Ever since the opening of the Atlantic Ocean during the Jurassic, the east coast of North America has been a "passive margin" - that is, a boundary between continental and oceanic plate where there is little in the way of active tectonics. We don't tend to hear about many large earthquakes along the eastern seaboard (although they have happened - in 1755 and 1886) and all the volcanic material is the eroded remains of the innards of magmatic system.
So, the news that an earthquake swarm has been causing a stir on the New Hampshire/Vermont border has been a bit of a surprise. Located near Wantastiquet, the swarm has been quietly building over the last few months from a few earthquakes a day to now hundreds of microearthquakes each day, all below magnitude 2. Geologists have been baffled by these earthquakes – they don't seem to have the same signature as the other mysterious Arkansas swarm – but the seismicity continues. There have even been reports of steam coming from the ground in the area (see top left).
Dartmouth University geologist Dr. Siegfried Farnon is now speculating that the earthquakes might be volcanic in origin: "It is not out of the realm of possiblilty that our passive margin in eastern North America might be changing back to an active margin, that is a subduction zone. This might be the first signs of this transition." His suggestion that we might be seeing the the start of a new "New England arc" is remarkable and something to watch closely. We already know that magma was found under New York City, so the re-initiation of volcanism in New Hampshire might be the start of something big – think about cities like Washington, Philadelphia and Boston all located in an tectonically-active area! Stay tuned!