Today is a break from the Devonian, and a thanks to listener Thatcher Hogan for suggesting the Adirondacks as a topic.
I mentioned the Adirondacks back in the Precambrian, because the rocks are part of the Grenville Terrane which collided with the Superior Craton, the ancient core of North America, about 1.1 billion years ago. But the modern Adirondacks are much younger than that old mountain-building event. The rocks were buried deeply – perhaps as much as 15 miles below the surface – during the Grenville Orogeny. They were also intruded by various magmas. The result of the high pressures and temperatures was a wide variety of both igneous and metamorphic rocks.
The Adirondacks of northern New York are a strange little range, almost circular in shape. It’s really a large dome, a circular geological uplift. The oldest rocks, uplifted the most, are in the center, with younger rocks draping the flanks of the dome. And this uplift is really quite young, beginning around 5 or 10 million years ago – just yesterday, geologically speaking – and continuing to the present. So the present mountains have nothing to do with the ancient Grenville mountains, and also nothing to do with the Appalachians – which today are relatively low, eroded hills, a remnant of the mountain building events of the Ordovician, Silurian, Devonian, and Carboniferous. So why are the Adirondacks there?
The circular dome shape suggests some kind of force pushing up from depth, and you get domes where things like magmatic intrusions, cylinders of molten rock, like the neck of a volcano but down within the earth, rise. They push the rocks above them up like your fist pushing up in the middle of a blanket. Salt, which is not usually molten in the earth, can flow plastically under pressure, and salt domes can do the same thing to the rocks they rise up against and through.
The Precambrian core of the Adirondacks is exposed because those old rocks were uplifted, along with a thick pile of younger sedimentary rocks. It’s probable that sedimentary rocks, including the Cambrian Potsdam sandstone (February 19) and strata from the Ordovician, Silurian, and Devonian were laid down over the region where the Adirondacks now stand. But those rocks were eroded off the rising Adirondacks, mostly in the past 5 or 10 million years or so.
It would have to be something big to rise from great depth to produce the huge dome at the Adirondacks. Not a salt dome, and not the small uplift around a rising magmatic intrusion. Those kinds of things make domes that are maybe one to 5 miles across, maybe 10 miles at most. The Adirondack Dome is 160 miles in diameter.
To be honest, we really don’t know why the Adirondack Dome began to rise, and why it continues to rise – by some estimates, one of the fastest-rising mountain ranges on earth, perhaps as fast as 1 or 2 millimeters a year, which is actually incredibly fast. There is controversy over uplift rate estimates, so stay tuned for more research on that.
The best guess – and it really is a guess – is that there was a hotspot beneath the Adirondacks. Hotspots are regions of relatively low-density mantle, many tens of miles within the earth, that tend to rise buoyantly through denser parts of the mantle. Such a blob, pushing up, could make the broad dome that we see in the Adirondacks. Hotspots are well known, especially those that get shallow enough that reduced pressure allows the hot rocks to melt. Then you can get volcanoes. There is a hotspot beneath Hawaii, one under Iceland, and one under Yellowstone. There are a few dozen around the world.
Out in the Atlantic Ocean there is evidence for a hotspot that the Atlantic Oceanic Plate has been moving over for some time. The track is represented by seamounts, essentially flat-topped eroded volcanoes that might have once been something like today’s Hawaiian chain. The hotspot that made those volcanic seamounts is called the New England Hotspot or Great Meteor Hotspot. Don’t let that name throw you – it has nothing to do with a meteor impact. The seamount that gives its name to the hotspot, Great Meteor Seamount, was named for the German research vessel Meteor, whose scientists discovered the seamount in the 1920s. It’s possible that the Adirondacks represent an uplift above that hotspot when it was beneath the continent. There are problems with that idea – like why is the only domal uplift at the Adirondacks? Maybe the hotspot hung out there longer. Or maybe the crust was a bit weaker there. There are problems with the timing, too – the hotspot would have been under the Adirondacks well before the modern uplift. So maybe the hotspot weakened the crust, so something else made it rise in the past 5 million years or so. Or maybe something else.
This is an interesting and enigmatic question, and the bottom line is we really don’t know for sure why the Adirondacks are there.
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Harry Hess was born May 24, 1906, in New York City. He spent most of his geological career as a member of the faculty at Princeton University, and he is considered to be one of the founding fathers of plate tectonics theory. He came up with the concept of sea-floor spreading, based on the discovery of mid-oceanic ridges and rift valleys. Even though it was not completely understood as a mechanism, it was the lynchpin in the idea of continental drift, which had been largely ridiculed, at least in the United States, because there was not only no mechanism to explain it, there was no evidence that even started to explain it. Mid-ocean ridges and sea-floor spreading provided that important basic starting point as a hypothesis, and work addressing that idea uncovered the evidence for the mechanism – convection currents of heat within the earth’s mantle that drive the motion of plates in the crust.
—Richard I. Gibson
Good overview of Adirondack geology
Map from USGS