A tiny scrap of zircon from an ancient beach in Australia bears witness to Earth’s earliest Epoch. 4.5 billion years ago, the planet we now know as home was just another ball of rock – a very hot ball of rock. At that time the Earth was still molten, superheated by collisions with a giant Mars-sized asteroid and countless meteors that circulated the proto-solar system before they settled into the belt where they now exist between Mars and Jupiter. The early ball of magma that was Earth took the impact of these giant space rocks with liquid grace, popping a proto-moon out the other side and then quickly, it now seems, began cooling enough to form a thin crust.
This crust, like a thin scab, was not safe from bombardments, however, and was continually battered down into the molten under-layers, shifting with new, angry, and vibrant tectonics, or being subsumed by planet-wide forces of gravity. The result? Until recently, we have had no record in the geologic sense of the actual age of our crust, the first solid, cool surface upon which life could get its start. Scientists had believed that there was nothing left of that first crust to be studied; or if there was, the fragments would not exist in a size and quantity to be dated. So it stood. The Earth had cooled at some point – we just had no idea when.
Until now. In 2001, geologists in Australia discovered and studied several tiny pieces of zircon crystal found buried within sandstone that once was one of Earth’s earliest beaches…and dated them to 4.4 billion years old. This date appears to indicate that the Earth was beginning to harden for the first time millions if not billions of years earlier than previously thought; just a bare few million years after the planet’s initial formation from the whirling, gravity- and friction-harried clouds of dust that made up our solar system. The incredible dating of these tiny scraps of Australian zircon bear silent witness to the state of our Earth in its earliest epoch of geologic time.
Here, along the small range of former coastline now called Jack Hills in the west of Australia, the unexpected has occurred; the highly unlikely proof of the age of Earth’s first solid crust. Pounded to bits by meteor strikes, sucked beneath the thin layer of which they were a part, and melted into magma, some bits of early Earth yet managed to survive and retain their crystalline structure long enough to keep a record of the formation of our earliest, three-billion-year-old sea. These bits of zircon bear witness to the beginnings of the conditions necessary for the fostering of the first life we know of in our universe.
Zircon is exactly the crystal one would expect to have survived this long. It is extremely hardy, and can be torn apart, pounded, squeezed, melted, incorporated into other structures, and generally abused for millennia of geologic time and still retain their original chemical structure. Better yet, zircon retains radioactive isotopes like uranium, which is excellent for accurate dating purposes since it decomposes to an end product which we call lead in an observable and organized fashion.
These ancient zircon crystals, barely large enough to be seen with the naked eye and ranging in color from transparent to blue to red, are scattered amidst the deposits of ancient sandstone in Jack Hills like a gift from a time so distant as to be unimaginable to us and a gift it is indeed.
As pointed out by geochemist John Valley of the University of Wisconsin-Madison who tested the crystals, science is incredulous and requires a great deal of evidence. The scientific method functions by proving things wrong. If enough attempts to prove a hypothesis wrong are to no avail, a premise is considered true until and if it is later disproven. Undaunted by the questions and doubts of colleagues, Valley and the other geologists in charge of the find set out to debunk the doubters. If they could not disprove their own theory, then it would stand any and all tests done by the skeptics.
The reservations of the doubters were well-considered ones. The uranium and subsequent lead atoms within the crystals could, when subject to heat and pressures, migrate over those billions of years, until they spread out over the entire structure or concentrated entirely in the tested patches, contributing to a false date. After all, with crystals that are almost microscopic in size, it is not as if there is enough rock there to do dozens of slices for testing.
In order to uphold their theory that these crystals really were a part of a first, 4.4-billion-year-old crust, the international group of researchers who found them came up with an ingenuous new testing method called atom-probe-tomography to disprove the a misaligned dating. They invented a powerful new instrument designed to x-ray across the entire grain of each crystal, pluck out, and test those uranium and lead particles one atom at a time.
The result? As published in Natural Geoscience, the data proved that lead atoms migrated in only one period, approximately 3.4 -billion-years-ago when superheated in the thermal environment of early Earth, but as they had to exist with their uranium ions intact before this event to have been subjected to this thermal event, the results proved rather than disproved their dating. According to Sam Bowring of MIT and Jim Mattison of UC Santa Barbara, there is no further skepticism that this method has yielded accurate results which have ended all significant doubt in the scientific community that the crystals’ age can now stand as previously established. They are the oldest pieces of the earth still extant, and dating from a time previously thought to be before the Earth had even cooled enough to bear a crust.
What does this mean to all of us?
If the Earth had cooled enough to produce a crust capable of supporting liquid water a mere 160 million years after it had been just another ball of hot liquid rock in a newly-formed solar system, then that pushes back the dating for when life could have begun here. The timeline established by these data support the theory of an early cool earth with a hydrosphere-a system capable of sustaining liquid water and the beginnings of an atmosphere-before the previous estimate of 4.3 billion years ago, and far before the first known samples of life found in stones near Pilbara, Australia that date back 3.5 billion years.
Further, as these crystals are likely to have formed due to the cooling effect of liquid water over the still-warm new crust, that means that geologically-heated young oceans would have been capable of supporting evolving life; a veritable bathtub for the bacteria that would become our ancestors.
At this point the only thing that could possibly be better would be to find a bigger, or possibly older, rock. Bigger crystals would give us more information about the mineral make-up of the Earth at that time. Older ones would blow the lid off of even this startling date for our first crust. There are some candidates for bigger hiding out near Hudson Bay in North America, though their dating is hotly contested. Some scientists think it unlikely that larger stones from this era will have survived the battering taken by the early crust, as stones with greater mass might have sunk back into the magma beneath while the smaller shards like those from Jack Hills could survive as dust or float above the tumult.
For now, the little scraps of crystals washed ashore on an ancient coastline in Australia beat out all comers by a handy 400 million years, which means that the surface of the Earth we live on, and possibly the beginnings of life, were jump-started in conditions we cannot even imagine, in a time we would have never considered before now. What is left, these microscopic shreds of zircon from a prehistoric Australian beach, have witnessed an untold epoch of the earliest Earth.
By Kat Turner