Geology of the Grand Canyon

That a chasm as deep as the Grand Canyon could have appeared over so short a time and display the billions of years of rock that it does is somewhat mind boggling. The Grand Canyon was formed over only six million years. While it sounds like a long time to us, it’s almost nothing in the life of the earth. At the bottom of the canyon rocks billions of years old tell stories we might never have known otherwise. It’s a lesson in the formation of the earth’s crust put beautifully on display thanks to the Colorado River.

But it wasn’t entirely the river’s doing. The foundation was laid millions of years ago when the ocean covered Arizona and surrounding land. Water brought heavy sediment from all over and as it receded, left the sediment behind it. Tectonic plate movement shoved the land together creating the high plateaus of the region. Finally, the Colorado River sliced through the rock to create the depth of the canyon we see today. It’s width is thanks to the erosion the ice age caused through the expansion and retraction of waters that pushed it apart giving it the step-wise topography we see today.

At river’s edge the rock on display is over one and a half billion years old. The stratification in the rock is literally a visual lesson in the geology of our earth. Eleven layers of the earth’s crust are visible from top to bottom and clearly defined. Most of these layers are made up of sedimentary rock, compressed sediment left behind by the water no longer there. It holds a wealth of information about our earth and what was happening at the time of its layer’s formation. Fossils tell stories of plant and animal life once found there. The two visible bottom layers are made up of metamorphic rock made by heat and pressure. Lastly, and not visible from every vantage point, there is one layer called a Supergroup. These are rocks that used to be sedimentary in formation but have morphed through heat and pressure into something different.

There is a billion year gap in the layers between the metamorphic rock at the bottom and the first layer of sedimentary rock. Over that time, thick layer of sedimentary rock was laid down and eroded away leaving a huge void of information about those years. It confounds geologists and is known as the Great Unconformity