We've avoided too much geology ... until now. With apologies to all who find rocks boring, this story will "blow your mind." First, let's find a present-day "model" of the geology we want to describe. Then we will apply it to what we observe in the field. You can judge whether it makes sense or not. Brio thinks it's pretty cool, as do we.
We are grateful to the book we have cited before, "Four Billion Years and Counting: Canada's Geological Heritage" for the interpretation of the Canadian geology.
Let's start with Mount St. Bruno, which we have noted in several earlier Montreal posts. It stands out on the horizon as you can see here. It is less than a thousand meters high, but it is noteworthy.
It is not alone. There are several other hills of similar stature in southern Quebec, including Oka, Mont Royal (which we climbed yesterday), Mont Saint-Hilaire, Mont Rougemont, all the way east to Mont Megantic, east of Sherbrooke, QC. What's involved with this "suite" of hills?
To find an applicable "model" of similar "chains" of hills, but on a much grander scale, we can go to the Hawaiian Island chain. In fact, the Hawaiian Island chain has a bend in it that tells a fascinating story associated with continental drift and a change in direction of the plate that carries this chain.
Looking underwater, you can see just how large the volcanoes are that form the base of these islands and, if they remain underwater, these seamounts.
In fact, Mount Everest, as every kid knows, is the largest mountain on Earth. But it isn't! Hawaii is if you measure it from the base where it sits on the seafloor to its top.
Why does this chain exist?
Imagine that there is a volcanic "hotspot" under the sea floor. Today, that hot spot is building Hawaii, just as it built Maui, Oahu, Kauai and Kaula and Nihoa. As the Pacific plate moves to the northwest, the existing mountain being built moves, too. Kauai used to be where Hawaii is now. In the future, there will be a new Hawaii and Hawaii will move to the northwest, carried by the plate's movement.
Go back several million years and do age dating on the seamounts at the "bend" in the first picture. You will date the time when the plate stopped moving northward to build the Emperor Chain as it moved over this hot spot and started building the Hawaiian Chain.
That's a model for today. Now, let's go back to Quebec and the northeastern North America.
This map comes from Four Billion Years and Counting: Canada's Geological Heritage.
You can see the line of hills with #1, Oka, being the westernmost hill. Now, imagine that after the mountain-building events of the Appalachian Orogeny and earlier events, all in the Paleozoic, the tectonic plates drifted over a volcanic hot spot under what is now the Atlantic Ocean. The oldest emplacement was Oka. These volcanic emplacements became younger as we move to the east, with Mont Megantic being the youngest, as we see with Hawaii today.
These mountains are collectively called the Monteregian Hills. They stand out, not for agricultural purposes, but as examples of old volcanoes drifting over an ancient hot spot. Because of the source rock for the volcanoes, there is an entirely different mineral suite associated with these "Hills," often making them prime collection points for crystals and mineralogy.
The book doesn't leave this suite of Hills there! No. It's appropriate to step back and look further to the east.
In northern New England, there are remnants of volcanoes in Vermont (Ascutney), New Hampshire (Monadnock, Chicora, Ossipee) and Maine (Agamenticus and others). Like the Hawaiian Chain, these volcanoes were active and tall. Ossipee is estimated to be over 10,000 feet tall when active.
Of course, the story doesn't end with what we find on land. There is an obvious string of dormant volcanoes stretching east from the New England Coast, known as the New England Seamounts. Some of the Seamounts are as young as 80 million years! You can now imagine how, as the Atlantic Ocean continues to open and the North American Plate continues to move west, that the Seamounts get younger and younger and they all started with Oka and Mont Royal in Quebec during the early Cretaceous, around 140 million years ago. (The dinosaurs went extinct at the end of the Cretaceous.)
Brio thinks this is all pretty cool. She hopes you do, too.
Cheers,
Brio
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