Hundreds of Millions of Years Ago, Continents Collided – In Connecticut

A series of earthquakes recently rocked parts of eastern Connecticut. I went looking for what caused the ground to shake and found evidence of some really epic geologic history.

Imagine Connecticut: mountains as high and as sharp as the Himalayas, volcanic activity, and ancient earthquakes shaking the ground – much more powerful than those we feel today. To understand how this was possible, we need to dial the clock back just a little bit… about 300 million years.

“Yeah! Oh, I wish we could get into a time machine and travel back,” said Maureen Long, geophysicist at Yale. She thinks a lot about how movements deep inside the Earth drive the motion of the continents on its surface. And several hundred million years ago, Connecticut was in the middle of a massive continental collision.

Back then, modern-day Africa and South America were clumped together into an ancient landmass called “Gondwana.” It had just plowed into our shoreline, colliding to form the super continent Pangea and, in the process, pushing up huge mountains.

“Basically it probably looked, at least in some ways, like modern-day subduction zones,” said Long. “Think South America and the Andes or the Himilayas.”

While that was happening above-ground, deep, deep beneath the earth a borderland between the two continents was forming. Today, geologists call it the Honey Hill / Lake Char fault system. (We’ll refer to it as “Lake Char” from here on.) It runs across the southeastern part of Connecticut, and then twists north, roughly following the I-395 corridor. (See the orange line in the lower right of the below map.)

For geologists, the rocks here are a kind of portal into the past.

“In geology, that’s one thing we do a lot. We substitute space for time or time for space,” said Phil Resor, a geologist at Wesleyan. I met Resor in East Haddam by Gillette Castle to walk along the banks of the Connecticut River – on one end of the modern-day Lake Char fault zone.

As a bald eagle soared overhead, we walked toward a gigantic cliff face that was covered in melting ice (see below). To me, it just looked like a big rock. But for Resor, it was evidence of a massive continental collision.



He pointed to the fine black patterns that flow throughout the hardened cliff. It almost looked like a liquid that’s frozen in place.

“That super-fine grain material actually is what we call ‘pseudotachylite.’ It was a melt – a frictional melt in the fault,” Resor said. “If you think about rubbing your hands together, you’ll get heat, right? So if you rub fast enough, you’ll raise the temperature to the point where you can actually melt the rock.”

Evidence of ancient earthquakes? Check. But what about evidence of that ancient continent called “Gondwana”?



For that, Resor (pictured below) and I went a few miles east, approaching the Lake Char fault zone from the other side. As we traveled, he explained that as Pangea broke apart about 200 million years ago, the Atlantic Ocean began to open up. A little piece of that ancient continent called “Gondwana” had broken off and was left behind, stuck to Connecticut. Geologists call this zone “Avalonia.”

As we marched through what-was-once Avalonia – immediately, we could see it. The rocks here were different: much lighter, almost gray.



“It looks, roughly – for a layperson, you’d probably look at this and say, 'Oh, it’s a granite,’” Resor said.

It’s actually called a granitic gneiss (pronounced 'nice’), and Resor said it couldn’t have formed on the North American continent.

As we stared at the large boulder (below), he laid out the evidence. The rock’s age doesn’t match up to volcanic activity known to have happened here. Fossil evidence found elsewhere in the region matches with fossils found in modern-day Africa, and there’s also the fabric of the rock itself. When heated minerals cool, they’ll lock in the magnetic field of where they are, which gives geologists a clue to their latitude at time of birth.

It’s called “paleomagnetism”

“And so, this block, the Avalon block is unique, compared to the North American rocks,” Resor said. “It’s unique in terms of the fossils, the geologic history and the paleomagnetic signature. It was recognized from early on that it was a unique terrain.”

It is so unique, in fact, that the Lake Char fault zone was actually name checked in an influential 1960s paper, one of the first to outline the theory of plate tectonics.

The region has remained fascinating to geologists ever since, especially for scientists trying to get a clue into the recent earthquakes that have rattled eastern Connecticut.

Glaciers, Ice, and Earthquakes

For this part of the story, we need to turn the clock back about 18,000 years, when Connecticut was covered in massive blocks of glacial ice.

“They’re heavy. They’re kilometers of ice,” said Maureen Long, the Yale geophysicist. “That weighs a lot, and they press down on the landscape beneath them.”

Imagine pressing your hand into a memory foam mattress. When you remove it, the foam will take a while to jump back into place.



Over thousands of years, that heavy ice may have also pushed out portions of the earth’s mantle, the layer of slowly-flowing rock that exists miles underneath the earth’s crust.



Now, Long said fast forward the clock to about 6,000 years ago. Those glaciers began to recede and melt, scouring the surface of our state. It carved out things like Long Island Sound and Martha’s Vineyard, but it also paved the way for changes to happen underground.

“The Lake Char Fault is not a fault that’s going to move for meters and form a mountain range. That part of it’s history is long, long past,” said Jeffrey Park, a geophysicist at Yale. “But it is still a zone of weakness. So if it’s stressed beyond its breaking point, it will slip a bit.”



Millennia ago, Park said, that liquid mantle began to very slowly flow back underneath our state.



“You’ve got old faults that are left over from all these dramatic plate collisions that occurred 400 million years ago. They don’t have very much to do, but they are responding to this very slowly gathering stress that is occurring,” Park said. “Because the glaciers are gone and the mantle is flowing back.”

That mantle flow, he said, could be one explanation for the small earthquakes recently felt in Connecticut. Or the mysterious Moodus Noises.

“A magnitude two earthquake corresponds to a fault zone that’s about as big as a football field that moves about a centimeter,” Park said. “A football field is pretty big, right? And if you try to move solid rock a centimeter, well, that requires a fair amount of force,” Park continued. “That gives you an idea of the amount of energy that gets released in an earthquake of that size.”

Geophysicist Maureen Long said we know a lot about surface geology and the history of rocks in Connecticut. (Above: the Lake Char fault zone intersects the Connecticut River in East Haddam.) But going forward, she hopes scientists can learn more about what is going on underneath the ground too.

“Presumably all of these processes that shaped the surface geology have also really shaped what the deeper structure looks like,” Long said. “To me, that’s a really interesting scientific question that’s still open. How have all these plate tectonic processes that shaped the surface geology, how have they shaped the deeper structure that we can’t see, but maybe we can measure using geophysical techniques?”

Last month, geologists deployed a number of seismic stations to Plainfield. The hope, Long said, is to “peer down” inside the Earth and better understand the next chapter of the Lake Char fault zone’s colorful history.

Listen to the radio story:

(Image Credits: Patrick Skahill, Wikimedia Commons, The Connecticut Geological Survey)

