Geology Reference
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superheated water. Such intense water-rock interactions served to hasten crustal cooling,
making way for deeper ponds, then lakes, then oceans.
The exact timing of the global ocean's formation is unknown, but tantalizing evidence
hasemergedintheformofEarth'soldestcrystals.SomeofthemostancientrocksonEarth
are three-billion-year-old layers of sediments in the arid sheep-ranching terrain of Western
Australia, known as the Jack Hills. The sand-size mineral and rock fragments that make
up those sediments would have eroded from vanished rock formations that must have been
much older. A tiny fraction of those sand grains, not more than one in a million, is made of
the mineral zircon—zirconium silicate (ZrSiO 4 ), one of the toughest materials in nature.
Individual zircon grains, typically smaller than the period at the end of this sentence,
first formed as a minor accessory mineral in igneous rocks. Imagine basalt solidifying out
of a melt that holds only a trace of the element zirconium. Most of the chemical elements,
whether rare or common, easily enter the crystal structures of pyroxene, olivine, and feld-
spar. But zirconium has no home in common minerals. Rather, it seeks its own kind and
thus forms isolated, tiny zircon crystals.
Several factors act together to make these easily overlooked zircon crystals a unique
source of insight about the earliest Earth. First, zircons can last almost forever (at least for
all of Earth history). A single crystal of zircon can erode from one rock (perhaps the ig-
neous host where it first crystallized), then become part of a sedimentary sandstone rock,
and then erode again and again and again for billions of years. The same individual zircon
grain can be recycled through a dozen different sedimentary rock formations.
Second, zircon crystals tell time because they readily incorporate the element uranium,
which can make up 1 percent or more of their atoms. Radioactive uranium, with a half-life
of about 4.5 billion years, is nature's ultimate stopwatch. Once a zircon crystal forms, its
uraniumatomsarelockedin,andtheybegintodecayatasteadyrate;halfofthemdecayon
averageevery4.5billionyears,eachoneultimatelytransformingintoastableatomoflead.
Theratioofdwindlingparentaluraniumatomstogrowingdaughterleadatomsprovidesan
accurate estimate of the zircon crystal's age.
Finally, two of every three atoms in zircon are oxygen, which provides clues about the
temperature of formation. Recall that one line of evidence regarding the Moon's formation
was the distinctive ratio of oxygen's stable isotopes: Earth and the Moon have identical ra-
tios of oxygen-16 to oxygen-18, which implies that they formed at a similar distance from
theSun.Inasimilarlineofreasoning,theratioofoxygen-16tooxygen-18inazirconcrys-
tal points to the temperature at which it grew: heavier oxygen-18-enriched samples point
toacoolertemperature offormation.Forigneousrocks,thistemperature canbeasensitive
indicator of the water content of the magma in which zircon crystals grew, because water
lowersthetemperatureatwhichcrystalsgrow.What'smore,isotoperatiosfromwaternear
Earth's surface tend to be even more enriched in heavy oxygen, so zircon crystals with ex-
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