Seven Sisters: Basalt and Bending Rifts

A few weeks ago I had the pleasure of walking the Seven Sisters trail in the Holyoke Range of western Massachusetts. The Holyoke Range is an unusual mountain range for the Eastern United States because it trends East-West. Most of the mountain linear ranges on the east coast like the Appalachians and Taconics formed as oceanic island arcs were smacked onto the side of the continent (here's a nice cartoon). So what's different about the Holyoke Range?

Overview of the Seven Sisters Trail (not sure how you get seven peaks...).

A few clues in the geology offer an explanation. The first is simply the rock type. While walking the Seven Sisters, you see a whole bunch of basalt. The trail continually goes up, over and around angular outcrops and talus slopes (image below) of basalt. Orogenies (a term that refers to the "smacking on" of island arcs described above) typically result in high grade metamorphic rocks and granitic magmatism, so what's all this basalt telling us?

A talus slope on Bare Mountain via Wikipedia

To figure this out, let's consider the broader geologic context of the area. The Holyoke Range lies at the northern terminus of the Metacomet Ridge that runs up the center of the Connecticut River Basin (see map below, the Metacomet Ridge is in red). By mapping faults and rock types, geologists have determined that the Connecticut River Basin is actually a continental rift that formed as Pangea began to break up 200 million years ago. Eventually, rifting in Connecticut stopped as extension shifted to the rift valley that would eventually become the Atlantic ocean, but not before erupting a considerable amount of lava in the center of the rift. This lava solidified into what is now the Metacomet Ridge.

So the Holyoke Range is essentially the terminus of the system of fissures that formed and erupted a whole bunch of lava that solidified as basalt. But why does it bend and turn east-west? Rifts are essentially propagating cracks and as a crack propagates it will take the path of least resistance, following weakzones (this is demonstrated nicely in East Africa, where the East Africa Rift splits into an eastern and western branch, wrapping around the strong Tanzanian craton). If the rift was following some pre-existing weakness, it could have turned if the weak zone happened to change orientation. So taking this all in, the east-west trend of the Holyoke Range is likely due the propagating Connecticut rift following some pre-existing weakness.

The Connecticut Valley Rift Basin - image source here.

As a final note - should you find yourself at the end of the Seven Sister's trail, looking for some food and a drink, drive on over to El Comalito and take your tasty burrito next door to The Moan and Dove. You will not be disappointed (if you doubt me, just ask @alkenones)!