Vishnu Schist
The ~1.75-billion-year-old metamorphic basement of the Grand Canyon's Inner Gorge — the dark crystalline floor beneath the Great Unconformity.
Also known as: Vishnu
Proterozoic — long before animals, a world of microbes and bare rock
The world back then
1.7 billion years ago, and there's nowhere to stand — you're miles down inside the crust, where old seafloor mud and volcanic rock are being cooked and crushed hot enough to flow like slow taffy. No air, no light, no surface. The only life on the entire planet is single cells adrift in a sea far overhead, and they'd never know you were there.
This rock spent more than a billion years grinding its way up sixteen miles to daylight so you could float past and glance at it. And here you are — on top of the world, in the sun, breathing air, riding water you can actually drink. That's a staggering piece of luck for a bag of mostly water. The Vishnu waited a billion years for a view you got for the price of a permit.
Recognize it in the field
- Typical color
- dark gray to black
- Weathers as
- Forms the dark, steep, polished walls of the Inner (Granite) Gorge.
- Grain
- medium-grained schist
Look for
- dark quartz-mica and pelitic schist
- near-vertical foliation, polydeformed
- relict submarine-sediment protoliths
- intruded by pale Zoroaster granite and pegmatite
- the crystalline floor beneath the Great Unconformity
How this rock was made
Deep-crustal metamorphism of island-arc sediments during collisions that accreted this crust to Laurentia ~1.7 Ga.
The setting: metamorphosed submarine sediments and volcanics (Yavapai orogeny)
A basement complex, not a bedded unit — exposed through the Upper and Lower Granite Gorges; the upper contact records ≥25 km (16 mi) of erosional unroofing.
In the rock record
↑ Younger — rests above
Where to see it
Regions: Grand Canyon (Inner Gorge)
- Upper and Lower Granite Gorges, Colorado River corridor
Float past it: Grand Canyon