Great Unconformity –Grand Canyon, Arizona
So just like intrusive igneous rocks, metamorphic rocks require great lengths of time to accomplish the uplift and erosion in order to be exposed at Earth’s surface.
So what do we make of this photograph? It shows a sequence of sandstone, shale, and limestone sitting on top metamorphic rock (called the “Vishnu Schist”) in the Grand Canyon. The sandstone was deposited right on top the schist.
Since sedimentary rocks, like sandstone, shale, or limestone, are deposited at Earth’s surface –and metamorphic rock forms beneath the surface, this photo shows that BEFORE the sedimentary rocks were deposited, the metamorphic rock (schist) had to have been uplifted and exposed. So all the time required to bring the schist to the surface had to take place before the sandstone was even deposited.
The surface of contact between the sandstone and the schist is called an unconformity because it is here that we see evidence for a great deal of missing rock record. The sandstone must be much younger than the schist –for the very reason that the schist first had to get uplifted and exposed at the surface before the sandstone was deposited on top of it. So… because the sandstone is so much younger, but it was deposited right on top the schist, there must be a gap in the rock record between them … an unconformity.
And here is where we see evidence for even LONGER periods of time. Overlying the sandstone? Thousands and thousands of feet of more sedimentary rock. And much of that sedimentary rock was marine… formed at sea level. It is now over a mile above sea level.
And the schist itself? The people who’ve studied it have determined that much of it was originally volcanic –which means that it originally formed at the Earth’s surface. So… over geologic time, it must have been buried to the depths needed to turn it into a metamorphic rock BEFORE it was uplifted and exposed.
So… how old is Earth? Some say 6 or 10,000 years… I think we’re looking at 10s of millions in this photo. And if we consider the numerical ages for these rocks, 1.7 billion is the age of metamorphism of the schist –its original volcanic rock must have been older!