Making gneiss

We often hear about Pangea, but earth has seen at least 7 supercontinents, including Gondwana, Rodinia, Pannotia, and my favourite: Ur. And as they roamed, the initially baby-sized continents kept gathering sediments, popping up volcanos and growing bigger.

Australia smashes Canada, makes mountains

Then, around 1.2 billion years ago (the Proterozoic eon for the keeners), teenaged North America collided with teenaged Australia as the supercontinent Rodinia was forming. You read that right: Australia was east of North America at the time. In that slow-motion smashup, the sedimentary rock layers and old volcanoes of North America’s east side got crunched and folded under intense heat and pressure, creating metamorphic rock like marble. Also new magma (liquid rock) pushed up from the mantle into cracks to create igneous rock like granite. The mountains built here in that continental fender-bender got as high as today’s Himalayas.

Snowball earth

Then 1 billion years ago those mountains stopped rising and started to slowly wear down. “Shortly thereafter,” things got mega-cold for a while in the Cryogenian period (700-600 million years ago), causing the aptly named Snowball Earth. While not everyone agrees on the details, many scientists believe that 3-5 kilometres of rock were ground down by two 10-million-year periods of earth-wide scouring ice sheets that were like a giant sander someone forgot to unplug.

The Cambrian explosion

Incidentally, many scientists think that when that huge volume of ground-up mountain poured into the ocean, a bunch of calcium carbonate became available to the soft and squishy ocean creatures––the only life on earth at the time. And that building material gave them new ideas about spinning shells and carapaces and spines to protect themselves from the bigger squishy things, spurring the Cambrian explosion. Not an explosion like a meteor hitting earth––more an explosion of creative and crazy new creature-bodies. And that was the birth of complex life here on earth. (Ever notice how sentences get dramatic as soon as you add “here on earth”? Try it, everywhere.)

The Great Unconformity

So the billion-year-old roots of those ancient mountains in eastern North America were laid bare. Then about 600 million years ago the climate heated up and melted the snowball, and the oceans rose. And all that sea sediment? It started to settle and form new rock (more on that in Sea snails and sediments), directly on top of the billion-year-old rock. In between those two layers would have been almost 500 million years of rock record, but instead it’s simply missing. It would have been 3-5 kilometres thick, but instead it’s zero. That absence is called, kind of strangely, the Great Unconformity. Scientists first discovered the gap in the Grand Canyon, where up to 1.6 billion years is missing. But most believe the absence is present (!) all over (or should I say under) the earth.

Rare non-thing

In most places the Great Unconformity is buried deep, but here in the Capital Region we can see it at the surface at Lac Beauchamp. Head out to visit, but don’t be disappointed when you just see some rock sitting on top of some other rock. You’ll have to use your imagination to conjure the absence: 500 million years of hard rock-building work––gone!

Part of ye olde Canadian Shield

And those billion-year-old mountains? You can visit their ancient worn-down roots in the Gatineau Hills and in Carp/Kanata. You’ll see igneous rock like granite and metamorphic rock like marble (metamorphosed limestone), quartzite (metamorphosed quartz sandstone) and gneiss metamorphosed granite or sedimentary rock

Here they’re called the Grenville basement layers, but they’re part of the much huger Canadian Shield, which can also be much older in places. Like the Acasta Gneiss in the Northwest Territories: 4.3 billion years old and arguably the oldest rock on the earth’s surface.

Go sit on some old rocks

Back in Gatineau-Ottawa, those old Proterozoic rocks are as smoothly scoured as you might expect, and they can get lovely and warm in the sun. Go find them at the many viewpoints in the Gatineau Park: It’s not everyone who can say they’ve had their lunch––or even napped!––on a billion-year-old rock.