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Where do you find old iconic rock stars? In Canada, of course!

Did you know that Canada is home to the oldest rock ever discovered? It’s the 4.03-billion-year-old Acasta gneiss. For most of us, four billion years is an unimaginable length of time. So how do we know this rock is in fact almost as old as Earth itself? Simply Science visits a highly specialized Geological Survey of Canada lab in Ottawa to meet a team of time-travelling rock star experts who figure this all out.

March 2023

The iconic Acasta gneiss, found in the Canadian Shield along the shores of the Acasta River in the Northwest Territories, is over four billion years old — making it the oldest known rock on Earth and the planet’s original rock star. To put that in context, the Earth is about 4.56 billion years old, so the Acasta gneiss was formed during its earliest days. And within its structure this ancient rock holds vital clues to the complex geological history of the planet.

Group of people sitting outside.

Left: Members of the GSC’s Geochronology Lab. Top right: a sample of the Acasta gneiss. Bottom right: A scientist performs precision work in the clean lab.

Despite its special status, to the untrained eye Acasta gneiss is a brownish grey, finely grained rock that looks, well, ordinary. However, researchers at the Geological Survey of Canada (GSC) see things differently. The exceptional age of Acasta gneiss is only revealed using special equipment designed for analyzing rocks at a granular level. And the GSC has long been known for its expertise in dating rocks. As far back as the 1960s, GSC scientists dated moon rocks collected during NASA’s Apollo 11 mission.

How do scientists date rocks?

The field of work determining the age of a rock is called geochronology, and the researchers who do it are geochronologists. Geochronology is a very accurate way to study a rock’s geological history. To date rocks, geochronologists study naturally occurring radioactive decay. Certain elements such as uranium or potassium transform — or decay — at predictable rates, over eons of time, into other, more stable elements such as lead or argon. Measuring tiny amounts of the original element and the product resulting from radioactive decay allows researchers to precisely calculate the age of a sample. Precise and accurate ages calibrate past geological events and answer important questions such as how an ore deposit forms and how it relates to other regions across the country and around the globe.

The thrill of discovery

A tiny mineral named zircon is found in many rocks, including the Acasta gneiss. It’s a fantastic time capsule that records and preserves a rock’s complex geological history. Zircon is essential to cracking the code of a rock’s age and the tiny zircon grains from ancient rocks are especially notable. They hold the only record we have of Earth’s early history and provide hints about the geological forces at work when the continents were created billions of years ago.

“It's kind of like CSI. At a crime scene, you might have a footprint and a broken window — you have pieces of what happened, but not everything is preserved,” says Dr. Bill Davis, head of the geochronology and isotope geochemistry team at Natural Resources Canada (NRCan). “The first question that always has to be asked in forensic work is, when did things happen and how fast did they occur? That's the element that we bring into the geological investigations that we’re involved in. We help put the pieces together in time, draw correlations and develop genetic process models for past geological events.”

The rock dating process: collect, crush and separate, select and analyze, individual grain analysis, and interpretation.

Rock dating, step by step

  1. Collect: The first step is for geochronologists and GSC geologists working in the field to collect rocks to test geological hypotheses.

  2. Crush and separate: The samples are brought to the lab, where they’re crushed and separated into grains so small, they resemble fine sand. Then, using water and gravity, the grains are separated by density. Zircon grains, the most valuable mineral for figuring out a rock’s age, are relatively dense so they sink.

  3. Select and analyze: The team selects a few grains of zircon for a closer look. Then, using a scanning electron microscope, geochronologists examine each tiny grain carefully for insights and then identify specific zones within each microscopic zircon crystal for further analysis.

  4. Individual grain analysis: Researchers analyze the smallest details in a grain using instruments developed specifically for such tasks. At this stage, they must work in special environmentally controlled labs, or clean labs, to determine such precise measurements.

  5. Geological age interpretation: The final step is to interpret the age of the rock by incorporating statistical analysis of the data as well as relevant geological factors.
People working in a lab.

Top photos: Analysis of the smallest rock grains requires special instruments. SHRIMP lab top left, images of zircon using a scanning electron microscope top right. Bottom photos: Precision work is done in the clean lab to ensure the integrity of all samples.

The team uses other equipment, such as the Sensitive High Resolution Ion Microprobe, or SHRIMP, which allows analysis at a scale many times smaller than the width of a human hair. This provides a more complete picture of the vast range of geological events that a single rock may have experienced over its lifetime.

“Some of the elements we analyze weigh as little as a few picograms,” says Dr. Davis. “To put that in context a picogram is one-trillionth of a gram, which is about the weight of the DNA in a single cell of a tiny hummingbird.”

The geochronology group also uses noble gas mass spectrometer, which measures minute quantities of argon in potassium-bearing minerals. This instrument helps unravel the timing of thermal events experienced by rocks on their route through the crust to the Earth’s surface.

Time travelling teamwork

The GSC’s geochronology lab plays a major role in unravelling four billion years of geological history that has shaped the continent — and the planet. Geochronologists help resolve complex questions and produce geological maps of regions across Canada through dating of rocks. While technical staff pay scrupulous attention to detail ensuring that no samples are contaminated in the process. They also happen to be experts at keeping sophisticated instrumentation running at the highest level.

And the team doesn’t just time travel to the past, their work also contributes to revealing Canada’s mineral and energy resource potential now and for the future.

For more information:

Age of the World's Oldest Rocks Refined Using Canada's SHRIMP: The Acasta Gneiss Complex, Northwest Territories, Canada Stern, R A; Bleeker, W; Geoscience Canada vol. 25, no. 1, 1998 p. 27-31 (GSC Cont.# 1997258) 

More about Acasta Gneiss

Historical background on the Geochronology Lab

The Sensitive High-Resolution Ion Microprobe (SHRIMP)

The Canadian Geochronology database 

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