Now, when you step below the Grand Canyon Supergroup rocks, or below the Tapeats Sandstone, depending on what trail you’re hiking, you will have gone back 1.7 billion years, 1700 million years! As you enter the Inner Gorge of Grand Canyon, and you find yourself suddenly surrounded by twisted shiny black schist and streaks and blobs of salmon-pink granite, you have truly entered Deep Time. Be prepared for a world that doesn’t even slightly resemble the one you know today, or even the worlds you’ve traveled through to get to this deep layer in the canyon.
The Inner Gorge is made up of metamorphic and igneous rocks, there are no sedimentary rocks to be seen anywhere. The Vishnu, Brahma and Rama Schists are dark brown to black, twisted and mangled metamorphic rocks, and the Zoroaster Granite is the rich pink and white granite that you see within the schists.
So how did this stuff form? The thing you have to remember is that metamorphic rocks and most igneous rocks don’t form at the earth’s surface. The kind of heat and pressure that it takes to metamorphose or melt rock happens way down deep, sometimes miles below the surface of the earth. So keep that in mind as we go.
About 1.7 billion years ago, our continent was much smaller than it is now – in fact it ended somewhere up around Wyoming. Instead, all was ocean in this region. Rolling in from the south like a massive conveyer belt was a huge piece of the earth’s crust. As this piece collided with the continent, it was forced back down into the depths of the earth, in a process known as subduction. This is currently happening right off the coast of Oregon and Washington as a little piece of the earth’s crust known as the Juan de Fuca Plate is being subducted under North America. As the plate was subducted, sediments of the sea floor, bits and pieces of ancient volcanic islands, and volcanic rocks of the crust were all heated and squeezed, and began to metamorphose into schists and gneisses. The different names, Vishnu, Rama and Brahma Schist, refer to what kind of rock provided the source for that schist. By looking at the types of minerals and features found in these rocks, geologists estimate that the rocks of the Inner Gorge were formed around 12 kilometers, or 7 miles below the surface of the earth! At the same time, some of the plate that had been taken deep down began to melt. This magma rose back towards the surface in big blobs, like the blobs in a lava lamp. The molten magma squeezed its way into the metamorphosing rocks around it, and began to cool and harden into veins and masses of pink granite.
What was happening on the surface while all this was going on 7 miles down? Well, we used to say that there was a huge mountain range forming up on top, but geologists now believe that the surface relief may not have been all that great. Instead, perhaps there were volcanic islands and low mountains forming from the eruptions of the magma that made it to the surface, and from the crumpling of the earth’s crust that occurs during collisions between crustal plates. What we do know is that by about 1.4 billion years ago, the collision seems to have stopped, and erosion took over as the dominant force in the region. By 1.2 billion years ago, enough stuff on top of the schist had eroded that it had risen back to sea level, kind of like the way an iceberg rises out of the water to balance the melting of its top. How do we know that the schist and granite had risen 7 miles to sea level by 1.2 billion years ago? Well, the rocks of the Grand Canyon Supergroup were all formed right around sea level and they sit right on top of the schist. Thirteen thousand feet (at least) of these Supergroup sediments were laid down on top of the schist, all right around sea level, so as they were deposited they were pushing the schist back down again. Then the Supergroup rocks (and everything underneath them) were broken and tilted along fault lines, so the schist was lifted up again. By 500 million years ago, we know that the schist was at sea level again. How? Because the Tapeats Sandstone records the advance of an ocean that laid a sandy beach right on top of the schist. All the layers on top of the Tapeats, including the thousands of feet of sediment that used to be on top of the rim and have now eroded away, helped push the schist back down again. Now, at the present day, the schist lies 2,000 feet above sea level. It’s been quite the trampoline ride for that schist and granite, hasn’t it!
As you wander around the Inner Gorge, perhaps on your day off at Phantom Ranch, be sure to examine the rocks of the Inner Gorge. The granite is filled with glassy gray quartz, shiny silver mica and gorgeous salmon pink pieces of orthoclase feldspar, a type of mineral rich in potassium. The schist often looks like it is formed in layers that are now tilted up vertical. Rather than original layers, these are lines that indicate that the schist was formed under extreme pressure. The pressure caused all the minerals in the rock to line up parallel to each other, and it creates a “layered” look. If you see a fresh surface of the schist, it will often look like it is filled with silver. The silver is huge amounts of mica, one of the most common minerals in this schist. Sometimes you’ll see metamorphic rocks in the Inner Gorge that look more banded than layered, with dark and light bands running through the rock. This is not a schist, but a gneiss, formed under even greater heat and pressure. Finally, look and see if you can find little tiny veins of granite or quartz that are folded into tiny tortured curves like a human intestine. These formed when the granite intruded into the schist as it was bending, and so the granite bent as well. Imagine yourself seven miles down, with the heat and pressure so intense that it causes the rock to twist and bend like silly putty, with veins of molten magma squirting up all around you. Quite a different world than the calm seas and wind blown dunes of later times!
As you sit cooling your feet in Bright Angel Creek, remember to look up and think of all the time you just passed through, literally billions of years of earth history. Ancient mountains and rivers, oceans and sand dunes, molten magma and shifting plates, all laid out for you to see and imagine and be a part of. Some people say they feel insignificant when they see themselves in Grand Canyon. But to me, it feels very significant to be a part of all that history, and we are. We humans are just the latest page in a book that has been written now for four and a half billion years, and will continue to be written long after we are gone. Pretty wild, eh? Now, go get some dinner!