Saturday, February 06, 2010

Geology of North Easton, Massachusetts: We're Still in West Africa.


Sitting on a chunk of West Africa, looking towards West Africa, thinking about snack cakes. Fred's Pond, North Easton, Massachusetts. Dec. 2009.

The most important thing to know about the geology of Easton, Massachusetts is that every rock you see is a part of West Africa.

Massachusetts has some of the most horrendously contorted and complex bedrock geology of any place on Earth.

But in Easton, it's fairly simple. All of Easton is underlain by a type of granite called the Dedham granodiorite, just like most of eastern Massachusetts from Boston to Cape Cod. It's named after the Town of Dedham, the type locality where it was first studied.

This Dedham granite is what we see all over town, especially in North Easton, where it appears everywhere in boulders scattered and dragged about by the last glaciation which ended 15,000 years ago. It's light gray, fine-grained, and is much harder than your head, which you know if you tripped over it running from the cops at midnight in Picker Field.

Radioactive dating of this granite show it cooled from a molten state about 620 million years ago. Chemical studies show this granite hardened fairly close to the Earth's surface, since it is mixed with surface volcanic lava of similar composition. Sit on any one of the millions of boulders or miles of stonewalls in Easton and you're sitting on a 620 million year-old chunk of Senegal, Liberia, or Sierra Leone from a time when no life on land existed on Earth and nothing bigger than your hand lived in the world's oceans.

All of eastern Massachusetts, from Route 495 to the Atlantic, is a chunk of West Africa which began to split from that continent around 600 million years ago and started moving towards ancestral North America, which at the time had a coastline alongside the Adirondacks in the north and followed the western side of the Appalachians to the south, neither of which existed then. This early, smaller continent we now call North America is called "Laurentia." At this time, Africa and much of Europe and Asia were part of one giant continent called "Gondwana."

A Winnebago-sized chunk of 620 million year-old Dedham granodiorite along Massapoag Ave. in North Easton in Borderland State Park. If you get up close it goes, "GRARWRRRRRR !!!"
Somewhere around 650-600 million years ago the westernmost end of Gondwana did an Ishmael, took a boat ride and began to split from Africa. As this splitting occurred, magma from the Earth's mantle surged up through the cracks, creating volcano chains and lava flows. Some of the magma never surfaced but formed huge pools and ponds underground where it slowly cooled into solid, fine-grained rock. This is the rock, the Dedham granodiorite, which comprises what we now call Easton. This made what my dad called "Hoobies." [1]

By recent and ingenious detective work using "paleopoles" in Massachusetts rocks, which use the alignment of tiny iron crystals in rocks as compasses to show where the rocks were in relation to the Earth's magnetic pole when they cooled, we can figure out where eastern Massachusetts ... err ... West Africa ... was at various times in the deep past. Using volcanic rocks from Lynn and Mattapan, which cooled after the Dedham granodiorite of Easton, it is believed that by 595 million years ago, what is now Easton had already left its mooring at Dakar, Senegal and was heading into fair seas toward the beautiful seacoast town of Binghamton, New York. Try the trilobites !!!
Sitting atop an outcrop of Dedham granodiorite between Shovelshop Pond and Fred's Pond in North Easton. The red arrow showing me gives an idea how big these boulders are.

Mom, Are We There Yet?

During the American Revolution, it took 3-6 months to get people, messages and mail by boat from England to Boston. The Dedham granodiorite of Easton took a much slower boat than Ben Franklin -- 200 million years slower. Cape traffic had nothing on this. Better bring board games for the kids !!!

To get a proper perspective for the ridiculous time scales we're talking about, think of this. The time from when the rocks of North Easton first cooled until they set sail from Africa is about the time it took a four-legged animal to venture into the ocean and become the largest animal to ever live on Earth, the blue whale.

Somewhere between 375 and 325 million years ago, Easton finally made landfall and glued itself to North America. This event is called the Acadian Orogeny, which is a fancy name for when one big chunk of the Earth's crust slams into another.

But Easton was not the sole passenger on this very slow ferry. Easton was part of a long, thin chunk of crust called Avalonia, which appears to have been of a similar size and shape to Japan. This chunk included most of coastal Maine, lots of coastal New Brunswick, the eastern half of Newfoundland, southernmost Ireland, a big chunk of England and part of Belgium. The word "Avalonia" comes from the Avalon peninsula of Newfoundland, where this whole mess was first deduced. But how?

As comedian Lewis Black would say, "Fossils."

Geologists have long made a habit of studying marine fossils and have spent a century carefully identifying and sorting them according to species, going back 600 million years. By doing this over and over, around the world, you can sort marine fossils (most no bigger than your thumb) into groups. And the groups tend to be sorted by region and habitat. From the earliest marine fossils we can find, there is a clear separation of species between North America and Europe and Africa. The little buggers are often quite similar, but the differences are there.

What confused 20th century geologists and paleontologists about eastern Newfoundland was that it contained marine fossils identical to those found in Europe and the British Isles but were clearly different from those found in nearby parts of North America. Similar findings were made with fossils in eastern South America and western Africa.

In 1912, a crazy guy named Alfred Wegener, noticing an oddly coincident fit between the coasts of South America and west Africa, theorized that perhaps the fossils on both coasts were so similar was because the two continents were once joined. His theory was called "continental drift," and he was loudly mocked and ridiculed, partly because Wegener could not devise any way continents could skate and slide willy nilly across the oceans like air hockey pucks. [2]

As late as the 1960s, the concept of "continental drift" was still mocked and derided by many esteemed older geologists as kooky carnival hoo-haa. By about the time I was born, in 1964, geologists were finally able to study in detail the seafloor of the Earth's oceans. What they found was that Wegener was right. However it wasn't the continents "drifting" around on top of the oceans like boats. They found that the oceanic crust does the drifting and the continents just ride along on top, like the cherry on a sundae in the dining car of a very slow moving train. They found through detailed mapping that the Earth's solid crust is broken into a few dozen thin sheets, or plates, and these plates are all doing a slow motion dance around and against each other, with some plates going under another, some going on top of another. And here's the kicker: some plates are split in the middle and move away from each other, creating new crust in the gap by undersea volcanoes. The secret is that the Earth's crust, which is solid and hard and brittle, rides on top of the Earth's mantle, much like an ice skate glides on the ice of a pond: the pressure from the crust helps to liquify the very top of the mantle, creating enough lubrication to move.

It's these cracks in the oceanic plates, called "spreading centers," that start the whole merry go-round moving. Convection cells of heat welling up from the bottom of the mantle appear to be the primary driver. And the largest spreading center on Earth splits the Atlantic in two, creating a crack from Iceland to Tierra del Fuego. The Earth's crust beneath the mid-Atlantic is literally splitting apart, with the left side moving west (toward us) and the right side pushing east to Europe and Africa. The continents, being made of lighter rock, sit atop these plates like rooftop luggage and go along from the ride.

Today the Atlantic Ocean is getting wider by a couple thumbnails (10-15 millimeters) each year. If you dial that rate back, you find that around 290 million years ago, Africa was just a hop-step off Plymouth Rock (which, incidentally is made of West African bedrock, ie. good old Dedham granodiorite). This whole body of knowledge is now called "plate tectonics" -- but a shorter name is "what actually happened," since there is today no question of its veracity. To geology, plate tectonics is Darwin's theory of evolution, or Newton's theory of gravity. Without plate tectonics, you can't explain anything except by fairies, leprechauns, trolls, Pukwudgees, and Uncle Weedo.

All Geology is Local

A glance at the Massachusetts bedrock geology map explains a lot of why Easton is the way it is. An west-east line that follows Rockland Street, Allen Road, Lincoln Street, Main Street and Torrey Street is where the Dedham granodiorite goes underground. North of this line it is on the surface. This is why North Easton is so damned rocky and South Easton is not. This is why North Easton has always been crappy for farming and South Easton is comparatively good. This explains why the brooks in North Easton are all fast-moving and the brooks in South Easton are comparatively swampy and sluggish. This is why North Easton has all the good sledding hills and South Easton has none.

If you go to Sheep Pasture along Queset Brook to the foundations of the old Ames mansion, you walk along a big outcrop of bare bedrock that runs north and south from Main Street. This is one of the most southerly exposures of the Dedham granodiorite in North America, or more correctly, that 620 million year old slab of West Africa that we live on.

One of the best and most enjoyable ways to see the geology of Easton is at Borderland State Park, specifically the Upper and Lower Granite Hills trails which run north from Leaches Pond to Mountain Road. This is the largest exposure of bedrock in town and is virtually all Dedham granodiorite. The rock is exposed in giant ribs like blue whales with massive boulders cracked and plucked out and scattered by the last glaciation. There is at least one cave along this trail, which my cousins Todd and Pete Heino and my brother Tim found when we were kids and crawled into as far as we could go. It goes back about 20 feet and then branches out. We brought a flashlight in one day and found a bat clinging to the ceiling of the last big chamber. Last summer I tried to find the cave, but couldn't.

At the highest point of Upper Granite Hills trail, if you are walking north toward Mountain Road from Leaches Pond, turn left and ascend a few hundred feet to what is probably the highest point in Easton, a glacially flattened knob of Dedham granite with small red oaks growing in its cracks. Climb halfway up one of these trees and you can see all the way to Cape Cod. There is a similar height of land along Canton Street near the Easton-Stoughton line next to Long Pond, just north of the Ames Rifle & Pistol Club.

Next: The mysterious, wet geology of South Easton.

[1] When he was a teenager, my dad, Allan E. Watts, was assigned by his father the task of digging out the cellar of their beach cottage in Mattapoisett with a shovel. In the cellar were several giant, rounded boulders about eight feet long, weighing a few tons. They didn't try to move them, not that they could. My dad called these "hoobies." In the era of 20-ton excavators, 80-house subdivisions, 100-acre clear cuts and high explosives, the word "hoobie" has become an anachronism.

[2] Professor Chet Raymo of North Easton, who teaches at Stonehill College, did a nice demonstration of this for our 6th grade class at Frothingham Hall in Easton in 1976 using a paper map and a pair of scissors. As he showed us, the key to get a near-perfect fit is to include the continental shelves. Chet's book, co-authored with his daughter Maureen Raymo, Written in Stone, is an excellent, non-technical overview of the geologic history of New England and Easton. Chet's book, The Path, is also very good.

NOTE: Because nearly all geological research and review papers are inaccessible on-line due to a paywall which charges $20 or more per paper, even for papers published 50 years ago, the summary above is based on abstracts of many research papers and is far less informative than I would prefer to present. This paywall system basically prevents anyone except professional geologists from learning anything about the geology of their own town or region. I'm not sure keeping people dumb is a good way to increase scientific literacy.

1 comment:

David said...

Thanks, I really enjoyed reading this - took me straight back to A level geology!

atb, David, via twitter, via pottery.