The Vishnu schist is part of the Vishnu complex in the exposed basement rocks of the Grand Canyon region. This metamorphic layer was formed by the intrusion of plutonic masses from under the crust and the deposit of sediment from an eroded mountain chain.
The oldest rocks in the Vishnu complex are deposits of hornblende and quartz that were laid down around 1.8 billion years ago. These rocks were originally part of a deep ocean trench, and they were subsequently overlain by sediments now known as the Brahma schist, which was laid down 1.75 billion years ago. Within a few million years of the Brahma schist deposit, volcanic activity added the felsic rock of the Rama schist. Together, these layers comprise the Vishnu schist that serves as the basement of the entire Grand Canyon area.
Schist is a metamorphic rock type that is commonly formed by the pressure of overlying sediments over a period of millions of years. The rocks of the Vishnu schist are typical of their type, having elongated minerals that can easily be separated into flakes. Some igneous rock is present in the Vishnu complex, though it represents an intrusion that took place considerably later than the original sediment deposits.