The Bones of the Earth

Rivière-du-Loup, Québec: August, 2018

This summer, while walking on the shore at Rivière-du-Loup, I was struck by the way in which the spectacular steeply-dipping sedimentary beds “dive” and become submerged beneath the scarp on one side and the water on the other, disappearing as they are covered by overburden or waves. This got me thinking about human perception, and our general lack of appreciation for the hidden geology beneath us.

La Pointe outcrop at Rivière-du-Loup

Today, more than half the people on this planet live in urban areas. For those of us in cities, the Earth is often just that: earth. Many of our biggest cities have been built in relatively flat, low-lying areas, and if we happen to notice a natural substrate below the parking lots, asphalt, buildings, and lawns, it is likely that we will only see soil, clay, or gravel.

Most of the time, most of us never think about what might be beneath that compacted, ashy, polluted soil. Do many city people maybe assume that the soil just “goes down all the way”? Is the Earth flat, or does it sit on the back of a giant turtle?

The faulted succession at Rivière-du-Loup represents deep-water sediments.

I live in Winnipeg, and in Winnipeg the land surface is clay: clay soil, Lake Agassiz clay deposited in an ancient glacial lake, river clay laid down as the Red River migrated across its floodplain. It is literally clay everywhere, and when I talk to people about what is beneath their feet, they are generally surprised to learn that there is anything other than clay in the immediate vicinity. But sure enough, if you go down 15 or 20 metres, through the soil, the Lake Agassiz clay, and the boulder clay below that, you will reach the weathered upper surface of Ordovician dolostone laid down in a tropical sea hundreds of millions of years ago. And if you continue down through the bedded dolostones and limestones, more than 200 metres deeper you will hit granites and gneisses. And those Precambrian rocks go down a LONG way.

Most of us are very good at ignoring any aspect of the world that doesn’t concern us immediately, but if you travel with an inquiring mind you can find clues in so many places about what lies beneath. If you go to the mountains or to a rocky seacoast, you are bound to see many cliffs made of solid bedrock. Or perhaps you live where you can walk up a nearby hill and see a spine of rock poking out in one or two places.

Spines of rock sticking out in one or two places: bedrock stands out on an island in the St. Lawrence near Rivière-du-Loup.

Do you assume that the solid bedrock only occurs in those places where you can see it poking out? On the contrary, the bedrock is like the bones beneath Earth’s skin, and you only see some of it because the skin is absent in those places. If you look at your hand, you can’t see the bones, the carpals, metacarpals, and phalanges. But you know that the bones are there. The bones of the Earth go “all the way down” – you might see them in just those few places, but they are always there beneath you – and the solid or somewhat solid rock goes right down through the crust and mantle until you reach Earth’s liquid outer core.

Earth’s bones are everywhere. Under the mountains, of course, are sediments folded and wrinkled like bedclothes, huge granite plutons, faulted gneisses, deep crust extending down tens of kilometres. That bedrock also continues under the prairies, with flat-lying or gently tilted limestones and shales. The continent’s crust continues out past the land’s edge along seacoasts and continental shelves, making its transition to thinner basaltic crust that underlies all the immense ocean basins.

Geologist and assistant examining outcrop at Rivière-du-Loup. (photo: Vicki Young)

Geologists go out and examine all those bits of Earth’s bones where they poke out, and it is (relatively) easy for them to figure out what is going on with the geology in those places. But it is important for us to understand the structure of the Earth everywhere, if we are going to locate drinking water, find oil and minerals, or keep people safe from natural hazards. This means that geologists also have to figure out what is below you in all the places where the bones are covered by skin. A century or more ago, we mostly knew what was going on below the surface if someone excavated a test pit to see if there was stone suitable for quarrying, or if an oil well or deep water well had been drilled. But then we realized that a section into the Earth could be pulled out almost intact, in the form of a drillcore. And that drillcore will tell us what is directly below, hundreds or thousands of metres straight down. When the geophysicists really got going, applying the study of earthquake waves, gravity, and many other phenomena to the understanding of the Earth’s structure, they were able figure out even more, far beyond the reach of direct methods.

In the past few decades, we have developed an understanding of the Earth’s structure that is much more complete than what was known when I started studying geology in the 1970s. This is important knowledge for everyone: whether we like it or not, we all depend on petroleum for our quality of life, we all need metals and other products made from minerals, it is essential to have consistent sources of water, and our cities and structures should be kept safe from natural hazards such as earthquakes and landslides. Our knowledge of the Earth’s structure takes many of the “unknowns” out of decision-making processes, as long as the decision-makers pay attention to it. In a democratic society, we are all decision-makers to at least some extent, so it is critical that we all become more aware of our hidden world. The bones of the Earth.

© Graham Young, 2019