A terrane is a landmass that has been added to a continent from a faraway place. We know this because the rocks that makeup terranes are different from those in the rest of the continent. Additionally, terranes can come from a few different places. Often, sediment from another plate will be carried onto a continent’s shore. Other times, whole islands will smash into them.
What is a Terrane?
We’ve already defined a terrane in basic terms. But, we can go a bit deeper. To start thinking about the characteristics of terranes and why they matter, we’ll look at an example.
The Avalon Terrane
Terranes are foreign pieces of land. They’re found as part of one continent but are from another place. Often times, the original place will be totally destroyed, and all that’s left is the terrane. Fortunately, we can use the terrane to learn more about where it came from.
Geologists study the Avalon Terrane for this exact reason. Here’s what it looks like: On the Eastern coast of the United States, we find strange rocks. They’re a whole lot different from the rest of the rock around them. What’s more, these rocks are cut off from others by faults (separations in the crust). So, they form a distinct, bounded region. We call it the Avalon Terrane.
Origin of the Avalon Terrane
Once geologists identified this strange area of rock, they had a lot of questions to ask. Most importantly: Where in the world did this thing come from?
Luckily, they got a huge hint. The same exact kind of rock is found all the way over in Britain, across the Atlantic Ocean! This might seem even further confusing. But, if we understand the theory of continental drift, it’s actually a very useful fact.
Although, there’s one extra thing we have to know to make sense of this. That is, scientists believe that the formation of the Atlantic Ocean is what broke up Pangaea. The ocean opened up right in the middle of the continent, splitting it into smaller ones. Two of them were North America and Europe.
Yet, they didn’t break off cleanly. In between them was actually another small continent: Avalonia. When Europe and North America separated, they each took a piece of Avalonia. Nowadays, they still have those pieces. In fact, they make-up the region known as the Avalon terrane.
So that was quite a story. But terrane formation isn’t always complicated. Usually, terranes are made out of little pieces of sediment. They come from subduction zones and are added to a continent over time.
If you didn’t know, subduction zones are where crust is being pushed into the mantle. We call the descending crust a ‘slab,’ and it’s always a piece of oceanic crust. Moreover, terranes only form when a slab is pushed under a continent.
As it descends into the earth, down to the mantle, sediment scrapes off of it. This sediment floats towards the continent where it may attach and harden into sedimentary rock. We call the resulting landform an accreted terrane.
Now, accreted terranes form all the time, and rather slowly. Yet, there’s a much more dynamic way for terranes to form.
Above subduction zones, there may be volcanic islands. They form as oceanic crust is subducted. But, they’re also destroyed when the ocean closes (when it’s totally subducted). At that point, the volcanic islands are forced into the approaching continent. The result is a pretty huge terrane.
One last thing to note is that people sometimes call terranes exotic terranes. What they mean by this is that terranes come from faraway places.
And that’s actually what makes them good evidence for plate tectonics. If pieces of faraway continents are the exact same, we can safely say they were together at some point. In that way, terranes are like puzzle pieces. Geologists put them together (on a map) to figure out what the world used to look like.
Other Great Resources:
Where Terranes Collide: The Geology of Western Canada:
Simple Video Explaining Terrane Accretion:
More on Accreted Terranes: https://commons.wvc.edu/rdawes/basics/exotics.ijtml