A scar in the stone
In the depths of the Grand Canyon, you can place a hand on a line separating two vastly different types of rock. Below the line is the Vishnu Schist, a dark, crystalline metamorphic rock that is between 1.75 and 1.73 billion years old. Above the line lies the Tapeats Sandstone, a horizontal sedimentary layer around 525 million years old. The surface between them represents a gap in the geological record of up to 1.2 billion years. This chasm in time is The Great Unconformity.
First described in this location by geologist John Wesley Powell during his 1869 expedition, this feature is global. It is a continent-wide, and in some ways worldwide, event where immense amounts of rock, and the history they recorded, were wiped away before younger sediments were deposited. At the Grand Canyon, the unconformity has two forms: a nonconformity, where the Tapeats Sandstone sits on older, uplifted igneous and metamorphic "basement" rocks, and an angular unconformity, where the flat Tapeats lies on top of the tilted layers of the Grand Canyon Supergroup.
The sheer scale of the missing rock is difficult to comprehend. The Vishnu Schist formed between 10 and 15 miles deep within the Earth's crust. For it to be exposed at the surface and then covered by the shallow marine sands of the Tapeats, miles of overlying rock had to be eroded away over an almost unimaginable timescale.
Competing causes for a lost eon
Geologists actively debate the forces responsible for erasing such a vast portion of Earth's history. No single explanation is universally accepted, and the cause was likely a combination of planet-scale events.
One leading hypothesis involves the "Snowball Earth" glaciations that occurred during the Neoproterozoic era, starting around 720 million years ago. During these periods, ice sheets may have covered nearly the entire planet. The immense scouring power of these continent-sized glaciers could have scraped away kilometers of rock, leaving behind a flattened surface upon which later Cambrian seas deposited sand.
Another major theory points to tectonic forces associated with the formation and breakup of the supercontinent Rodinia. The collision of continents to form Rodinia created massive mountain ranges. The subsequent, prolonged period of tectonic uplift and rifting as the supercontinent broke apart would have exposed huge volumes of crustal rock to erosion. This process, occurring over hundreds of millions of years, could account for the removal of such a thick sequence of rock before the land eventually subsided beneath the waves of the Cambrian seas. Recent thermochronology research suggests the erosion may have happened in a series of events at different times in different locations, rather than one single global cataclysm.