Reading Rocks - The South Mountain Batholith
Barren tracts of granite are a signature landscape in Nova Scotia, most famously at Peggy’s Cove. But where did all that granite come from? Today’s geological story takes us to a basin in ancient Africa and 15km beneath the earth’s surface to learn how this igneous rock is formed and why it’s stayed so resilient.
EPISODE NOTES
More than just about any episode in this series, this one took me back to my childhood in the east coast. Scrambling over granite on a foggy day is the most nostalgic thing I can think of.
TRANSCRIPT
So far, as we’ve travelled geologically across the east coast, there’s been one thing uniting our stops. Whether the Sillery formation at Chaudierre Falls, the White Head limestone at Grand Falls, or the Triassic sandstone in the Bay of Fundy, it’s all been sedimentary. That is, formed by the gradual build-up of sediment layers that are then compressed by enough pressure to turn them into rock.
But there’s an iconic region of Nova Scotia that has a very different origin. Maybe most famously at Peggy’s Cove, you’ll find these vast, bare, sweeping formations of granite. And granite… is not sedimentary. So, where did all this come from?
Well, to start with: ancient Africa. Or rather, the Gondwana terrane that would eventually become Africa, 470 million years ago.
Here, mud and sand gather in a basin and are compressed into… yes, sedimentary rock. I know. But stay with me! Because that sedimentary rock is getting hauled across the globe by plate tectonics over tens of millions of years… to finally smash headfirst into North America.
It’s an ‘unstoppable force meets immovable object’ type deal where neither plate yields and the earth’s crust thickens as they mash together. Some of the rock is forced up into mountains. Some, like our African sedimentary rock, is forced down, deeper into the earth, where it is put under tremendous pressure and tremendous heat in a process called orogenic melting.
Temperature increases 25-30 degrees celsius every kilometer you descend. And the African sedimentary rock ends up 10-15 kilometers beneath the surface. There, it melts into magma.
Now, if you’re faster at math than me you may have noticed a bit of a problem with that. 30 degrees temperature increase per kilometer times 15 kilometers depth gets us… 450 degrees celcius. There are pizza ovens that get that hot. And, not suggesting you try this, but I’m pretty sure if you put a rock in one of those pizza ovens… it wouldn’t melt.
On its own, no it would not. But what actually happens down there is aided by pressure and water. It’s the difference between ‘dry melting’ rock which requires temperatures double the ones we’re talking about, and ‘wet melting’.
The rock is being squeezed - not just from above but from the sides as the two plates continue to crush into each other. Friction and deformation of the rock produces additional heat. And water-bearing minerals like mica go through dehydrating reactions, releasing water that weakens chemical bonds and helps the rock melt at a lower temperature.
So thanks to those specific conditions, our sedimentary rock does indeed become magma. And when that magma rises back up, thanks to continued tectonic activity, it cools over several million years and multiple intrusions toward the surface. In doing so, it becomes a totally different type of rock: no longer sedimentary, igneous, which simply means it originated as magma.
Then, as part of the Acadian Orogeny that lifted up the Appalachians, this massive intrusion of granite arrives at the surface. It covers 7300 square kilometers of what would become Nova Scotia.
And it’s fully exposed thanks to a combined effort of erosion and glacial stripping. Granite is an exceptionally hard rock, so it sticks around as softer rock above is scoured away. The glaciers of the last ice age help scrape off that surface layer. You can see some of their aftermath in these ‘erratics’, massive granite boulders sitting conspicuously by themselves: they were dragged across the landscape and deposited there by glaciers.
So that’s how you get these beautiful, stark landscapes across the southern half of Nova Scotia - now called the South Mountain Batholith. Batholith just meaning ‘intrusion of igneous rock’
And it’s worth zooming way in on that granite to appreciate it on a micro as well as macro scale.
It gets its specific texture from the gradual cooling process as it rose. Different minerals crystallize at different temperatures, including deposits of tin, tungsten and uranium.
The cracks, crevices and lines of white in the rock are evidence of late-stage magmatic intrusions. When pressure on the solidified granite makes it crack and fissure, more magma can fill in those cracks, and then cool quickly due to its smaller volume, into lighter crystals and aplite rock.
The granite also shows a distinctive pinkish, reddish colouration in the right light due to its potassium feldspar content, and a high quantity of biotite mica can make it sparkle.
I’m going to sound like a broken record, but geology is amazing. Not only can we trace each of those minerals and properties of this rock back to tell us about how this granite formed, rose and cooled… but we can look at it as a whole and see a journey across the planet and 500 million years long.
Like, I have no idea how we do that, I’m just the messenger. But being able to analyze this granite in its geological context here, and be able to say ‘oh yes, it started as sedimentary rock in Africa 470 million years ago that circumnavigated the globe, sunk, melted, rose, cooled and then was scraped clean by glaciers’ …that is nothing short of a miracle.