vishnu Schist

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.

Metamorphic rocks and most igneous rocks don’t form at the earth’s surface so the kind of heat and pressure that it takes to metamorphose or melt rock happens miles below the surface of the earth. About 1.7 billion years ago, all was an ocean in this region. As the ocean collided with the continent, it was forced back down into the depths of the earth, in a process known as subduction. As the plate was sub-ducted, 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.

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 7 miles below the surface of the earth! Molten magma squeezed its way into the metamorphosing rocks around it, and began to cool and harden into veins and masses of pink granite. In the meantime, on surface perhaps there were volcanic islands and low mountains forming from the eruptions of the magma that made it to the surface. 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.

500 million years ago, we know that the schist was at sea level again because the Tapeats Sandstone. 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 a loop on time. It's happening all over again and it will happen again!

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. 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. 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. Also, 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. More Hitthetrail

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