The Oldest Stuff on Earth

The Cosmic Pathway (coolest name for an exhibit ever) at the American Museum of Natural History spirals down through thirteen billion years of universal happenings and drops its visitors off at the entrance to the Gottesman Hall of Planet Earth. The Hall of Planet Earth appeals enormously to my inner geology geek, with displays ranging from chunks of black smokers (hydrothermal vents on the ocean floor where sulfide deposits build up and where life, it is theorized, may have begun on Earth) to lava samples from around the world to parts of an ice core from Greenland that’s been crucial to our understanding of climate change. Thrilled as I am to get to see these things, I am always a little disappointed by the specimen that I think should be the coolest part of the hall: a zircon crystal, found in Australia in the 1990’s, that is one of the oldest known pieces of Earth, at 4.3 billion years.

Zircon is an impossibly resilient mineral that scientists love because it contains uranium, which is unstable and breaks down to lead over a known time period—thus, we can use radiometric dating to find out how old the zircon crystal is. In fact, the radioactive material inside zircon crystals sometimes destroys the mineral itself over millions of years by emitting radiation, so that the mineral's structure is destroyed and the crystal is discolored (picture: right). If you catch sight of one of these so-called ‘metamicts’ in the field, it looks just like a tiny radioactive bomb went off—a brownish dot surrounded by little fractures in the surrounding rock.

In the Hall of Planet Earth, however, the zircon crystal just looks like a little lump of rock, which for some reason disappoints me. I suppose it seems that a remnant of Earliest Earth should be more obviously imbued with some quality of specialness—some sort of iridescent, diamond-in-the-rough sparkle (perhaps the Hall of Planet Earth, while appealing to my inner geology geek, fails to appeal to my inner Lord of the Rings geek).

At any rate, the zircon is special because it is one of the oldest materials on Earth, having formed just a couple hundred million years after the planet itself (Earth’s estimated age is about 4.6 billion years). While the 4.3 billion year-old zircon was certainly a fun find for science, the mineral had out-survived the rest of the rock with which it formed—in other words, while the other minerals in the rock melted and/or deformed and became new rock again, the zircon mineral remained solid, eventually re-incorporating as part of a newer, younger rock.

What scientists have been searching for, then, is rock that is as old as the zircon crystals inside—rock that formed just as the Earth was settling out into layers and cooling to a temperature that allows rock to solidify (the geologic word for this earliest eon of Earth is the Hadean: Earth then was a fiery, violent mess replete with extraterrestrial bombardments and volcanic activity). Because Earth’s crust is so often created and destroyed in the generative and destructive processes of tectonics and erosion, really old rock is tough to find-- the oldest rock in New York City is about 1.3 billion years, and a good deal of the land below us is much younger.

Scientists working in northern Quebec on the Hudson Bay believe they may have found some of this very old rock—a little exposure of Earth’s proto-crust. The dated material in this rock (it’s an area composed mostly of a rock named faux-amphibole, whose chief mineral component is Cummingtonite….I remember the day in mineralogy class when a fairly humorless professor lectured on this very mineral-- suffice it to say that we geology students, ever fond of questionable-sounding jargon, had a tough time keeping our faces straight) is about 4.28 billion years old. If the rock itself is as old as some of the minerals inside, scientists have an opportunity to take a close look at some of the first crust that ever formed on Earth. In doing so, they can begin to answer questions about how crust formed back then and to compare Earth’s early geologic processes with the processes occurring today.

If the rock does turn out to be proto-crust from 4.28 billion years ago, I hope the Hall of Planet Earth gets a chunk of it-- even if it’s not much to look at.