Enviornment Down to Earth with Naila Moreira: Reading the rocks

Geologist Mark Brandriss of Smith College is bent almost double on the shoulder of Route 5, his bare hands buried in vines. He’s yanking invasive plants off the face of a rock cliff just south of East Street.

“It’s just ravenous, this stuff. This is the thickest overgrowth I’ve ever seen here,” he says. “Gotta keep the outcrops clear until I retire, and then it’s someone else’s problem.”

He pauses, breathing hard on this 96 degree July day.

“It breaks my heart,” he adds.

Brandriss researches how igneous rocks — or rocks that cool from a melt — form deep within the earth. His own field sites are located far away in Scotland, Alaska and Canada, but he relies on local rock outcrops like this one to teach students the history of the Valley via an introductory geology class that is entirely based in outdoor fieldwork. The students learn how to read the story embedded in the rocks around them.

Today, Brandriss has offered to take me on a whirlwind day trip to show me — and readers of this column — what he teaches them.

“How do you take a bunch of rocks and make a map out of them?” says Brandriss. “How do we learn about the landscape and what happened here?”

Earlier this year, I wrote about the Pioneer Valley’s recent geologic history via a field trip with UMass geologist Julie Brigham-Grette. Brigham-Grette showed me how, some 15,000 to 10,000 years ago, the sediments of the flat regions around us were laid down by glacial Lake Hitchcock.

By contrast, Brandriss is showing me the hard rocks that form our ridges and outcrops, such as the Mount Tom and Holyoke ranges, and what scientists now know about them. These rocks, which formed not tens of thousands, but hundreds of millions of years ago, dictate the shape of our Valley.

Many locals know that dinosaurs once roamed our region, leaving footprints behind. Those footprints, left in marshland muds, became fossils and can be seen at sites like Amherst College’s Beneski Museum or the dinosaur footprint roadside turnoff on Route 5.

Fewer people are familiar with what created the various types of bedrock we see here today.

“Do you realize there was volcanic activity here in Massachusetts?” says Silas Kopf, a Northampton woodworker who has studied with Brandriss. “This seems like a really stable part of the earth, we don’t have earthquakes, we don’t have volcanoes — but we used to. And I think the average person on the street would be amazed to hear that.”

Kopf started out as one of those “average people on the street.” Then his neighbor, Smith professor Amy Rhodes, challenged him to take her introductory geology course half as a joke. As he started a partial retirement from his lifelong woodworking career, he decided to take the plunge.

“I have a bunch of friends who have audited at Smith,” he said. “As I was pondering a slowdown in professional life, I thought, maybe that’s something I should do. Once I did it, I realized it wasn’t a huge burden.”

He ended up continuing his geology education, taking a course with Brandriss as well as upper level classes from paleontology to the geology of archaeological artifacts. He even traveled to Oregon to explore volcanic geology with local geologist Richard Little, author of “Dinosaurs, Dunes, and Drifting Continents: The Geology of the Connecticut River Valley” (a slim, info-packed book I highly recommend).

For Kopf, the new ways of looking at the land around him were revelatory.

“It’s remarkable to look at rocks that most people would say, that’s just a rock, and with the right binoculars on, it becomes much more fascinating — if somebody can explain to you what the earth processes were that took place to make that rock what it is,” he said. “And frankly that was my original hope — to be able to walk around Mount Tom and know what rock I was looking at.”

So what is it, up there?

One remarkable feature of the Pioneer Valley is that its overall appearance today somewhat mimics how it looked 200 million years ago – during what geologists call the Jurassic period of the longer Mesozoic era. At that time, we also had a valley here, with rivers snaking below steep mountains and marshy regions supporting animals and plants.

“The modern Connecticut Valley is basically the outline of the Mesozoic rocks,” Brandriss says. Today, “erosion has created a valley, while in the Jurassic it was a valley due to tectonic movements.”

Those tectonic movements were the splitting apart of huge geologic plates that once underlay the supercontinent Pangaea, made up of all the continents on earth stuck together. Around 200 million years ago, the tectonic plates that carry America and Europe began their multi-million-year breakup in our region.

As they separated, vast regions of rock tilted down into the growing space between the continents, creating a valley floor. Eroding rock from highlands on either side spilled into the valley basin.

“It’s the fate of basins to be filled with sediments,” Brandriss says.

Shifted by rivers, dumped into lakes, the earliest of these sediments turned into the New Haven Arkose (also called the Sugarloaf Arkose further north), a type of coarse sandstone.

A chunk of this arkose usually has tiny, reddish grains of sand stuck together, with larger lumps of rock embedded in it like raisins in a Christmas pudding.

As the valley continued to widen, volcanic activity began.

Why? The hard tectonic plates that carry continents and oceans rest on the flowing mantle of the earth deep beneath. The mantle isn’t liquid: imagine, instead, that plates float on it like bathroom tiles on a bucket of silly putty. As the tiles holding Pangaea broke apart and separated, the mantle flowed upward to fill the gap.

As the hot material of the mantle rose, some of it melted a little as pressure dropped. That molten rock poured onto the surface of the rift valley.

As Pangaea split up, huge amounts of lava bubbled out. In our area, these lavas cooled into a mighty deposit, the Holyoke Basalt, on top of the New Haven Arkose.

The valley kept widening, tilting the rocks within it still more to leave a layer of cooled, tilted basalt atop tilted beds of sandstone. Because sandstone erodes faster than hard, resistant volcanic rock, the ragged edge of the Holyoke Basalt now sticks up above the landscape.

The crest of both the Holyoke and Mount Tom Ranges are made of this blocky, reddish-gray stone that tends to break into the majestic columns seen along the cliffs of Mount Tom.

Remarkably, according to Brandriss, geologists are now learning that these outpourings of lava might have played a catastrophic role in life’s evolutionary history.

“The volcanism may have caused one of the biggest mass extinctions on earth,” he says.

When volcanic lava emerges onto the earth’s surface, it belches out carbon dioxide, a gas that traps the sun’s heat on earth. Today, most excess carbon dioxide is produced by humans via the burning of fossil fuels like oil and gas. But when volcanoes go into overdrive, this dangerous “greenhouse effect” can be produced naturally.

Our lavas, said Brandriss, formed part of a massive area of Atlantic volcanism that includes the Palisades in New York and is seen on four continents. Recent evidence tying the magmatism to extinction has mounted, including the fact that leaf pores seen in fossils shrank, a reaction to excess carbon dioxide. In 2017, researchers at Oxford University found high levels of mercury, a marker of volcanic atmospheric deposition, in sediments at exactly the time of the extinction.

At the Triassic/Jurassic boundary, scientists posit, carbon dioxide likely doubled, and many living beings couldn’t adapt to rapidly rising temperatures. About a quarter of marine life and as much as 60 percent of four-legged land species went extinct.

After the extinction event, the geologic cycle repeated, but less vigorously. The East Berlin Formation, made of silts and muds deposited in lakes and slow-moving streams, eroded off the mountainsides onto the Holyoke Basalt. In turn, it was overtopped by the explosive volcanic ash of the Granby Tuff and the flowing lavas of the Hampden Basalt.

Brandriss and I stood on the observation tower of Goat Peak in the Mount Tom State Reservation to view this smaller layer of basalt, which now forms Little Mountain just northeast of the Mount Tom summit. Holyoke Basalt was once quarried nearby for crushed roadbed material.

Finally, we concluded our trip through geologic time by driving to the dinosaur footprint pull-off on Route 5 alongside the Connecticut River.

At that time in the Jurassic, our area was hot and marshy, a valley with mountains far off to the sides. Muds in these swamplands were stomped on by roaming dinosaurs and imprinted by long reedy plants. Hardening into rock, the muds became the Portland Formation, the youngest stone visible in our region today.

Over millions of years, the valley would fill in completely due to erosion. Only quite recently would glaciers, and then the Connecticut River, carve it out again into the Pioneer Valley we know today, freeing a Mesozoic outline back into view.

“You don’t have to know a lot of geology. You just need to know some basic geologic principles and you can work out a lot for yourself,” Brandriss told me as we pondered the long history of the earth at our feet. “You can recreate a vivid landscape of the past just by looking at a bunch of rocks.”

Naila Moreira is a writer and poet who often focuses on science, nature and the environment. She teaches science writing at Smith College and is the writer in residence at Forbes Library. She’s on Twitter @nailamoreira.

Author: Going Green

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