Hanover
Though the blob may reach the surface in the form of giant, pressure-releasing volcanoes, there’s no need to abandon the region — the upwelling is tens of millions of years old, and will likely take millions of years more to present any kind of danger to surface-dwellers.
“It’s something like a pot just before it comes to boil, where some water is rising and some is sinking” said Leslie Sonder, a Dartmouth professor of earth sciences who studies tectonic plates for a living. “It’s a very chaotic pattern. I suspect there are places where blobs are sinking down, and some places where blobs are rising up.”
This hot rock pocket, thought to be a couple of hundred miles long and 100 miles wide, is largely located beneath central Vermont and western New Hampshire, according to the study, which was conducted by the Department of Earth and Planetary Sciences at Rutgers University-New Brunswick, and was recently published in the online journal Geology. A significant part of the heated area also seems to be in New York, and in parts of Massachusetts.
In some places, the top of the upwelling is as close as 60 miles to the surface of the Earth — equivalent to the distance between the Vermont towns of Springfield and Bradford.
Sonder said that, while the study will help to explain New England’s peculiar geology, the discovery will not change predictions about regional earthquakes, which are based on past quake events, such as the magnitude 4.1 Vermont-based tremor in 1962 that damaged the support structure beneath the Statehouse dome in Montpelier, or the June 1638 New Hampshire-based magnitude 6.5 temblor that threw people to the ground and shook stone chimneys to pieces.
“The past history is the past history, regardless of whether we have this mantle of upwelling,” Sonder said. “It doesn’t change our assessment of seismic hazard.”
Part of what’s made it difficult to assess New England’s quirky earthquake profile is that the underlying causes are shrouded in mystery — no one knows for sure whether local quakes are caused by a rebounding of the Earth’s crust that’s been, in geologic time, suddenly relieved of the pressure of Ice Age glaciers, or whether they’re in response to the pressure of a tectonic plate slipping beneath the East Coast, or some combination of those two things.
The Rutgers study, which was helmed by lead researcher Vadim Levin, gives a little more insight into processes that are buried so deeply in the planet that they’ve never been directly observed. Though rock seems very solid to us, Levin said, over the course of geologic time, it flows.
“The apt analogy I can offer is that of a very sluggishly overturning fudge or syrup,” Levin said.
The researchers relied on data produced by a series of seismographs scattered throughout New England, including one located in the basement of Dartmouth’s 160-year-old Shattuck Observatory.
The cylindrical instrument, about 10 inches high and 6 inches in diameter, is in direct contact with the bedrock, where the tiniest vibration is recorded and sent to Weston Observatory at Boston College in suburban Boston.
A proposal by Dartmouth administrators to build a 750-bed dormitory in College Park, where the observatory is located, has drawn concerns about the possibility that the two-story brick building will be razed or relocated.
Sonder said one of the things that makes the data produced by the station so valuable is that it has been consistent over time.
“They had to use long-term records,” she said. “This is one of the few long-term records that’s available. If they relocated it, the data might be similar, but not exactly the same as this particular station. Even if they managed to save the observatory, the data quality would be damaged by the construction and extra traffic.”
Levin and his colleagues used a nifty bit of detective work to draw their conclusions about the existence of the hot rock, its boundaries, and its relative age.
They used the seismic data to tease out how quickly sound waves are traveling through the earth. One of the factors that they use is the level of “anisotropy” — the extent to which the bedrock, stretched like taffy over time, is oriented. Most of the rock around here is oriented in a northwest-southeast direction because of the direction the North American plate is drifting over the Earth’s mantle. Differences in the speed of sound waves coming from different directions allow researchers to figure out where the rock has that orientation.
“The technique is maybe only 20 years old, and it depends on a lot of high-quality data,” Sonder said.
But the researchers found a big, irregular-shaped absence of anisotropy in the bedrock, where sound waves traveled more slowly than they would have if the rock had that orientation. Sonder said that’s a sign that the shear of the rock has been erased, because the upwelling heat has wiped out the orientation.
“It is hot material there, hotter than other places at the same depth,” Sonder said.
Levin said the hot area is not molten, but that it is hundreds of degrees hotter than the surrounding rock.
Last year, Arizona State University researchers used the same method to identify two much larger continent-sized upwellings beneath the Pacific and Atlantic oceans, and a similar situation is thought to be fueling Yellowstone National Park’s volcanic activity and hot springs. In New England, the trapped heat hasn’t yet made itself felt at the surface, leading the researchers to conclude that New England’s warm blob is relatively young.
The paper isn’t the first to suggest that New England is sitting on a geologic woodstove, but it’s the first successful effort to pin down its boundaries.
“It took it from being a proposal on the order of ‘maybe’ to ‘probably,’ ” said Sonder. “It’s likely.”
Matt Hongoltz-Hetling can be reached at mhonghet@vnews.com or 603-727-3211.