Geology Reference
In-Depth Information
tremely high oxygen-18 contents have been interpreted as having interacted with surface
water.
In this way, zircon crystals from Earth's earliest rocks can survive many cycles of
erosion and deposition, while they preserve details of the age, temperature, and water con-
tent of their original environment. All that information gleaned from crystals barely large
enough to see without a microscope!
The bottom line is that many individual zircon crystals from the Jack Hills of Australia
aremorethan4billionyearsold,withoneaged,ancientsandgrainclockinginataremark-
able 4.4 billion years. That oldest zircon crystal—indeed, the oldest known surviving solid
fragment of Earth—has a surprisingly heavy oxygen isotope composition. Some scientists
conclude that 4.4 billion years ago, when Earth was only about 150 million years old, the
surface was relatively cool and wet: hence there were oceans.
Other experts aren't so sure. They point out that zircon crystals can be incredibly com-
plicated: that 4.4-billion-year-old grain, as well as virtually all its slightly younger com-
panionsfromtheJackHills,hasanancientcrystalcore.Butdetailed mappingofeachindi-
vidual crystal reveals concentric layers of younger zircon grown around old layers. It's not
uncommon for a single grain to display a billion-year range of ages from core to rim, with
correspondingly complex variations in oxygen isotopes. If the older core was altered dur-
ing more recent pulses of crystal growth, then the true character of Earth's ancient surface
might be obscured.
Whatever the eventual outcome of the zircon story, most experts agree that not much
more than one hundred million years after the Big Thwack, Earth had become a brilliant
blue water world with a mile-deep encircling ocean. From space it would have appeared as
an ultramarine marble, swirled with white wisps of clouds to be sure, but predominantly a
breathtaking blue. (The ocean's color arises from simple physics. The sunlight that bathes
the surface comprises all the colors of the rainbow—reds, yellows, greens, and blues—but
water absorbs the red end of the spectrum more easily, so our eyes perceive the predomin-
ance of the scattered blue wavelengths of light.)
And what of the land? Today the continents occupy almost a third of Earth's surface,
but at the dawn time of our planet, during the hellish Hadean Eon, continents had not yet
formed. The primordial blue ocean was broken only by isolated steaming volcanic islands
thatpokedabovethewaves.Theircone-shapedcontoursandnarrow,rubblyblackbeaches,
randomly dotting the globe from the poles to the Equator, were the only features to break
the watery monotony.
As we think back to Earth's earliest, globe-spanning ocean, we wonder what it was like.
Was it hot? Probably at first, given the still-cooling global magma ocean beneath. Was it
fresh or salty? Salt is perhaps the most distinctive property of modern ocean water, but it
might seem reasonable to assume that Earth's first ocean started out fresh, with few dis-
