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When viewed from the top, the natural monument seems beautiful and very awe-inspiring. However, it\u2019s when it\u2019s 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\u2019d see at your last wedding party\u2014filled with layers upon layers of rock, stacked one atop the other.<\/p>\n\n\n\n
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Here, Earth\u2019s 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\u2014mainly 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\u2019s 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\u2014a testament, if any, to the sheer power of Earth\u2019s geological forces, which are capable of warping even tons of rock like it was a sheet of paper.<\/p>\n\n\n\n
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.<\/p>\n\n\n\n
This remains true in most parts of the canyon\u2014up 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\u2019 worth of rock\u2014rock that, quite simply put, isn\u2019t there. This geological anomaly is referred to as the \u201cGreat Unconformity,\u201d and it has puzzled scientists for decades.<\/p>\n\n\n\n
The Great Unconformity<\/h2>\n\n\n\n
The rock anomaly isn\u2019t 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\u2014as if the nearly 1 billion years\u2019 worth of geologic time separating the two was never actually there.<\/p>\n\n\n\n
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\u2014zircon<\/em>\u2014forms 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\u2019s embedded in experiences huge tectonic forces or vast temperature changes. This mineral also contains traces of uranium\u2014a 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\u2014which is likely very close to when the zircon in the rock, and thus the rock itself, was first formed.<\/p>\n\n\n\n It\u2019s 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\u2019t react with the elements in the rock around it. And, being a gas, it has a tendency to escape the rock matrix\u2014especially when the atoms in the matrix are mobile, like when it\u2019s still molten. Thus, when helium is generated from radioactive decay when the rock is already cool, there\u2019s 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\u2019t form at the same time at different places.<\/p>\n\n\n\n Much like what the title suggests, scientists still don\u2019t 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<\/em> around 750 million years ago. (And yes, Pangaea<\/em> wasn\u2019t the first one; it wouldn\u2019t show up until at least 400 million years after Rodinia rifted apart.)<\/p>\n\n\n\n Despite this, experts mostly agree on the consensus that Rodinia\u2019s split isn\u2019t the sole <\/em>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\u2019s all a matter of asking the right ones. Geologists are still hard at work determining our planet\u2019s turbulent past, studying the rock one piece at a time.<\/p>\n\n\n\n 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\u2019s not just something pretty and amazing\u2014you\u2019re staring at what could be a billion years\u2019 worth of Earth\u2019s history, and viewing it all at once.<\/p>\n\n\n\nSo, What Caused It?<\/h2>\n\n\n\n
Bibliography<\/h2>\n\n\n\n