Geology Rocks!

Professor Steven L. Goldstein ’76, GSAS’86 started our first Zoom call with a simple question: “How much do you know about geology?”

When I responded that I only knew the barest basics and gave him a weak description of plate tectonics, he was prepared. A quick screenshare later, I was getting a crash course from Goldstein’s class (which he co-teaches with Professor Sidney Hemming) “Earth’s Environmental Systems: The Solid Earth,” flying through a billion years of Earth’s history in 30 minutes. For the uninitiated:

♦ ~1.3–0.9 BILLION YEARS AGO

Rodinia, an ancient supercontinent, covers much of Earth.

♦ 640 MILLION YEARS AGO

The Iapetus Ocean opens up near Rodinia’s proto-North America landmass.

♦ 500–440 MILLION YEARS AGO

The Iapetus Ocean closes and a large volcanic landmass collides with what is now the Eastern coast of North America.

♦ ~330–260 MILLION YEARS AGO

The Pangea supercontinent is assembled.

♦ 200 MILLION YEARS AGO

Pangea begins to break up.

♦ 180 MILLION YEARS AGO

The Atlantic Ocean begins to form.

♦ 160 MILLION YEARS AGO

Africa drifts away from North America.

♦ 20,000 YEARS AGO

Northern North America is covered by a glacier during the last Ice Age.

And then we come to the present, a beautiful August day in New York City. Goldstein and I met in person a few days after our call to take a field trip to Central Park, where this billion years of movement, creation and destruction is on full display, if only you know where to look. And Goldstein, the Higgins Professor of Earth and Environmental Sciences, knows exactly where to look.

Raised in Maryland, Goldstein describes himself as “a totally accidental Columbia student”; he transferred to the College in his junior year, looking for the perfect blend of a strong science department and good general studies. After a brief stint at Harvard to earn an M.A., he returned to Columbia to earn an M.Phil. and a Ph.D., and, after a decade at the Max-Planck-Institut für Chemie in Germany, joined the Lamont-Doherty Earth Observatory in 1996. On August 1, the longtime Columbian added another feather to his cap: He began as interim director of LDEO, the scientific research center of the Columbia Climate School.

In a message to the LDEO community announcing Goldstein’s appointment, interim dean Jeff Shaman spoke to Goldstein’s unique qualifications, including his deep background in research — he has published more than 170 peer-reviewed papers — and awards celebrating “his outstanding contributions to the fields of volcanology, geochemistry and petrology.” Goldstein’s popular teaching methods were also noted: “... he has been recognized for excellence in teaching across Columbia and received the 2022 Great Teacher Award from the Society of Columbia Graduates,” Shaman wrote.

“What I really like to do is go out in the field, look at the rocks and learn about the rocks,” says Goldstein. I asked him to take me on one of the same excursions he leads for students, to showcase the geological history that can be found right in New Yorkers’ backyard.

To kick things off, it’s important to know that Central Park is full of schist — “Manhattan Schist” to be exact, a type of rock made of metamorphosed sand, silt and mud. The Manhattan Schist was formed roughly 500–440 million years ago from sediment dumped in the Iapetus Ocean. You also need to know that the Hudson River is technically a fjord, carved by a glacier during the last Ice Age.

Armed with my newfound geologic knowledge and Goldstein’s decades of expertise, we headed into the park. “It’s going to be like riding a bike,” says Goldstein, adding that once you know the secret history hiding in the rocks, “you’re going to see it everywhere.”

---

Field Trip: Stop 1

Sedimentary Bedding and Continental Collision, Magmatic Dikes and Glacial Striations

Standing on Umpire Rock, a short walk from the park’s Columbus Circle entrance, you can see evidence of sedimentary bedding in the rocks’ lines. The sedimentation was cyclical, with coarse, sandy layers alternating with fine clay layers, slowly eroding over millions of years into the Iapetus Ocean.

Goldstein: “The way erosion works is that when it rains, it erodes stuff that’s above the ocean and deposits it into the ocean. That’s how the Manhattan Schist formed. When you deposit stuff into the ocean, it settles down in flat, horizontal layers below the water line.” ►

Goldstein: “These rocks were just happily sitting beneath the ocean in flat, horizontal sheets — like a piece of paper — and then the ocean closed about 450–500 million years ago and a continent roughly the size of present-day Japan slammed into North America. It buried those sediments. Imagine what would happen if a continent slammed into something; it would mangle things. So here we see evidence of both events: the deposition of these rocks as flat sedimentary rocks, and folding of the sedimentary layers from the continental collision.” ►

After the soft sediments were deeply buried, heated and baked into hard rock, molten rock (magma) cut dikes (sheets of rock that cut across older rock beds) in the rock. You can see the lines of the lighter-colored magma cutting across the sedimentary bedding.

Goldstein: “This line is actually coming from a magma that formed below, came up through here and went right across it after the entire area got folded up [in the continental collision]. You can follow it as it goes right across the layers.” ►

Glacial Striations can be seen along the sedimentary bedding, where the slow flow of glacial ice from the last Ice Age carved out paths in the rock.

Goldstein: “These are my favorite glacial grooves in New York City; you can see that they are long, wave-like, wide grooves. If you look closer, you can see smaller grooves all going in the same direction. What was formed here 20,000 years ago is still pretty much the same.” ►

---

Field Trip: Stop 2

Erratic Boulders

Erratic boulders are rocks that were transported from somewhere else; the erratics in Central Park are evidence of the glacier that covered Manhattan during the last Ice Age. Erratics can be easily identified; they are clearly different from the schist that comprises most of lower Manhattan.

Goldstein: “Glaciers can carry things long ways — even if they’re heavy — because glaciers don’t care how heavy these things are. So glaciers can carry rocks and then when glaciers melt, they deposit them. You can see that this boulder is totally different; you don’t see any indication of sedimentary layering — that rock came from somewhere else.” ►

---

Field Trip: Stop 3

Roche Moutonée and Plucking

A Roche Moutonée is a geological feature formed by glaciers moving over rocks, characterized by a smooth, shallow sloping backside and a steep drop-off.

Goldstein: “This is a small example of this glacial feature that we can see in lots of different places, like Camel’s Hump in Vermont or Lembert Dome in Yosemite National Park. The shape of it slopes gradually upward, in the direction of glacial movement, and then falls into a cliff.” ►

A Roche Moutonée is also defined by its craggy frontside and “plucking” — pieces of rock ripped off the stone.

Goldstein: “During the Ice Age, the glacier was moving right across this rock going over our heads; there was a downslope here and the bottom of the glacier was actually water. When the pressure decreased because there was more space from the downslope, the water turned into ice, then the ice filled these crevices and expanded in the cracks. As the glacier continued moving forward, it plucked the rocks off.”