The Earth is very very very old. Roughly 4.54 billion years old, or 4,540 million years old, or 4,540,000 thousand years old (however you would like to think of it). Prior to the development of radiometric dating to give us numbers and absolute dates we mostly used fossils and other principles to obtain relative dates of rocks. So all of our subdivisions of time are based upon the fossil record. The Precambrian refers to an immense span of time (4,540 million years ago to 542 million years ago) where the rocks contain very few fossils due to the fact that organisms never developed hard parts that could be fossilized until later in Earth’s history.
Zoroaster Granite and Vishnu Schist
At the bottom of the Grand Canyon we find rocks that formed during the Precambrian, two of which are the Zoroaster Granite and Vishnu Schist. In general a granite is defined as an igneous rock that forms from the crystallization of magma while a schist is a metamorphic rock that forms when shale is subjected to heat and pressure.
These rocks formed roughly 1,700 million years ago when a tectonic collision occurred between an older section of our continent that stretches from Southern California to Wyoming (dubbed Wyomingland) collided with a volcanic island arc. This collision of Earth’s plates provided the heat and pressure to morph the shale that formed the Vishnu Schist while producing the magma that formed the Zoroaster Granite. The Zoroaster literally intruded into the Vishnu as magma before cooling and solidifying to become a rock itself. We came across the first exposures of these rocks within the inner gorge of the canyon along side Bright Angel Creek:
This is a spectacular example where you can see huge sections of the Vishnu schist that broke off into the magma chamber before the magma cooled and solidified locking them into the place they are now.
By observing that there are inclusions of the Vishnu inside of the Zoroaster you can determine which rock is older than the other. The Vishnu must have been there first for the magma to intrude into it and break off pieces.
The Great Unconformity
Further upstream we got some impressive views of an interesting natural phenomena called an unconformity. In fact, this is known as the Great Unconformity which is represented here by an angular unconformity. The Great Unconformity was first identified in the 1800s by John Wesley Powell, a one-armed civil war vet who was the first person to lead an expedition down the Colorado River and later became the second director of the USGS. Here is a picture of the unconformity:
Can you spot it? Does it help that Kristi is pointing directly to it? Clearly there are beds of tilted rock dipping towards the right but are truncated at the top by horizontal bedding of a different rock. What in the world would cause this to happen? These are two different types of sedimentary rocks sitting right on top of each other at different angles. Would natural processes deposit the original sediments like this right on top of each other? The answer is no. The two rocks are not conformable. The “line” separating the two is an erosional surface which represents a gap in time that we call an unconformity. The rock unit on top is known as the Tapeats Sandstone that’s roughly 520 million years old while the unit on the bottom is known as the Dox Formation that’s roughly 1,120 million years old so there’s somewhere around 600 million years of missing time in between the two.
Here’s how this works:
Pretty cool right? Unconformities are found throughout the Grand Canyon and all throughout the world. Sometimes they are hard to identify but this one stands out as a prominent feature along the rim of the inner gorge.