Ever imagine what Ontario looked like billions of years ago? Geologist Henry Halls suggests you imagine taking a bus ride from Sault Ste Marie to Wawa in Northern Ontario: along blasted road cuts you see the pink granite rocks of the Canadian Shield interleaved with vertical black stripes. These stripes are called dykes, giant cracks that were filled with molten lava 2.4 billion years ago. Four hundred million years later, the compression of colliding continental plates caused massive earthquakes that split the Canadian Shield along a line from James Bay to Wawa, called the Kapuskasing Zone.
I recently met with Halls, a professor of geology at U of T’s Mississauga campus, as he explained his recently published work. Halls uses a technique called paleomagnetism to study dykes in the Canadian Shield and has discovered an interesting region where the Earth’s crust has literally risen from the underlying ground in Eastern Ontario.
Paleomagnetism studies fossil magnetism in rocks that form, for example, when lava cools. The direction of the magnetization is parallel to the Earth’s magnetic field at the time the rock cooled. Magnetism is carried in these rocks by the mineral magnetite, which acts like thousands of little magnets orienting themselves with the field, like a compass needle.
Here’s the fascinating bit: over the last several millions years, Earth’s magnetic field has reversed its polarity every 500,000 years on average, so south becomes north and vice versa.
In order to using paleomagnetism to study polarity reversals, Halls and his graduate students have taken over 2,500 core samples from more than 400 of the dykes in the Sudbury region.
This period of volcanic activity that produced those dykes lasted about 30 million years. During their formation, they probably resembled rising subterranean rivers of red-hot molten lava.
By analyzing the intersecting patterns of dykes and their magnetic polarity, Halls determined only one reversal of the Earth’s field took place over the entire 30 million years of dyke formation. This suggests that in the early history of our planet, magnetic reversals occurred less often than they do now.
Halls showed the Canadian Shield underwent a major period of deformation along the Kapuskasing Zone, in which the Western half of the shield has shifted towards the north and rotated counterclockwise with respect to the Eastern half, and that the whole shield, at least the part occupying northern Ontario, has been tilted a few degrees southward.
The counterclockwise rotation may reflect a splitting of the Canadian Shield, a weakness that has been exploited ever since and on which Hudson Bay has formed.
All this deformation occurred two billion years ago. Today, it is remarkable how geologists use magnetism to unearth our land’s past.
It is humbling to realize the age—so much older than our earliest history—of the land we walk on today.
