Vermont's patiently cataclysmic geologic origins
I interrupt our regularly scheduled polar bear gallery with a little primer on Vermont geology, for the Green Mountain Boys and Girls out there wondering how the place ended up looking like this.
The view from the Camel’s Hump summit captures a geological story a billion years in the making. What seem like picturesque hills and pastoral valleys are actually the remains of extinct mountains, ancient seas, shredded continents, and titanic collisions. The evidence of this ancient chaos survives in the stone underfoot.
Stone must form before a story is written in it, and stone forms in three ways. Sedimentary rock forms when particles— eroded sand, dissolved insects, decomposed plants, or even volcanic ash— collect and cement together over thousands or millions of years. Igneous rock forms when molten stone seeps upwards from Earth’s superheated, molten innards (the mantle), cooling at or near the surface into solid material. Metamorphic rocks are harder, warped and folded versions of their sedimentary or igneous precursors, atomically strengthened by the heat and pressures endured in mountain building.
These three rock types compose about a dozen major plates, arranged like a cracked eggshell over the earth’s mantle. The plates constantly drift and collide, powered by the convection oven below, in a phenomenon called plate tectonics. Plate collisions form mountains, while rifting plates create ocean basins and rewrite continental borders. Interacting plates create most of New England’s geologic features.
1 billion years ago: The story begins during a time when North America’s plate boundary ran a couple hundred miles inland of today’s coastline. Two continents smash together along this border and buckle the crust edges upward into a range of Himalayan proportions called the Grenville Mountains. The force of the collision sutures the plates together for the next half a billion years. While the region waits for the next big tectonic event, the colossal Grenvilles slowly erode into obscurity. The ereoded sediment later becomes the source material for much of Vermont’s bedrock.
500 million years ago: the supercontinent finally splits down the middle of Vermont and the two sides drift in opposite directions. The crust near the boundary stretches and sags before separating entirely. Water pools in the depression, creating a shallow ocean called the Iapetus Sea that persists for the next hundred million years. The sea collects eroding Grenville sediment, dissolved carapaces of now-extinct sea life, and falling ash from volcanoes erupting in the midst of the tectonic turbulence. That sediment, thousands of feet thick, cements into the dominant sedimentary bedrock across Vermont.
450 million years ago: an incoming westbound landmass collides with the North American plate. The Iapetus Sea squeezes out of existence as the incoming landmass surfs overtop the edge of North America. The leading edge of the landmass scours and compresses the Iapetus-derived sedimentary rocks westward like a plow pushing snow down the driveway. This terminal “snow bank” is the proto-Green Mountain Range. After about 100 million years, the incoming plate finally stalls out as the Green Mountains push up against the monolithic roots of old Grenville bedrock (which would later rise again as the Adirondacks). These events collectively describe the mountain-building period called the Taconic Orogeny.
350 million years ago: just as the Taconic activity settles, a third plate rear-ends the stalled landmass on its eastern side. Because the landmass is still blocked to the west, the eastern side crumples like an accordion, piling new bedrock against and atop the proto-Green Mountains, rising the range to twice its present-day height. This second collision period describes the Acadian Orogeny.
The compressive forces during both orogenies mold some of the Iapetus sedimentary bedrock into their metamorphic versions. Whether the bedrock formation is sedimentary, metamorphic, or variations thereof depends only on the degree of squashing. Road-cuts along the interstate through the Green Mountains (where compression was greatest) expose cross-sections of these warped rock layers.
While the tectonic plates slide over one another during the orogenies, the submerged plate boundaries plunge into the mantle from the weight of the plates overtop. The pressurized crust melting in the furnace cooks upward through fissures and faults, forming chains of volcanoes and magma plumes at the surface. These igneous domes, exploited today as granite quarries, punctuate Vermont’s abundant Iapetus bedrock.
200 million years ago: the landmass so involved in shaping Vermont drifts away. The plate rifts well east of Vermont, forming today’s eastern seaboard. The rift widens and births the Atlantic Ocean. After half a billion years of activity, Vermont is finally removed from the tectonic collision zone.
Vermont is now taking a well-deserved break from cataclysmic mountain-forming, but the landscape is still periodically molded by a different process. Glaciers in the last hundred thousand years eroded mountaintops, pulverized bedrock into soil, relocated loose bedrock, and cut rivers valleys throughout the state. If topography is a sculpture, then bedrock is the putty, tectonic plates are the artists, and glaciers add the final touches. In the next billion years, that sculpture will be melted and reworked into future chapters in Vermont’s geological story.
|Some gneiss schist.|