In the first three parts of this series, we discussed the interesting geology of Minnekhada and area and how it likely came to be. We looked into recent history (geologically speaking) and a much more distant past. I teased the fact that the rocks that underly Minnekhada may not even be from here. Given their age (over 100 million years old), and what we know about plate tectonics that seems almost certain. There is however another, more recent innovation that supports this, and that is what is know as paleomagnetic analysis.
This is a deeply technical field of study, which I don’t pretend to fully understand, but here’s an attempt to explain it at a high level; Specialized geologists study the orientations of magnetic minerals in rock samples. The locked in magnetic axis of these minerals point towards the magnetic poles during the formation of the rock. This can tell us a lot about where on earth the cooling occurred, based on the calculated position of the magnetic north pole at the time. Many tests have been done on rocks in the Coast Mountains and the results shocked many in the field. It turns out the granite under our feet was likely formed off the coast of what is now Baja, Mexico. This “Baja BC” hypothesis is still controversial, and not all geologists agree, but it is kind of fun to think we can visit Mexico without even leaving home.
As mentioned previously, much of British Columbia is not part of the original North American landmass, or craton. It has been created by a series of collisions over tens of millions of years, a result of plate tectonics on a global level. Think of it like a very slow moving conveyor belt of land masses from various other parts of our planet, all eventually running into North America. The Coast Mountains are an important part of that story.
A generalized overview of the accreted terranes of B.C., Yukon, and Alaska. Minnekhada lies at the bottom extent of the red CPC(Coast Plutonic Complex) area. The purple WR (Wrangellia) terrane came later. Credit: opentextbc.ca
So now, as I complete this four part series, I’ll try to come back to Minnekhada and summarize the main parts of the story;
Mya = Millions of Years ago
- > 115 Mya: A volcanic island arc forms somewhere in the Pacific, well south of our current location. Like the Hawaiian island chain, much of the land mass is below sea level. Tectonic forces start to move the island chain towards what is now the Pacific Northwest.
- 115 to 70 Mya: The island chain accretes (joins) the North America craton, and becomes the Coast Belt, joining other terranes that have already docked against North America and before some still to come. Intrusive granitic rocks are thrust upwards, as the overlying cover rocks are slowly eroded, creating the Coast Mountains. In places, volcanism re-activates.
- 22 to 5 Mya: The early Fraser River carves the Fraser Canyon.
- > 2 Mya: Continuing tectonic changes along the Cascadia Subduction Zone allow the Cascade Ranges to form, pushing up through the existing terrane.
- 2 .5 Mya to 12,000 years ago: multiple glacial advances and retreats carve and erode the Coast and Cascade mountains and create U-shaped valleys and other features, including the Pitt River valley.
- < 12,000 years ago: The modern Fraser River extends its delta and fills in glacially carved valleys with silt.
To put this in a more “down to earth” way, when you are standing at the top of the High Knoll at Minnekhada, you are standing on granitic rock that formed under a volcano somewhere out in the Pacific Ocean and then moved both laterally and vertically to its current location. Note the cracks, cuts and abrasions in the rock, caused by a combination of thrusting and glaciation. Note the steep sides and the talus slope (boulder pile) below the high knoll, caused mostly by glaciers but also by repetitive freeze/thaw cycles slowly breaking apart our wonderful viewpoint. Incidentally, the fire three years ago may, for a short time, accelerate that process.
From the knoll, if you were to look down and to the east, you would see the Pitt River, slowly winding through an almost perfectly flat landscape, called a polder. That’s mostly the story of the Fraser River around the end of the last ice age, and part of that story is, well, pretty dramatic.
And to understand the last part of this fascinating story, complete with some exciting new details, be sure to attend our Year End Wrap-up event for volunteers on Nov 12, as our special guest, Geologist Dr. John Clague presents “OUT OF THE ICE AGE – A geologist’s story of our youthful landscape”. I hope to see you there.
A view of the Pitt River from the fire damaged High Knoll. Photo Steve Smith
Steve is the Secretary and Vice Chair of the Minnekhada Park Association and a very active volunteer, who proudly wears the “science nerd” badge.