Grand Canyon’s Great Unconformity-Part 1
Written by Wayne Ranney, Grand Canyon Geologist and Author
Whether you’ve taken one or seven Grand Canyon river trips or are preparing for your first, you might have heard of a geologic attraction called The Great Unconformity. It often seems as if this feature is more famous than the Canyon’s more well-known and named rock layers, and is talked about as much as how Grand Canyon was formed. In spite of its popular moniker some people are confused about what The Great Unconformity represents and why it’s mentioned so much in the Canyon. In its simplest description, The Great Unconformity (or TGU) is a large gap in the geologic record that is the result of long periods of erosion where rocks were removed during a long interval of time.
Here’s an Analogy to Help Explain
Suppose you are reading a book where the plot centers on the childhood of a young girl. As the story unfolds the girl first rides a tricycle, then grows up a bit to ride a two-wheeled bike. You turn the page and suddenly the same character is teaching her grandson how to ride a bicycle. Wait a minute? What happened to all of that time that certainly must have happened in between? Well, the time itself hasn’t gone missing, but somehow the record of what happened during that time is missing. On the shelf, the book looks normal with no interruptions evident in the pages but the gap in the story doesn’t make sense. Forming a hypothesis causes you to search for evidence and a possible answer to the mystery. Looking more closely at the page numbers reveals that page 51 is followed immediately by page 177. The gap in the page numbers shows that someone physically removed some pages (the book technically has an unconformity!).
The Great Unconformity is a Very Large Gap in the Rock Record
In Grand Canyon, TGU is represented where the horizontal layers of the Tapeats Sandstone rest directly on top of the crystalline and more vertically oriented Vishnu Schist and Zoroaster Granite. The schist and granite we can touch today in the floor of the Canyon formed between 10 and 15 miles deep in the Earth’s crust. This means that before the Tapeats Sandstone could wash in over them, those 10 to 15 miles of rock first had to be removed. This is a mind-boggling concept that helps explain why TGU is such an attraction in Grand Canyon! And it begs so many other questions: how do we know that this specific amount of rock was removed, when did it happen and where did all of that eroded rock go? The answers help illuminate why a river trip in the Grand Canyon can be such a life altering experience.
How was this Specific Amount of Rock Removed?
Regarding the thickness of rock removed, the answer comes from garnet minerals found in the Vishnu Schist. These garnets form only within specific temperatures and pressures that today are located 10 to 15 miles in the subsurface. So even though the Vishnu Schist is exposed at the surface today, we know these same rocks had to have once been buried by that much material – how else could the garnets form. As to the age of the Vishnu Schist and Zoroaster Granite geologists again look at certain minerals within the rocks that document their age of crystallization. These minerals contain radioactive elements that break down into other elements at known and constant rates. Laboratory techniques measure the ratio of the original element with the by-product element, yielding the age that the rocks formed. Generally, these crystalline rocks are about 1,700 million years old, or said another way 1.7 billion years old.
The Tapeats Sandstone is dated using the same method and is much younger, being “only” about 508 million years old. This means that TGU represents a whopping 1,200 million (or 1.2 billion) years that is missing from the rock record. Keep in mind that this does not represent missing time, rather it is a missing part (or gap) in the record for that period of time. When the 10 to 15 miles of crust was being eroded away, not only was part of the crystalline rock record being destroyed, but no other rock could be preserved during this long period of erosion.
And Where did it all Go?
As to where all that eroded material went – well, we know it did not go to outer space! And while you might think I’m just being funny with that answer, remember that eroded material is typically washed downstream into a basin where it collects as sediment and then buried to become sedimentary rock. Since this happened such a long time ago, these sediments have not been identified (or were likely eroded as well). They could have been buried deep enough to become metamorphic rocks or even melted completely to become igneous rocks. In turn, these could have been uplifted once again wherever they were located, and then eroded once again.
These concepts highlight the notion that our planet is the ultimate recycler – rocks of any type are uplifted into mountains, where they are eroded to become sediment, and washed downstream into a basin to become sedimentary rocks, then perhaps buried deeply enough to be metamorphosed or even melted. Endless cycles of uplift, erosion, sedimentation, metamorphism, or melting. Repeating endlessly throughout time.
You Can See the Great Unconformity on a Grand Canyon Rafting Trip!
In Grand Canyon, the best place to see and touch TGU is at River Mile 121 in Blacktail Canyon. Here TGU is at eye level only a short walk into the mouth of the canyon (it is often hundreds or a thousand feet or more above your head while floating on the river). And it is said that if you are lucky enough to visit Blacktail Canyon and run your fingers along this hairline contact between the crystalline rocks and the Tapeats Sandstone, you’ll never get a failing grade in a geology class again!
In Part II we will examine if anything can be known from the 1.2 billion year gap in the rock record in Grand Canyon.
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