25: The Oldest Rock on Earth

If you’re just tuning in, you might be surprised that it’s taken 25 episodes to reach Earth’s oldest rock- you’d expect them to appear much earlier in the podcast. If you’ve been around this whole time, you’ll know there are a few reasons for this. First: the oldest rocks on Earth were not the first rocks on Earth. The first 600 million years of our planet’s history have been erased from its’ memory. To be fair, you probably don’t remember when you were a baby, either. Any rock laid down in Earth’s earliest days has been ground to sand or melted into magma: fuel for younger, fresher rocks.

In a few lucky cases, a tough crystal from those early rocks can survive the journey. The best examples are zircon crystals from the Jack Hills of Western Australia, though others exist across the world. Geologists have pored over every conceivable aspect of these crystals to tease out Earth’s first days. We can also look at ancient rocks from other worlds- meteorites and moon rocks that haven’t been ravaged by time. Together, these morsels gave us a lot of info about the Hadean world from Season 1, but there’s only so much they can tell us.

The other reason it’s taken this long is that these stories require a bit of background. For example, to tell the tale of life’s origins, you need to know what an organic molecule is, what a protein is, what RNA is, what a cell is, and how all these pieces were formed. To me, it’s far more educational and far more fun to work from the ground up instead of assuming you know everything. We also get to meet scientists asking the same questions you are right now, questions like: what is the oldest rock on Earth?

On that note, let’s return to today’s episode. We’re currently sitting 4 billion years ago, February 15th on the Earth Calendar. As we learned last episode, 4 billion marks the beginning of a new chapter: Season 2, the Eoarchean, a chapter defined by Earth’s oldest rocks. These rocks will tell us more detailed stories of the early Earth and give us a chance to stretch our legs a bit. Last season, we were focused on the Jack Hills of Western Australia. In Season 2, we will travel from pole to pole in our search for ancient rocks. Today, we’ll learn where the oldest rock was found, what type of rock it is, and exactly how old it is. Without further ado, let’s break out our hammers and do some digging.

Part 1: The Surveyors

Canada is the world’s second-largest country, with 9 million km2 of land. That’s a lot of rock to research, and it’s taken a long time to catalog. Last episode, we met Sir William Logan, who founded the Geological Survey of Canada in 1842. The survey is still around today and has many functions- to protect Canada’s environment, to search for economic resources, and to protect citizens from natural hazards. Many surveyors walk the land, making measurements and collecting field samples. In short, the Geological Survey’s job is to survey geology- it’s right there in the name.

One goal of a field survey is simply to learn what’s below our feet- what rocks are there, and how old they are. Despite centuries of advances such as GPS, drone technology, and good hiking gear, Logan would still recognize today’s field techniques. Scientists hammer at rocks for an ID, sketching mountains and making beautiful geologic maps.

After Logan’s time, Canadian geologists headed north and west, slowly mapping the country over several generations. Our story begins in the 1980s, 140 years after the survey was founded, and thousands of miles from Logan’s stomping grounds.

If you thought Western Australia was remote, it has nothing on northern Canada. Only 200,000 people live in Canada’s northern half. For reference, imagine if half of the United States or Europe only had the population of a small city. In the middle of this region sits the Northwest Territories- a land of vast pine forests, windswept tundras, and many, many lakes.

In 1981, the Geological Survey sent researchers to the heart of the NWT. Their goal was to map an area more than 100,000 km2, as large as Connecticut or Jamaica. The nearest town was dozens of miles away. Several First Nations live in the area, most belonging within the broader Dene community. Mapping this expanse took nearly a dozen researchers and seven summers to complete. Most of the mapping was done by just three people: Janet King, Andre Lalonde and Marc St-Onge. Together, these researchers would hike mile after mile through the Northwoods, identifying any rocks they found and putting them on the map.

This sort of work has its’ ups and downs- I can tell you from personal experience. I haven’t worked in the NWT, but I have done similar exploration in remote northern Ontario, being flown into the woods by bushplanes far from the nearest settlement. The thing about forests is there’s not a lot of rock to observe- all those pesky plants get in the way. If you’re lucky, you can simply pick up a piece of moss and roll it back like a blanket, but most times, the roots and soil are too thick to penetrate. Instead, you’re left with isolated windows of stone peering up at you through a sea of green and brown. You can hike for miles without seeing any rocks at all. Lakes and rivers are your friends- stony shorelines provide more extensive outcrops to study, and a place to take a dip when you get too hot.

At the end of a sweaty, mosquito-filled day, it might seem disheartening to put a few isolated dots on a huge map. However, these points turn into a sort of game- Battleship or connect-the-dots, trying to solve what story these rocks are telling us. If it was easy, everybody would be doing it. To me and other geologists, the rewards are worth it- the excitement when the story starts to make sense, a quiet lunch on a canoe in the middle of a huge lake, the cry of the loons under the Milky Way. It can be a tough job, but it’s one of my favorite parts of geology.

Year by year, piece by piece, the Geological Survey connected the dots across the NWT and pieced together a much larger picture. So what did they find, and how old were these rocks?

Most rocks in the NWT are pretty old- the late Archean, around half the age of the Earth. The geologists found ancient lava flows, beaches, and even fossilized bacteria, stuff we’ll see in Season 5. But in a few spots, much older, gnarlier rocks peeked up from below. These rocks had clearly been squeezed and tortured into completely different shapes. Before these rocks could be dated, the geologists had to figure out what they were originally, and how they had changed over time. Let’s join them and put our noses to the grindstone.

Part 2: Under Pressure

Way back in Episode 2, I introduced three recipes to make a rock. Here’s a quick summary. One recipe starts with lava or magma and cools it down into a solid mass. As you can imagine, these rocks form around or under volcanoes- they’re called igneous rocks, from the same word as ignite or “to set on fire”. The best igneous examples are granite and basalt.

Recipe two starts with smaller pieces in water: pebbles, sand, mud, or crystals. As these smaller pieces are buried, they’re squeezed and cemented together into a solid rock. Such rocks usually form in oceans, lakes, and rivers- they’re called sedimentary rocks, since they start with loose sediment. Sandstone and limestone are great examples.

Finally, you can take one rock and pressure-cook it into a completely new rock, with different minerals and textures. Such rocks usually form deep underground, squeezed by mountain-building or cooked by nearby magma. They’re called metamorphic rocks, just like a caterpillar morphs into a butterfly. You don’t hear about metamorphic rocks every day, but we’ll fix that soon enough.

All Eoarchean rocks in Season 2 are metamorphic. As you can imagine, the older a rock, the more likely it will be squeezed or cooked into something else. But some rocks are more cooked than others- it all depends on the original rock, how hard, how hot, and how long it was cooked. Earth’s oldest rocks are at the extreme end of this metamorphic pressure-cooker. When rocks are buried more than a few kilometers deep, pressures become greater than the deepest ocean, and hotter than a tandoori oven. If we get much warmer, rocks will melt back into magma, but that’s not our destination today. Rocks that are left down here for millions of years, just on the edge of destruction, begin to warp and twist into completely new shapes. To see how, let’s put a granite down here and see what happens.

A piece of granite has crystals of white, black, and pink randomly scattered around- check out your kitchen countertop or walk around your local city to find a good example. If we place this speckled granite deep below the surface, several things start to happen.

If pressure is applied in a specific direction, minerals also begin to rearrange themselves. Some crystals are stronger and hold under pressure, others are more pliable and contort themselves to fit in. Lighter and darker minerals will separate into distinct layers as they’re squeezed apart.

If we bring this rock back to the surface, it no longer looks like a granite anymore. Instead of a random speckled pattern of pink, white, and black crystals, those minerals have now been contorted into dark and light stripes like a zebra or a tiger. The rock is so different that we must give it a new name: gneiss (nice), that’s g-n-e-i-s-s, from an old German word meaning “spark”.

The first and last time we met gneiss was in Episode 2. There, I mentioned that the oldest rock on Earth was a gneiss, which brings us back to Canada.

As the surveyors trekked across the NWT in the 1980s, they found a wide stretch of striped gneiss the size of London or Los Angeles. This gneiss was centered on the Acasta River, so they called it the Acasta Gneiss Complex. Along with her survey colleagues, Janet King is credited with finding and describing the Acasta Gneiss Complex, earning her PhD from Queen’s University in Ottawa. Most folks refer to these rocks simply as the Acasta Gneiss, but that’s not exactly accurate- the word Complex is there for a reason. There are actually three different flavors of gneiss in the Acasta area.

Each of the three Acasta gneisses tells a different story about a different Eoarchean rock. We don’t have time to get into those stories and especially not their names, but here’s a primer. The three gneisses in the Acasta Complex are: a dark gneiss, a pale gneiss, and a blend of the two. All three were once igneous rocks, forming in magma chambers deep underground. We’ll learn more later, but for now, keep the idea of a molten hot pocket in your back pocket.

To finish out our story, let’s return to the northwoods and learn just how old the oldest rocks are.

Part 3: The SHRIMP Strikes Back

As Janet King and the other surveyors first walked across these different gneisses, they didn’t know they were walking on the oldest rocks on Earth. They just knew these rocks were older than the other rocks they saw, and worth asking their age. To do so, they needed to search for zircon crystals. We’ve talked about zircons and their use as geologic timepieces twice on the show: in Episode 3, the Dating Game, and in Episode 10, the Oldest Thing on Earth. For a more detailed explanation of how we date ancient rocks, I invite you to check those two episodes out.

Otherwise, here’s a one-minute summary: zircons are tough crystals that form in molten magma chambers underground, just like the parents of the Acasta Gneisses. As zircons crystallize, they trap uranium atoms, which slowly decay over time. The less uranium in a zircon, the older the crystal is… usually. Zircons can develop cracks and extra layers which complicate this story. To get the best measurements, you need the best equipment.

Phew. All right, with that recap, let’s return to the Acasta Gneiss.

One of the later surveyors, Samuel Bowring, was an expert in dating zircons. When the project began in 1981, Bowring was a PhD student in Kansas; a decade later, he would become a geology professor at MIT in Massachusetts. In 1989, Bowring, Janet King, and their team published an article in the journal Nature, the highest place a geologist can go. Inside one of the pale Acasta gneisses, the team found zircons at least 3.84 billion years old, sometime before February 28th on the Earth Calendar. But they didn’t know exactly how much older it was. For that information, they needed the best equipment possible. They needed a SHRIMP.

In Episode 10, we met a team of Australian researchers led by Bill Compston. Their goal was to hunt down ancient zircons using a new machine nicknamed the SHRIMP, which used tiny lasers to gather samples. Three years before Bowring’s paper, the SHRIMP had already made waves by discovering the oldest crystals on Earth- the Jack Hills Zircons in Australia, more than 4 billion years old. Note- the Jack Hills has the oldest crystals, but not the oldest rocks. There’s a big difference between zircons in the Jack Hills and the Acasta Gneiss, a difference Bowring and Compston were keenly aware of.

The Jack Hills Zircons, while incredibly important, have been displaced from their original homes. They first formed in magma chambers, molten pockets of rock deep underground which cooled into igneous rocks like. These rocks were eventually ground down into sand, with only the tough zircons surviving. In the Acasta Gneiss of Canada, however, that last step didn’t happen. Sure, the Acasta rocks were twisted and contorted into gneiss, but they were still solid rocks, larger pieces of the ancient world. In short, if you could give an age to an Acasta zircon, you could give a date to an entire rock, something you could grab with two hands.

Bowring teamed up with Bill Compston and Ian Williams in Canberra, feeding Acasta zircons to the hungry SHRIMP. Just one month after his Nature paper, Bowring pushed the Acasta date back to 3.96 billion years old, February 18th instead of February 28th. A decade later in 1999, the same team would push even farther back to 4.03 billion years, February 12th on the Earth Calendar. However, they were just beaten to the punch by, of all places, the Geological Survey of Canada. These researchers, Richard Stern and Wouter Bleeker, were not from the original 80’s field crew but had something the old crew didn’t: a brand-new Canadian SHRIMP machine. The different machines looked at different rocks, but found the same age, 4.03 billion years old.

In future episodes, we’ll learn that the Acasta Gneiss has a complicated, tortuous history, and that just one stone can have pieces with very different ages and stories. It’s not correct to call the whole thing 4 billion years old, but we can say that the oldest pieces are. The other pieces are stories for a different day.

Summary:

The oldest rocks on Earth are two samples of gneiss from Canada’s Northwest Territories, in the lands of the Dene people. Both samples are made from thin bands of black and white crystals, resembling a bar code at a grocery store, and are labeled BNB95-103, and SAB94-134. The rocks were first formed as magma cooled into granite-like rocks deep underground, which were then buried and pressure-cooked into these striped gneisses. The original magma chambers cooled 4.03 billion years ago, February 12th on the Earth Calendar.

Now that we know the age, location, and name of Earth’s oldest rocks, it’s time to learn more about their stories. Next episode, we’ll meet the different types of gneisses in the Acasta area, and learn how each might have formed.