This text initially appeared in  Knowable Journal.

When Maureen Lengthy talks to the general public about her work, she likes to ask her viewers to shut their eyes and consider a panorama with unimaginable geology. She hears plenty of the identical options: Iceland, the Grand Canyon, the Himalayas. “No person ever says Connecticut,” says Lengthy, a geologist at Yale College in New Haven in that state.

And but Connecticut—together with a lot of the remainder of japanese North America—holds vital clues about Earth’s historical past. This area, which geologists name the japanese North American margin, primarily spans the U.S. japanese seaboard and a bit farther north into Atlantic Canada. It was created over a whole lot of hundreds of thousands of years as slivers of Earth’s crust collided and merged. Mountains rose, volcanoes erupted, and the Atlantic Ocean was born.

A lot of this geological historical past has grow to be obvious solely prior to now decade or so, after scientists blanketed the US with seismometers and different devices to light up geological buildings hidden deep in Earth’s crust. The ensuing findings embody many surprises—from why there are volcanoes in Virginia to how the crust beneath New England is weirdly crumpled.

The work may assist scientists higher perceive the sides of continents in different elements of the world; many say that japanese North America is a kind of pure laboratory for finding out comparable areas. And that’s vital. “The story that it tells about Earth historical past and about this set of Earth processes … [is] actually basic to how the Earth system works,” says Lengthy, who wrote an in-depth have a look at the geology of japanese North America for the 2024 Annual Evaluation of Earth and Planetary Sciences.

Born of continental collisions

The majority of North America right now is made from a number of totally different elements. To the west are comparatively younger and mighty mountain ranges just like the Sierra Nevada and the Rockies. Within the center is the traditional coronary heart of the continent, the oldest and stablest rocks round. And within the east is the lengthy coastal stretch of the japanese North American margin. Every of those has its personal geological historical past, however it’s the story of the japanese bit that has just lately come into sharper focus.

For many years, geologists have understood the broad image of how japanese North America got here to be. It begins with plate tectonics, the method wherein items of Earth’s crust shuffle round over time, pushed by churning motions within the underlying mantle. Plate tectonics created after which broke aside an historic supercontinent often known as Rodinia. By round 550 million years in the past, a fraction of Rodinia had shuffled south of the equator, the place it lay quietly for tens of hundreds of thousands of years. That fragment is the guts of what we all know right now as japanese North America.

Then, round 500 million years in the past, tectonic forces began bringing fragments of different landmasses towards the long run japanese North America. Carried alongside like elements on an meeting line, these continental slivers crashed into it, one after one other. The slivers glommed collectively and constructed up the continental margin.

Throughout that course of, as an increasing number of continental collisions crumpled japanese North America and thrust its agglomerated slivers into the sky, the Appalachian Mountains had been born. To the west, the japanese North American margin had merged with historic rocks that right now make up the guts of the continent, west of the Appalachians and thru the Midwest and into the Nice Plains.

By round 270 million years in the past, that motion was achieved, and all of the world’s landmasses had merged right into a second single supercontinent, Pangaea. Then, round 200 million years in the past, Pangaea started splitting aside, a geological breakup that fashioned the Atlantic Ocean, and japanese North America shuffled towards its present place on the globe.

Since then, erosion has worn down the peaks of the once-mighty Appalachians, and japanese North America has settled right into a principally quiet existence. It’s what geologists name a “passive margin,” as a result of though it’s the fringe of a continent, it isn’t the sting of a tectonic plate anymore: That lies hundreds of miles out to the east, in the midst of the Atlantic Ocean.

In lots of elements of the world, passive continental margins are simply that—passive, and fairly geologically boring. Consider the japanese fringe of South America or the shoreline round the UK; these aren’t locations with energetic volcanoes, massive earthquakes, or different main planetary exercise.

However japanese North America is totally different. There’s a lot happening there that some geologists have humorously dubbed it a “passive-aggressive margin.”

The japanese fringe of North America, working alongside the U.S. seaboard, accommodates fragments of various landscapes that attest to its complicated delivery. They embody slivers of Earth’s crust that glommed collectively alongside what’s now the east coast, with a extra historic mountain belt to their west and a piece of much more historic crust to the west of that.

That motion contains comparatively excessive mountains—for some purpose, the Appalachians haven’t been solely eroded away even after tens to a whole lot of hundreds of thousands of years—in addition to small volcanoes and earthquakes. Latest eastcoast quakes embody the magnitude-5.8 tremor close to Mineral, Virginia, in 2011, and a magnitude-3.8 blip off the coast of Maine in January 2025. So geological exercise exists in japanese North America. 

“It’s simply not following your typical tectonic exercise,” says Sarah Mazza, a petrologist at Smith School in Northampton, Massachusetts.

Crunching information on the crust

There’s a lot happening there that some geologists have humorously dubbed it a “passive-aggressive margin.”

After which scientists acquired even higher optics. Lengthy and different researchers started placing out further seismometers, packing them in dense traces and arrays over the bottom in locations the place they needed even higher views into what was happening beneath the floor, together with Georgia and West Virginia. Group members would discover themselves driving across the countryside to rigorously arrange seismometer stations, hoping these would survive the snowfall and spiders of a yr or two till somebody may return to retrieve the information.

The strategy labored—and geophysicists now have a significantly better sense of what the crust and higher mantle are doing below japanese North America. For one factor, they discovered that the thickness of the crust varies from place to position. Elements which can be the stays of the unique japanese North America landmass have a a lot thicker crust, round 28 miles. The crust beneath the continental slivers that hooked up afterward to the japanese edge is far thinner, extra like 15 to 18 miles thick. That distinction in all probability traces again to the formation of the continent, Lengthy says.

However there’s one thing even weirder happening. Seismic photographs present that beneath elements of New England, it’s as if items of the crust and higher mantle have slid atop each other. A 2022 examine led by geoscientist Yantao Luo, a colleague of Lengthy, discovered that the boundary that marks the underside of Earth’s crust—sometimes called the Moho, after the Croatian seismologist Andrija Mohorovičić—was stacked double, like two overlapping pancakes, below southern New England.

The outcome was so stunning that initially Lengthy didn’t assume it may very well be proper. However Luo double-checked and triple-checked, and the reply held. “It’s this super-unusual geometry,” Lengthy says. “I’m unsure I’ve seen it anyplace else.”

It’s notably odd as a result of the Moho on this area apparently has managed to take care of its double-stacking for a whole lot of hundreds of thousands of years, says Lengthy. How that occurred is a little bit of a thriller. One concept is that the unique landmass of japanese North America had an especially sturdy and thick crust. When weaker continental slivers started arriving and glomming on to it, they squeezed up and over it in locations.

How the Moho is working

The power of that collision may have carried the Moho of the incoming items up and over the older landmass, leading to a doubling of the Moho there, says Paul Karabinos, a geologist at Williams School in Williamstown, Massachusetts. One thing comparable is perhaps taking place in Tibet right now because the tectonic plate carrying India rams into that of Asia and crumples the crust in opposition to the Tibetan plateau. Lengthy and her colleagues are nonetheless making an attempt to work out how widespread the stacked-Moho phenomenon is throughout New England; already, they’ve discovered extra indicators of it beneath northwestern Massachusetts.

A second stunning discovery that emerged from the seismic surveys is why 47-million-year-old volcanoes alongside the border of Virginia and West Virginia might need erupted. The volcanoes are the youngest eruptions which have occurred in japanese North America. They’re additionally a little bit of a thriller, since there is no such thing as a apparent supply of molten rock within the passive continental margin that may very well be fueling them.

The identical course of may very well be taking place in different passive continental margins, Mazza says. Discovering it beneath Virginia is vital as a result of it exhibits that there are extra and alternative ways to gas volcanoes in these areas than scientists had beforehand thought doable. “It goes into these concepts that you’ve extra methods to create soften than your customary tectonic course of,” she says.

Lengthy and her colleagues wish to see whether or not different elements of the japanese North American margin even have this crustal drip. One clue is rising from how seismic power travels by way of the higher mantle all through the area. The rocks beneath the Virginia volcanoes present a wierd slowdown, or anomaly, as seismic power passes by way of them. That may very well be associated to the crustal dripping that is happening there.

Seismic surveys have revealed an analogous anomaly in northern New England. To attempt to unravel what is perhaps taking place at this second anomaly, Lengthy’s staff at the moment has one string of seismometers throughout Massachusetts, Vermont, and New Hampshire, and a second dense array in japanese Massachusetts. “Possibly one thing like what went on in Virginia is perhaps in course of … elsewhere in japanese North America,” Lengthy says. “This is perhaps a course of, not simply one thing that occurred one time.”

Lengthy even has her eyes on pushing farther north, to do seismic surveys alongside the continental margin in Newfoundland, and even throughout to Eire—which as soon as lay subsequent to the North American continental margin, till the Atlantic Ocean opened and separated them. Early outcomes recommend there could also be important variations in how the passive margin behaved on the North American facet and on the Irish facet, Lengthy’s collaborator Roberto Masis Arce of Rutgers College reported on the American Geophysical Union convention in December 2024.

All these discoveries go to point out that the japanese North American margin, as soon as deemed a little bit of a nap, has way more going for it than one may assume. “Passive doesn’t imply geologically inactive,” Mazza says. “We dwell on an energetic planet.”

This text initially appeared in Knowable Journal, a nonprofit publication devoted to creating scientific information accessible to all. Join  Knowable Journal’s e-newsletter.

Lead picture: The White Mountains in New Hampshire owe their dramatic topography to a whole lot of hundreds of thousands of years of geological exercise, together with collisions that constructed up continental crust and raised mountain ranges into the sky. Credit score: CO Leong / Shutterstock.

• Alexandra Witze

  Posted on April 11, 2025

  Alexandra Witze (@alexwitze.bsky.social) is a science journalist in Boulder, Colorado. In school, she modified her main from chemistry to geology after happening a category subject journey in japanese Massachusetts and mountain climbing round rocks all day.