|What is the oldest chasm on the Earth? I have been to Ausable Chasm in Keeseville, NY and know the rock is 500 million years old. Is there an older chasm? Thanks for your help! |
|Question Date: 2020-05-14|
Thanks for an intriguing question and for introducing me to Ausable Chasm, which I hadn't heard of before, despite having gone to school in New York.
This is going to sound nit-picky, but your question can be read in one of two ways: what is the oldest chasm (i.e., gorge or canyon on Earth), or what gorge cuts into the oldest rocks?
In geology we use something called the principle of cross-cutting relationships, which states that a geologic feature cutting another geologic feature is necessarily the younger of the two features. At Ausable Chasm, the Ausable River cuts the 500 million-year-old Potsdam Sandstone, so we know immediately that the gorge is younger than 500 Ma. Indeed the gorge is much much younger than that, probably no more than a couple of million years at most.
In the sections of the drainage where the slopes are steepest (cliffy), I suspect the feature is even younger. That's because nothing remains static in geology--although mountains are uplifted and canyons carved, erosive processes work to knock those features down, grinding down the sharp features first.
So, the very act of cutting a canyon or pushing up a mountain, insures that feature's ultimate destruction. The mountains in New York are low and rounded because they're old and deeply weathered, while those in the Himalaya are high and sharp because they're young, and indeed still growing. Same with a canyon. The Grand Canyon is deep and cliffy because the Colorado River has only cut through those strata quite recently (a few million years). With time those cliffs will collapse and erode, and the Grand Canyon will look ever more like the Mississippi drainage.
The Colorado River, in fact, cuts rocks that are almost two billion years old, so in that sense, the inner gorge of the Grand Canyon, "is older" than Ausable Chasm.
The oldest widely exposed rocks on Earth are roughly 4 billion years old. I'm sure in a few places on Earth very young canyons cut rocks even older than those of the Grand Canyon, but off the top of my head, I don't know what the best candidates are.
As for what the longest-lived canyon/chasm on Earth is, I don't know that either, but all will be quite young, geologically speaking.
Sorry for such a non-definitive answer.
"Geologically simple, the Ausable Gorge was carved out of 500 million-year-old Potsdam Sandstone from the Cambrian Period. Since the end of the Pleistocene Epoch ice age 10,000 years ago, the movement and subsequent melting of glaciers created a series of caves and tunnels, which the Ausable River linked up and exploited on its one mile (1.6 km) journey to Lake Champlain. During this time, the headward erosion of ancestral Rainbow Falls led to its location today, where the visitor's center was built."
So the carving out took place very recently geologically speaking as water from melting glaciers worked its way to local base level. That is what carved the chasm. The chasm is carved in Cambrian 500 million years sandstone called the Potsdam sandstone. There are many such features all over the planet Earth.
The most spectacular example would be the colors do river Grand Canyon… here the rocks at the base are billions of years older than the Potsdam sandstone.
The Grand Canyon was created mainly due to tectonic uplift… the ancestral Colorado river was already there and as the land went UP the river kept cutting down.
The oldest canyon in the world is the Grand Canyon. At the Ausable Gorge, the river runs through the Potsdam Sandstone (Cambrian in age, and famous for sweet jellyfish fossils!). So the rocks there ARE very old - but, the canyon isn't! Off the bat, what we can tell is that if the rocks are 500 million years old, the canyon must be younger than that, because the rocks had to exist before the river could run through it!
The rocks at the Grand Canyon are anywhere from 250 million to 2 billion years old. So, using the above logic, we know thatthe Grand Canyon must be younger than 250 million years old.
So,how old are they? The Grand Canyon started forming ~17 million years ago, when the Colorado Plateau (the name geologists give to that part of the land where the Colorado River has cut through) began to move upwards (we call this tectonic uplift). That uplift is still occurring today, so the Grand Canyon has been forming for the past 17 million years!
The Ausable Gorge formed from a similar process (water cutting through rock), but for a different reason! After the last "ice age", the glaciers which covered much of North America started melting. All that meltwater (the Ausable River) carved a canyon through the Potsdam Sandstone. So, even though the rocks at Ausable Gorge are older than the youngest rocks of the Grand Canyon, the Grand Canyon started forming long before the Ausable Gorge did (~10,000 years ago or so)!
That is a rather difficult question, because there are two different ages you could be referring to when you discuss the age of a chasm. One is the age of the rocks themselves, and the other is how recently the chasm was cut into them. For instance, while the rocks of Ausable Chasm are indeed quite old, they were probably only exposed in the past few tens of thousands of years.
To make things even more complicated, since younger rocks are deposited on top of older ones, and erosion cuts from top to bottom, the oldest rocks in the bottom of a chasm are the ones that have been most recently re-exposed. Also, the longer a chasm exists, the more the surrounding land is eroded, so that it eventually becomes more of a valley than a chasm. Finally, I do not believe that there is any one official definition of a chasm, so to search through all of them, you would also have to consider landforms described as canyons, fissures, rifts, and gorges, etc. That said, the best example I can find of old rock in a chasm is the Elves Chasm Gneiss, which is the oldest exposed rock in the Grand Canyon. This rock is approximately 1.84 billion (1,840 million) years old, over three times the age of the Potsdam Sandstone found at Ausable Chasm. On the other hand, the Grand Canyon was only eroded about 6 million years ago, so the "chasm" itself is still fairly new.
The age of the rocks at the bottom of the chasm do not tell you how old the chasm itself is. For example, the Grand Canyon in Arizona is between six and seventy million years old depending on whom you ask; however, the rocks at the bottom of the Grand Canyon are nearly two billion years old. This because the Grand Canyon was carved into the Earth by the Colorado River, gouging out a valley that it flows through.
However, to wager a guess at your question: erosion on Earth tends to destroy chasms by filling them in with sediment. This isn't the case on other planets that have less atmosphere. Thus, I would expect that Valles Marineris on Mars is older than any chasm here on Earth.
There are rocks on the surface of the earth that are more than 3 billion years ago.
The oldest rocks that are part of the Vishnu Basement Rocks is the Elves Chasm Gneiss.
That wikipedia article says: "The Brahma Schist has been dated to about 1.75 billion years ago. The felsic metavolcanic rocks that comprise the Rama Schist have yielded an age of 1.742 billion years ago." And it says the Elves Chasm has some rocks "dated at about 1.84 billion years ago".
This article here says "the Ausable Gorge was carved out of 500 million-year-old Potsdam Sandstone from the Cambrian Period." It agrees with you ;-]
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