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Blasts from the Past: Ontario’s Volcanic History

Wednesday March 19, 2014
The geological history of Ontario is complex. This image shows the simplified rock geology of Ontario. The pink colour represents granite formed by melting of material within the Earth. The green-coloured rocks formed from ancient volcanoes and are called greenstone belts. The pink and green rocks formed 2.5 to three billion years ago and together form the old Precambrian Shield rocks. Most of the metals that have been mined in Ontario come from the greenstone belts. The yellow coloured area represents sedimentary rocks that formed by erosion of the Precambrian Shield rocks. Geologists estimate that the yellow-coloured area formed 2.4 to 1.1 billion years ago. The brown colour represents different rock types that formed by melting of the Earth, by volcanoes, by erosion, and that were all shuffled on top of each other about one billion years ago when two great land masses ran into each other. The blue-grey coloured rocks are made of limestone, dolomite, and shale, which formed 500 to 100 million years ago. Many of these young rocks formed in a warm ocean, close to the equator.

There have been recent reports about volcanoes around the world that are spewing fiery rock into the sky.

These reports have caused some people to ask me if there are any volcanoes in Ontario. The answer is: “Yes… and No”.

The geology of Ontario is complex. It is that complex geological history that holds the hidden record of ancient volcanoes that used to dot the land we now call Ontario.

If you walked from Kingston, to Sault Ste. Marie, to Thunder Bay, to Kenora, up to Sachigo Lake, over to Webequie, down to Longlac, over to the Detour area by the border with Quebec, and back to Kingston, you would see many things, but you would not see tall volcanic cones or active smoking volcanoes. Instead, you would see a flat-to-hilly landscape and rocks of complex colours.

Such a journey would take you around the outer boundary (more-or-less) of an unusual area of rock called the Precambrian Shield. The rocks of the Precambrian Shield are old. They range in age from one to three billion years old. These ancient rocks are important to Ontario because virtually all of the metals that are mined here come from them.

The Precambrian Shield is made of rocks called granites, which are often white and pink in colour. Granites formed from melted rock created deep in the Earth.

The melted rock rose towards the surface of the Earth as a liquid blob, but that liquid cooled slowly and froze before it reached the surface.

There is another rock in the Precambrian Shield that is green or white in colour called volcanic rock. As opposed to granite, the volcanic rock formed from melted rock created deep in the Earth that spilled on and above the ground, where it cooled and froze quickly.

This is where volcanoes formed.

Different types of volcanoes were created from different types of melted rock, but most of the volcanoes formed on the bottom of ancient oceans — oceans that covered the surface of the Earth one to three billion years ago. Most of this volcanic rock is called basalt.

Today, when you look at the old basalt volcanic rock, you see it has a green colour.
Geologists call these areas greenstone belts.

These ancient greenstone belts are the rocks that contain most of the metals, such as copper, nickel, gold and chromite that have been discovered in Ontario.

Greenstone belts are the remains of ancient volcanoes that dotted the Earth so long ago.
Since that time, powerful geological forces wore down the tall volcanoes, stacked slabs of the Earth on top of each other, and buried them up to 50 kilometers below the surface of the Earth where they were squeezed and cooked. Over time, the remains of those ancient volcanoes eventually reappeared on the surface of the Earth exposed by erosion caused by rain, wind, freeze-thaw, and glaciers.

So, Ontario does have the remains of volcanoes, but the geological processes that created them have been silent for the last billion years.

And these processes will not start up again in our lifetime...or a million lifetimes, so do not feel the need to cancel your spring hunt, or move your hunt camp if it is located on greenstone belt rocks in fear of a new volcanic eruption.

Perhaps after several hundred million lifetimes, new volcanoes may once again emerge to dot the landscape of Ontario, but for now, the Ontario beneath our feet remains quiet.

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