Rocks and Trees

Rocks and Trees and waterfalls in the Canadian Shield

Often when we tell people that we are headed to the east people ask, “Why would you do that?  All you will see are rocks and trees and more rocks and more tree.” Then often, they add, “Boring!”  I would say if you think Northwestern Ontario is boring that you are probably boring.

Shortly after heading east down the Trans-Canada Highway we saw evidence of something that intrigues me.  That is the Canadian Shield. I kept thinking about this as I travelled east.  I love to see it, but my viewing pleasure is enhanced by what I have learned about this astonishing Canadian Shield.

We experienced snow on day 1 and 2 of our trip to the east

When I was at the Grand Canyon we were amazed that the oldest rocks at the bottom of the canyon  were 1.7 billion years old. That seemed like a lot, but those rocks are youngsters compared to the rocks in the Canadian Shield!

The shield contains some of the oldest rocks on the planet.  Many are 2 billion years old. Some are nearly 4 billion years old! The Shield goes well beyond Canada’s borders. Nearly 2/3 of North America is part of the shield.  It stretches from the Arctic to Mexico.

To geologists rocks seem to be alive since they tell us so much about where we and our world came from.  The earliest mountains on our planet are about 3.9 billion years old. The continents of our planet  have always been migratory. They travel like rafts on the surface of the earth. This is part of the system of tectonic plates. When continental plates collide parts of one plate can be added to the other. This is referred to as continental accretion. The plate tectonic process began soon after the earth was created. The Shield was assembled over billions of years. Since then many successive mountain ranges have been created and worn down over deep time. As a result geologists often look for ghosts of ancient mountain ranges. The Shield is one of these ghosts–the worn down stumps of very old mountains.

Colliding continents created immense volcanic activity which in turn created many rocks including  gneiss.  Gneiss is a highly metamorphosed rock that is composed of distinct bands of alternating pinkish granitic rock and darker more iron rich rock. It is very common in Canada. This rock shows the intense deformation of rock that occurred at great depths under the surface of the planet when crustal rocks collided to create the crustal mosaic that constitutes the oldest part of our country.  Where it is now exposed at the surface over large parts of the country these rocks are evidence of deep erosion of mountains and the removal of vast volumes of rock over the millions of years that followed these collisions creating a relatively flat Shield surface with which we are familiar.  The rocks we see are often the very deep roots of what were at one time very high mountains.  Contemplating such immense erosion gives one a profound sense of the power of time.

Gneiss is produced when 2 plates squeeze against each other and the igneous rocks are heated to extreme temperatures as a result of the friction and the pressures are enough to create mountains. The rock at that point is as soft as toothpaste.  The heat is so intense and so extreme that rocks are dehydrated  and produce water streaming from a burning rock.  The rock that is forced out is mineralized water.

Even though the Canadian Shield contains some of the oldest rocks on the planet, from time to time sojourner rocks have arrived from far off places. Some of these have even arrived from other worlds. For example, at Sudbury about 1.8 billion years ago a meteorite rocked the earth, digging deep beneath the surface at that time to create what we now call the Sudbury Basin.  The Basin is so deep it can be seen from space.

The force of that blow was awesome.  In fact it exceeded the force of all of our nuclear weapons combined!  The Sudbury Basin close to where we travelled is the second largest known impact crater on our planet. It is 62 km long and 30 km and 15 km deep.  To consider the force that created such an impact is a humbling task. Railway engineers who were constructing the Canadian Pacific Railway discovered it accidentally in 1885.   Another surprise meteoric visitor from outer space created the crater that later filled with rock. We now call that crater West Hawk Lake.

The last two million years in the Shield have produced great fluctuations in climate. From time to time, this generated massive continental ice sheets that came and went over the continent wearing down mountains, depositing rock materials, and retreating again, only to arise again later.  The changes created by these forces were also awesome.  These ice sheets were HUGE.  Imagine ice sheets 2 km. deep above where we were driving. It really was difficult to comprehend.

I love these little islands of rocks and trees

         My friends probably think I’m nuts, but I was thinking about such things as we drove through the Shield. It is much more than rocks and trees.

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