inside Earth鈥檚 crust.
鈥淭he tiny crystal is so hard that it will last until the end of the Earth鈥�
When it returns to the surface over a billion years later, pushed up by tectonic forces, the lands and oceans have transformed. Some of the cells that float in the oceans have evolved to harness energy from the sun and are pumping oxygen into the air, which rusts any unstable minerals on the continents. The bare landscape around the zircon is now a pattern of browns and reds.
The crystal is washed into the sea once more, briefly trapped in a slimy mat of microbes, then buried for a third time. For 2 billion years, the tectonic plate it is trapped inside drifts across the globe, crosses the equator, then is crumpled as continents collide. The zircon is sheared and cracked. It holds together 鈥� just. Part of its atomic clock is reset by migrations of uranium and lead; more scars that will one day tell the tale of its extraordinary voyage. As mountains rise above, the rocks around it begin to melt. New zirconium ions in the magma cover the ancient gem in a fresh crystal coat with its own atomic clock.
Erupted to the surface for a third time, the battle-scarred and healed zircon lies on a humid, tropical landscape. The skies are alive with birds and in a nearby forest a new animal cry has joined the chorus: the first primate! In a flash of monsoon rains, the crystal is washed into a new river, a new sea and is buried again.
The next time it sees the light of day, 65 million years later, circumstances are very different. A hammer and chisel pound at its rocky bed and the crystal faces its fiercest test. Collected by a human, it is cut on a diamond wheel, probed by an ion beam. The human reads the dates on its atomic clocks, deciphers its long history from its many scars 鈥� and, content with the knowledge, throws it away.
Diminished but still resilient, the zircon travels on towards the sea again, and another underground sojourn. Who would bet against it surviving to the end of the Earth?
Read more about your billion-year history in these seven fantastic journeys of the stuff that makes you
(Image: Martin Rietze/WestEnd61/Rex Shutterstock)
This article appeared in print under the headline 鈥淭o the depths of the Earth and back again鈥�
