The rocks that make up the Vishnu Complex, also called the Vishnu Basement Rocks, are old. Really, really, really old: about 1 3/4 billion years. Much of Canada is covered by similar rocks that are just as old or older, but other than up there you don’t see rocks like this in North America except under extraordinary circumstances. It is estimated that the Vishnu rocks were at one time over 16 miles under the surface of the earth, where they were cooked (metamorphosed) by extreme heat and pressure into the form we see them in today. Some huge geological process, likely the collision of two continents, raised them up fairly close to the surface over a billion years ago. But in the over a billion years since then, the Vishnu Complex rocks were buried under miles of sedimentary rocks. The upper formations in the Grand Canyon, starting with the Tapeats Sandstone and heading on up from there, are only the very bottom of a huge stack of formations that had to be eroded away to expose the Vishnu Complex. About 60 million years ago, relatively recent in geological terms, the whole Grand Canyon area was uplifted as part of the process that created the Rocky Mountains, shoving the Vishnu Complex rocks up to around the level that they are at now, but that still left thousands of feet of sedimentary layers on top of them. Finally, about six or seven million years ago, very recent in geological terms, something happened (just what is still a matter of debate) to make the Colorado River start cutting through those top layers to form the Grand Canyon, and finally exposing the Vishnu Complex rocks at the very bottom of the canyon as we see them today.
Here, standing on the Tapeats Sandstone rim of the inner canyon, near the Tonto Trail between Indian Garden and Monument Creek, we are looking down at the Vishnu, seeing it as it appears through much of the inner canyon, a mottled mix of craggy rocks, mostly black Vishnu Schist but with lines and splotches of pink Zoroaster Granite. The Vishnu Schist formed first, part of the mass of rock cooked deep in the earth. Sometime later, but geologically not very much later, volcanic magma pushed into it, filling up all the nooks and crannies, and cooled and crystallized into the granite we see today. Here you also see the Tapeats Sandstone smoothly layered on top of the Vishnu like icing on a cake. The Tapeats was deposited on top of the Vishnu by an ancient sea over 500 million years ago, when the Vishnu was already over a billion years old. We know that the formations in the Grand Canyon Supergroup were deposited on top of the Vishnu after a gap of several hundred million years, but there may have been many more formations layered on top of that. But if there were any additional formations they are long gone, ground down and washed away before the Tapeats formed, leaving the Great Unconformity, the huge gap in time between the Tapeats and everything underneath it.Not many Grand Canyon trails go up and down through the Vishnu. Most of those that do are in the main corridor and have blasted their way through. Here we are on the Bright Angel Trail in the Devil’s Corkscrew.
The trail makes an easy descent through the Vishnu, thanks to a wide, gently sloping trail that has been blasted out of the Vishnu cliff faces. You can see the trail further down where it has reached the floor of Pipe Creek Canyon, which it follows for another mile before reaching the Colorado River at Pipe Creek Beach. From there the route to Phantom Ranch follows the River Trail upstream along the Colorado, which again is an easy trail, but again was blasted out from the Vishnu. The North and South Kaibab trails also required plenty of dynamite, including a 100 foot tunnel blasted through the Vishnu to reach the southern end of the Black Bridge.
Above and to the left of the trail in the picture is a giant mass of granite I call the Pluton. The Bright Angel Trail passes right by it just before it enters the Devil’s Corkscrew, and you can easily scramble up on top of it for fine views. Imagine having to scramble down the sides of the Pluton to get to the floor of Pipe Creek Canyon if the park service hadn’t kindly blasted out a convenient trail for you.
A number of other trails and routes follow stream beds through the Vishnu to reach the Colorado River. Some of these routes are easy, but others are very difficult, requiring rappels or difficult bypasses to get around pour-offs. Here we are on one of the easy routes, the Granite Rapids Route from the Monument Creek Campground on the Tonto Trail down to Granite Rapids on the Colorado River. Notice the dark ridge, or fin, of Vishnu on the right. These routes pass one fin like this after another on your left and right, as they twist their way down canyon to the river. Every time I go down one of these routes I’m constantly asking myself – is this the last fin? – but there always seems to be another.
And then there are routes that simply scramble straight up and down through the Vishnu. Here we are on the lower portion of the Utah Flats Route, having scrambled up a route that begins right by the bridge over Bright Angel Creek far below you at the bottom of the picture. Another name for this section is the Banzai Route; picking your way down it with a heavy pack feels like a kamikaze dive. But the route does get you up to this spot for a wonderful look down at Phantom Ranch. You can, however, if you are not a glutton for punishment like me, follow a much easier route up the Clear Creek Trail, another trail blasted through the Vishnu, on the other side of the canyon to reach another lovely viewpoint above Phantom Ranch.Here we are at a spot just off the Clear Creek Trail about four miles east of Phantom Ranch. I remember wandering over to this spot, with its fine view of Zoroaster Canyon, to shoot a panorama. I was startled when I looked down at my feet, which were surrounded by crystals sparkling in the sun. I realized that the crystals were quartz crystals within granite, and I was standing on Vishnu while looking down at Tapeats Sandstone. This spot is on top of what over 500 million years ago was an island in the middle of the sea that formed the Tapeats Sandstone. You can see the Vishnu rising up on the left side of the picture with the Tapeats on the right, divided by an ancient shoreline.
To read more detailed geological information about the Vishnu Complex, visit the excellent Wikipedia page for the Vishnu Complex.