The famous and absolutely stunning Grand Canyon in Arizona formed largely within the last
6 million years due to the carving of the Colorado River.
Yet, this canyon, which is over a mile deep in some areas, is also the site of one of
Earth's longest and greatest geologic mysteries, and no I am not referring to the Grand Canyon
itself.
As, due to the erosion of many rock units by water, segments of rock ranging from 270
million to 1.8 billion years old are exposed for all to see, providing a fascinating geologic
record.
Yet, if we zoom in to a specific layer of rock you will notice that all of a sudden
what was once flat rock in overlying layers suddenly becomes tilted and is not sedimentary
rock but rather metamorphic and igneous rock.
This sudden change in rock represents what is known in geology as an unconformity.
Here, the 1.7 billion year old Vishnu Schist is directly overlied by the 520 million year
old Tonto Group sedimentary rock which largely consists of dolostone, shale, and sandstone.
Thus, approximately 1.2 billion years of Earth's geologic record, representing about 26% the
entire age of the Earth is seemingly missing.
And, this is not an isolated occurrence.
Not only is this unconformity found across the Grand Canyon, but also in the Black Canyon
of the Gunnison in Colorado, to Grand Teton National Park, New Mexico, Nevada, and beyond.
In fact, although the extent of the seemingly missing geology varies, a large swath of Earth's
geologic history is missing across a fairly wide area of the planet.
And this, is the mystery of the so called "Great Unconformity" which was known to scientists
all the way back in 1858.
So, why would so much of Earth's geologic history be missing in what is now the American
southwest?
Although the exact cause is still debated to this day, the author of this video will
discuss one hypothesis which he believes is the reasoning behind the existence of the
Great Unconformity.
And, to clarify, this period of Earth's geologic history is not missing around the entire planet.
For example, here is a 1.4 billion year old granite from California.
And, here is a copper nugget I mined from the upper peninsula of Michigan which dates
to around 1.05 billion years old.
Both of these pieces date to between the 1.7 billion and 520 million year old gap of the
Great Unconformity.
For context, unconformities generally only form for one of two reasons.
Either one, a long period occurs without any rock deposition, or two, a period with substantial
erosion occurs, thus seemingly erasing a thickness of rock from the geologic record.
In the case of the Great Unconformity involving the Grand Canyon, the latter was almost certainly
the case.
While Earth is 4.54 billion years old, if you were to go back in time, there would only
theoretically be enough oxygen to breathe as a human for about the last 550 million
years.
Before this period, and before the Cambrian explosion where a widespread radiation of
complex lifeforms occurred, there was a period of Earth's geologic history known as the Cryogenian.
Whereas the average global temperature today is 13.9 degrees Celsius or 57 degrees Fahrenheit,
during the Cryogenian the average temperature dropped to as low as -10 degrees Celsius or
14 degrees Fahrenheit.
As a result, glaciers were found across much of the planet, including at higher points
on the equator.
In fact, if cryogenian conditions were to return today, everything that wasn't between
25 north and 25 south latitude would be covered in more than a kilometer of ice.
This means the entirety of the United States's 50 states minus Hawaii, most of Algeria, two
thirds of Australia, and all of Morocco would be covered in ice.
While some hypotheses suggest that earth was even colder, with a fully global spanning
ice sheet to the point that our planet looked somewhat similar to Jupiter's moon Europa,
I do not believe this was the case as there was probably a zone of cold but not frozen
water and land at or near the equator.
During two long term so-called "slushball Earth" periods between 715 and 660 million
years ago, and 650 and 635 million years ago, large ice sheets swept across the planet.
This carved deep into the crust, removing between 2 and 5 kilometers thick of rock.
This loose material was then swept into the oceans over time, where it was destroyed due
to eventual plate subduction, as almost all oceanic crust is destroyed within 200 million
years of its creation.
Thus, the cause of the great unconformity was a nearly planet spanning ice sheet which
removed the last 1 billion years of existing rock units at the time, which made Earth's
most recent ice age look insignificant by comparison.
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