The Great Unconformity: Searching for the Earth’s Missing Time at the Grand Canyon

The Grand Canyon is an important key in understanding the Earth's geologic history, with layers of rocks stacked upon one another as they were laid down millions of years ago. However, there is a gap between rock layers which represent a billion years in some places.

Missing Layers

In 1869, while exploring the Grand Canyon, geologist John Wesley Powell noticed a gap in ages between rock layers. A rock layer, about 520 million years old, was sitting on top of rocks that were 1.4-1.8 billion years old.

It was Clarence Dutton, who gave the discrepancy the name "The Great Unconformity" in his 1882 book "Tertiary History of the Grand Cañon District." It is also assumed that the Great Unconformity was so-named because of the large interval of time it represents.

The mystery even grew when other places around the world like North America were found to have missing rock dating from before 550 million years ago. Before that, up to a billion years of rock and history were missing from the Earth.

There are two major unconformities of the Great Unconformity. Around 500 million years are missing below the Unkar Group while up to 1.3 billion years are missing in the spot, where Cambrian strata overlie Vishnu Basement rocks.

When Dutton coined the term, he was not yet aware of the age of the rocks on both sides of the unconformity. In his book, Dutton refers to the Tapeats Sandstone as "lower Carboniferous quartzites." At some areas in the Grand Canyon, the rocks under the Great Unconformity are Proterozoic sedimentary rocks. Since the ancient sedimentary rocks do not occur in Southern Nevada, it is suggested that these rocks are probably "Silurian" and could be younger than initially thought.

What Caused the Gap?

There are many other unconformities which can be observed locally in various places of the world. However, it is rare to have one that represents the same kind of gap in time from more than a billion years ago to around 500 million years ago.

In 2019, it was proposed that the gap was due to "snowball Earth," the idea that the surface of our planet froze from pole to pole. This theory suggests that the missing rock can be explained by the growth and retraction of glaciers which scoured the rocks as they were taken into the seas.

Since the study was published, it was challenged by other scientists. A 2020 study examined the geology of Pikes Peak where the older rocks made their way to the surface before being eroded around 717 million years ago. This is believed to be the likely cause of the gap other than the Snowball Earth.

In a collaborative study, a team of researchers led by Barra Peak from the University of Colorado, Boulder, theorized that the rock layers were lost during a tectonic disturbance brought by the breakup of a supercontinent. They suggest that while the Great Unconformity is present in rocks from around the world, the reason for its presence may differ in every place.