Every other Friday on Morning Edition NHPR’s Sam Evans-Brown tracks down answers to questions about the environment and outdoors for our listeners in a segment we call “Ask Sam.”
David from Denver, Colorado asks: I studied geology ... and we learned extensively about the long and complex geological of the New England region. And I can’t help but wonder what was the maximum elevation achieved in New Hampshire and New England generally?
Note: This edition of Ask Sam originally aired in November, 2018.
Are you ready? Time to get your plate tectonics hat on!
To present an extremely condensed and simplified version of the history of the Appalachian mountain range, geologists believe the chain was born around 450 million years ago. (That’s pre-dinosaurs, by the way… it’s around the time that plants first started to appear on land.)
A shift in plate tectonics meant and the ocean that was off the east coast of what would become North America, called the Iapetus Ocean, started to close. Over the course of hundreds of millions of years small land-masses—island chains, subcontinents and the like—collided with the east coast in wave after wave of mountain-building events, or orogenies.
The Big One (the one you’ve probably heard of) came about 320 million years ago when (pre) Africa and (pre) Europe collided with the (pre) North and South Americas and formed the supercontinent Pangea, with the Appalachian Mountains right down the center of it. It would have been heady times for the Appalachians, sitting at the peak of their glory, the center of the all land on earth.
The question one could naturally ask is this: how do we know they were so tall? Well, we have a couple of strategies. Let’s start with the most general and work our way to the most precise.
- Just Lookin’ Around The World
We know these mountains were formed by two continents colliding, and when we look around for two continents colliding, and what we see is the Himalayas. When continents collide, the crust of the earth thickens and folds back on itself.
“Imagine a train wreck or two cars colliding: they’re basically stacking on top of each other and that gives you twice the height,” explains geologist Catherine Riihimaki, associate director of the Council on Science and Technology at Princeton.
This is why you will often hear people say that the Appalachian Mountains must have been as high as the Himalayas are today. These were big continents colliding! Certainly the Appalachians MUST have been as high as the Himilayas!
Well...not so fast.
- Lookin’ At The Types of Rocks Lyin’ Around
It can help to think of mountains (and the earth’s crust) like icebergs floating on top of the ocean (the earth’s semi-plastic mantle). For everything that’s sticking up in the air, you need a commensurate amount of rock down in the earth providing the buoyancy that keeps the rocks “afloat.”
The further down you go, the hotter and more pressure rocks are subjected to, which changes their structure in predictable ways. So, in not-exactly-correct layman’s terms, the way they melt tells us how how deep the mountain’s roots were, which gives us a more precise estimate of how high they must have been.
Since the rocks that are at the surface of the Appalachians today are deeply metamorphosed, we know that the ancient Central Pangean Mountains (break that one out at your next dinner party to impress the crowds) had some very deep roots.
- Lookin’ At Where They Went
If the Appalachians were so gosh-darn big, where did they go, exactly? Well, they eroded, and all that eroded junk settled into the low areas that surround the mountains, called the foreland basin.
The original Appalachian Mountains were so old that geologists believe they wore down to a completely flat plain, and that a more recent tectonic event lifted up the ancient roots of the mountains for us to scramble up on top of today.
But Carmie Garzione of the University of Rochester explains that using computer models to mathematically pile the material in the foreland basin back onto where it must have been when it was atop the mountains, we can get an even more precise idea of how big a mountain range once was.
“And so we can see that the volume of sediment must have eroded a very large, thick, high-elevation mountain range,” she says.
But, was it as tall as the Himalayas? Not quite.
“The Appalachians at their height, were about the scale of the Andes,” says Frank Pazzaglia of Lehigh University. While the Himalayas are around 6,000 meters tall and top out around 8,000 meters, most of the peaks in the Andes are more like 4,000 meters, with the high peaks up in the 6,000 meter range. Though given the enormity of geologic time, this still amounts to an educated guess. Pazzaglia notes, “it’s really impossible to know.”
Now, since our question asker was specifically interested in New Hampshire, we have to point out that the evidence suggests the highest points were probably just off the East Coast (the tallest of the Appalachian peaks likely crumbled into the Atlantic when Pangea split into separate continents) down off the Delmarva Peninsula. Here in New England we were more like the foothills.
Some Words of Comfort
If hearing that the available evidence suggests we weren’t quite Himalayan in scale here in New England makes you sad, here’s some consolation. The Himalayas are remarkable, because they are probably about tall as mountains are physically capable of being on earth. Once mountains reach a certain height, they start to shrink just as fast as they can grow, which seems to be the case up there in Tibet/China.
As mountains get pushed up by continents colliding, they get tall enough that glaciers start to form on top. Those glaciers do a number on rocks, and the higher the mountains the more vigorous that glacial eroding. “So think of it like the mountain ranges are a stick of butter and the glaciers are a warm knife,” says Riihimaki.
What’s more, the taller mountains get the hotter things get down at their root. “When you get to a certain thickness, you heat middle and lower crustal rocks to high enough temperatures that they likely flow,” says Garzione, “Brie cheese is a great example: put the brie out on the counter and when the center warms up, it’s going to start flowing outward.”
If you’re still looking for bragging rights, the Appalachian chain was almost certainly a more extensive mountain range than the Himilayas, which makes for a really long walk.