Mar
15
2013

The Acasta gneiss: Earth's oldest surface rock

Acasta Gneiss: at approximately 4.03 billion years-old, the tonalite gneiss is the oldest rock exposed on the surface of the planet.
Acasta Gneiss: at approximately 4.03 billion years-old, the tonalite gneiss is the oldest rock exposed on the surface of the planet.Courtesy Mark Ryan
I recently attended a geology seminar sponsored by the Geological Society of Minnesota. The event took place at Macalester College in St. Paul, and was led by Jeff Thole, laboratory supervisor and instructor in the college's Geology Department. Jeff is extremely knowledgeable and enthusiastic about geology, and in the course of cramming a semester's worth of geology into the two hour lab, he mentioned that he had in his office one of the oldest rocks in the world: a nice chunk of Acasta gneiss. After finishing his talk about the rock cycle, and as everyone began examining the variety of rock types spread out on lab tables in several rooms, Jeff brought out the chunk of ancient gneiss for everyone to see.

Found on an island in the extreme and very isolated northern regions of Canada's Northwest Territories, the Acasta gneiss has been radiometrically dated to be upwards to 4.03 billion years old! That's a number that's not very easy to comprehend. The Earth itself is estimated to be just a half-billion years older, so the Acasta gneiss (pronounced nice) is some of the very earliest crustal rock still existing on Earth's ever-changing surface. For a rock unit to withstand 4 billion years of the rock cycle - where the forces of erosion and plate tectonics are constantly at work wearing down, reworking and remelting rocks - that's quite a feat if you think about it.

To give you a better idea of the vast amount of time we're talking about here, let's first reduce it to a more comprehendible time-frame. If you were able to take a single photograph of the Earth each year for those 4 billion years (4,000,000,000 photos) and then made a time-lapse video of all those photos (at 30 frames/photos per second), and started watching the video today, it would take you more than 4 years of constant, around-the-clock viewing to watch it from start to finish. You'd still be watching it in 2017, when non-avian dinosaurs suddenly go extinct about three-and-a-half weeks before the end of the video. We modern humans wouldn't appear for the first time until sometime in the show's last couple hours.

Acasta gneiss outcrop location: The exposure is about 180 miles north of the town of Yellowknife in Canada's Northwest Territories.
Acasta gneiss outcrop location: The exposure is about 180 miles north of the town of Yellowknife in Canada's Northwest Territories.Courtesy D-Maps.com
But back to the rock itself. The ancient gneiss is named after the Acasta River, located east of Great Bear Lake, where the outcrop was first found in the 1980s. The exposure is about 300 kilometers (180 miles) from Yellowknife, so the only practical way to get there is by float plane.

Composed mostly of the minerals quartz and feldspar, the Acasta gneiss was formed during the Hadean, the earliest eon in Earth's history. Its composition leads geologists to surmise that it was probably formed from highly metamorphosed granite subjected to unimaginable heat and pressure. The exact origin of that granite is unknown, but its presence indicates continental crust (and surface water) were probably already present in those very ancient times.

AGE BEFORE BEAUTY

Morton Gneiss: Minnesota's younger but prettier "Rainbow Granite" (seen here in a quarry near Morton, MN) has been used to decorate buildings around the country. It's also a popular material for countertops and cemetery headstones. The banding and severe swirling evident in the rock indicates that, at some point, it went through a period or periods of high heat and enormous pressure.
Morton Gneiss: Minnesota's younger but prettier "Rainbow Granite" (seen here in a quarry near Morton, MN) has been used to decorate buildings around the country. It's also a popular material for countertops and cemetery headstones. The banding and severe swirling evident in the rock indicates that, at some point, it went through a period or periods of high heat and enormous pressure.Courtesy Mark Ryan
It may interest you to know that Minnesota has its own ancient gneisses exposed in outcrops in the Minnesota River Valley. The most well-known is the gneiss that's quarried around the town of Morton, Minnesota. At nearly 3.6 billion years old, Morton gneiss is not quite as ancient as the Acasta rock but what it lacks in age it makes up for in beauty. Known in the construction trade as Rainbow Granite, polished panels of the banded and severely swirled Archean-aged-aged migmatitic gneiss can be found decorating building facades throughout the country.

TECTONIC VS MARKET FORCES

An enterprising miner from Yellowknife has filed a claim on the Acasta gneiss site, and has been trying to market the ancient rock. This doesn't set well with many in the geological community, who think the rare outcrop should be preserved for scientific study. They also say the prospector could be misrepresenting the public since not all the rock in the exposure dates back to 4 billion years, and it's very expensive to validate the age of any one piece.

THE DATING GAME

So how exactly has the Acasta gneiss been dated so precisely? Zircon crystals found in the rock's mineral structure trap uranium in their lattices when they form and can act as timekeepers through measuring the decay of the uranium into lead. The half-life of uranium is a known number (4.47 billion years for U-238; 704 million years for U-235), so measuring the ratio between number of parent atoms (uranium) to the number of daughter atoms (lead) allows for a very precise estimation of age. But even zircon crystals aren't immune from 4 billion years of exposure to the elements. Things like naturally occurring radiation can damage or alter them and thus skew the measurements. But by using an instrument called the Sensitive High-Resolution Ion Microprobe (aka SHRIMP) researchers are able to focus a beam of oxygen ions on a tiny unaffected segment of the zircon' s surface, remove atoms from it, and then analyze their isotopic composition. The SHRIMP was developed at Australian National University.

Jeff Thole's sample was given to him by a geologist from the Geological Survey of Canada, which purchased a SHRIMP and used it to date the Acasta rocks. It should be noted that an older Canadian rock unit supposedly exists in the greenstone belt east of Hudson Bay, but there's still some contention regarding this, since the method of radiometric dating isn't the same that was used to date Acasta samples.

Whether the Acasta gneiss is the remaining crust of a protocontinent that existed when the Earth was still a relatively young, hot mass of accreted material remains a mystery at this point, but scientist named the time the Hadean for good reason: back then it must have been literally Hell on Earth.

LINKS
Field trip to Acasta Gneiss outcrop
Acasta Gneiss info by Carl Zimmer
Geologists balk at prospector's plan
More Morton gneiss info

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