Intro: Oh, Canada!
The drive from Montreal to Quebec City looked at first to be an unremarkable straight-line drive along the marching lines of pylons carrying hydroelectric power west and long rows of pig farms. The fertile farmland along the St. Lawrence was a perfectly uniform neon green carpet only interrupted by the farmhouses and squat concrete businesses lining the highway. Then, all of a sudden, Mont Saint-Hilaire popped up on the horizon. It looked ridiculously random, like a gargantuan pine-covered bear had curled up to rest for a moment while ambling from the Adirondacks to the Laurentians. And it wasn’t alone! As we drove past Mont Saint-Hilaire we saw another huge hill towering over the flat agricultural landscape far in the distance- Mont Rougemont.
These isolated hills have the fun-to-say moniker of “monadnocks”, named after the Abenaki name of the type example in New Hampshire.
Mont Saint-Hilaire, Mont-Royal in the center of Montreal, and their fellow Monteregian Hills in Quebec are actually parts of a much larger family of igneous intrusions stretching from Triassic kimberlites in Nunavut to the Miocene Great Meteor Seamount in the Northern Atlantic- all children of the Great Meteor Hot Spot. David W. Eaton & Andrew Frederiksen reconstructed a map of its path in a 2007 article published in Nature. I’ve shown a figure from it below to show the extent of the extended family of plutons, which are igneous intrusions that formed as buoyant magma rose through cooler overlying layers of rock.
It’s a diverse family. Pangaea started to break apart 200 million years ago, and the earliest of the plutons formed approximately 178 to 147 million years ago as the magma rising off of the hot spot created the Churchill Kimberlite deposits in Nunavut and Ontario. Unlike the famous diamond-bearing kimberlites of Africa or even those further west in Canada, these kimberlite tubes ferried up spinels and olivine from deep within the mantle to the surface. Little topographic relief remains from these intrusions, as they are located under what was ground zero of the Laurentian ice sheet during the Pleistocene.
As the North American continent floated northwest during the breakup of Pangaea, the stationary hot-spot within the mantle created a south-east-trending series of volcanic necks punching up through the overlying sedimentary rock. Next up were the Monteregian hills around 129 million years ago. These hills are mainly composed of syenite, which is an alkali-rich magma with a composition similar to granite but impoverished in quartz. (citation)The cool thing about a hot spot is that the magma that forms the intrusions is influenced by the diversity of rocks that it melts through. Mont Saint-Hilaire is a mineralogist’s dream – over 277 varieties of minerals have been cracked out of the geodes of Mont-Saint-Hilaire, many new to science!
The sturdy igneous rocks of these monadnocks are much more resistant to weathering than the surrounding sedimentary rocks and so stand proudly above the surrounding area that has been washed away by rivers, cracked by frosts, and bulldozed by ice sheets.
As the steady expansion from the mid-Atlantic ridge North America northwest away from Europe and Africa, the hotspot came to rest under New England and form the White Mountains of New Hampshire that give monadnocks their name around 100 million years ago. After that, the hot spot ran out of North America above sea level and started to create the New England seamounts between 80 and 100 million years ago. All was quiet for a few million years while the Great Meteor hot spot took a well-deserved break from its creative streak, ducked under the mid-Atlantic ridge, and then belched out the Seewarte Seamounts on the African plate around 15 million years ago.
Despite the diversity of forms of the intrusions and the changing composition of the magmas depending on the overlying rocks within the Great Meteor Hotspot Track, studies such as Zurevinski, Heaman, and Creaser 2011 showed that the isotopic composition of strontium and neodymium in the rocks are similar enough for them to all to be driven by the same hot spot deep beneath the crust.
Hot spots are tricky beasts to study, especially when their handiwork lies too deep beneath the sea to get a rock sample to analyze for mineral similarities to suspected igneous relatives. The Hawaiian hot spot steals the spotlight, of course, since everybody loves a good explosion from a distance. There’s no proof that the Monteregian hills ever went ballistic like that, as the remaining rocks look more like volcanic necks that cooled calmly far beneath the surface. On the other hand, this slow geologic process gave us all the explosion of colorful minerals found in Mont Saint-Hilaire!
Twenty minutes passed by, and Mont Saint-Hilaire and its comrades faded into the rear-view mirror, and we reached Drummondville.From there to Quebec City the pig farms were once more the highlights out the window, with the exception of the melodramatic billboards admonishing us all to avoid the perils of texting while driving. Very Quebecois.
Up next: from the Great Meteor Hot Spot Track to the actual meteor that hit the Charlevoix! Oh, Canada! Part 2: Impact