Travelers to the United States of America, or even those aspiring to travel there one day, will most likely know of the Grand Canyon. Located in northwestern Arizona and labeled as a U.S. National Monument back on January 11th of 1908, this vast crevasse was created as the Colorado River coursed through the rising Colorado Plateau—perhaps as early as 6 million years ago. This process, slow as it may seem at first glance, eroded the rock as the waters of the river passed over it, deepening and widening this “scar” on the Earth. What it left behind, luckily for us humans to see, was a strikingly beautiful geological formation; it laid bare hundreds of millions of years of geologic time for anyone to see. What would otherwise require multiple machinery and quite possibly hundreds of years’ worth of digging was instead done by nature. Granted, it took quite a bit more than a few hundred years to do so, but it has left behind a treasure trove of geological curiosities that will keep geologists busy for hundreds of years. And the process hasn’t stopped—the canyon is still getting deeper, as the Colorado River still courses through its deepest parts, slowly eating away more rock in the process.
When viewed from the top, the natural monument seems beautiful and very awe-inspiring. However, it’s when it’s viewed from the side that it truly piques the interest of those looking for a bit more than just pictures when visiting the Grand Canyon. When viewed facing its edges, the Grand Canyon unravels like a tall cake you’d see at your last wedding party—filled with layers upon layers of rock, stacked one atop the other.
Here, Earth’s history is opened like a book waiting to be read. As geologic time passes, older rock is constantly buried under newer rock at the top. Here, layers of rock can be compressed and contorted, perhaps by the weight of the rock above it. The rocks can also vary in consistency, quality, and size—mainly due to the differences of what that layer of rock used to house at the time when it was at the top of the layer. It’s these differences in what that layer of rock went through that led to this distinct layering. Some were simply stacked one atop the other, like pancakes for breakfast. Others have been tilted to oftentimes outstanding degrees of tilt—a testament, if any, to the sheer power of Earth’s geological forces, which are capable of warping even tons of rock like it was a sheet of paper.
Thus, the deeper you go, the older the rock becomes. It stands to reason, then, that given a relatively pristine vertical slice of rock from the canyon unperturbed by past geological events, there should be a continuous line of rock that constantly gets older the farther you go from the surface.
This remains true in most parts of the canyon—up to a point. There exists an anomaly, sandwiched between layers of rock; the two layers, when dated, reveal that they should be separated by up to a billion years’ worth of rock—rock that, quite simply put, isn’t there. This geological anomaly is referred to as the “Great Unconformity,” and it has puzzled scientists for decades.
The Great Unconformity
The rock anomaly isn’t just isolated to the Grand Canyon; other parts of the world have this strange rock gap, too. In fact, in the Ozark Plateau, another geological area shared by the U.S. states of Arkansas, Missouri, Oklahoma, and Kansas, it exists as a gap between a layer of 1.4-billion-year-old granite, and 500-million-year-old sandstone that sits right above it—as if the nearly 1 billion years’ worth of geologic time separating the two was never actually there.
To determine how old these layers of rock are, geologists can use several techniques. One of which, radiometric dating, takes advantage of the fact that one of the most ubiquitous minerals in these rocks—zircon—forms while the rock is still molten as magma. These minerals are very stable and hardy, often resisting changes to itself while the rock matrix it’s embedded in experiences huge tectonic forces or vast temperature changes. This mineral also contains traces of uranium—a radioactive element. Thus, scientists can determine what the age of the rock matrix is based on what elements are present in the rock, and how many of it there are; as uranium undergoes radioactive decay, its reaction products can escape into the rock matrix surrounding it. From the elements that are identified, scientists can know at what stage of radioactive decay the uranium inside it is in; and since we know how uranium decays, we can trace the by-products back to when the uranium first started decaying—which is likely very close to when the zircon in the rock, and thus the rock itself, was first formed.
It’s this technology that led scientists to this great anomaly, and it will likely help lead scientists to the answer for it, too. In fact, in a recent study published in 2021, scientists tracked the helium present in the rock of the Grand Canyon. Helium, being an inert gas and one of the by-products of uranium radioactive decay, simply doesn’t react with the elements in the rock around it. And, being a gas, it has a tendency to escape the rock matrix—especially when the atoms in the matrix are mobile, like when it’s still molten. Thus, when helium is generated from radioactive decay when the rock is already cool, there’s nowhere left for the helium to escape, and it stays embedded in the matrix. Scientists can then use the amount of helium left in the rock to determine its age. Using this method, the researchers determined that there are differences in the time gap presented by the anomaly across different parts of the canyon, meaning that the gap didn’t form at the same time at different places.
So, What Caused It?
Much like what the title suggests, scientists still don’t have a uniform answer to this question, as there is much work to be done in order to fully understand what is going on. One of the major culprits is likely the breakup of the supercontinent Rodinia around 750 million years ago. (And yes, Pangaea wasn’t the first one; it wouldn’t show up until at least 400 million years after Rodinia rifted apart.)
Despite this, experts mostly agree on the consensus that Rodinia’s split isn’t the sole explanation for the Great Unconformity. Likely, several geological events occurred during this tumultuous span of geologic time of roughly a billion years, which may be likely related to a supercontinent breaking apart into pieces. Like most unanswered questions in science, it’s all a matter of asking the right ones. Geologists are still hard at work determining our planet’s turbulent past, studying the rock one piece at a time.
At the very least, knowing all this gives us a new perspective on places like the Grand Canyon, or perhaps at other geological features around you that look like layers of rock, like a cliffside or a mountainside. That’s not just something pretty and amazing—you’re staring at what could be a billion years’ worth of Earth’s history, and viewing it all at once.
Bibliography
- Joel, L. (2018, February 5). Erasing a Billion Years of Geologic Time Across the Globe. Eos. Retrieved September 13, 2021, from https://eos.org/articles/erasing-a-billion-years-of-geologic-time-across-the-globe
- Pappas, S. (2021, August 27). A billion years of geologic history is missing from the Grand Canyon. LiveScience. Retrieved September 13, 2021, from https://www.livescience.com/great-unconformity-supercontinent-breakup.html
- United States Geological Survey. (n.d.). Grand Canyon Geology. United States Geological Survey. Retrieved September 13, 2021, from https://www.usgs.gov/science-support/osqi/yes/national-parks/grand-canyon-geology
- Peak, B. A., Flowers, R. M., Macdonald, F. A., & Cottle, J. M. (2021). Zircon (U-th)/he thermochronology reveals pre-great unconformity paleotopography in the grand canyon region, usa. Geology. https://doi.org/10.1130/G49116.1
