One of the cooler things about rocks, among many cool things, is that they allow us to look back into the past. The further down in a rock's layers you go, the older the rock.
Studying layers, and where these layers merge, can tell us about local geology as well as big events in our planet's history, such as the time it rained for around 2 million years. Sometimes, though, geologists find a gap in the record, where layers of rocks are sat on top of older rocks, with no middle-aged layers in between.
Known as "unconformities", these can be the result of no new layers of rock being laid down during the intervening years, or rock being laid down and then eroded or washed away by geological processes.
Unconformities are relatively common, but every now and then a big one is found that takes some explaining.
The Great Unconformity
Exploring the Grand Canyon in 1869, geologist John Wesley Powell noticed a gap in ages between layers of rock. Sitting on top of rocks aged 1.4-1.8 billion years old was rock that was just 520 million years old. While a huge mystery in itself, earning the name The Great Unconformity, the mystery grew when other sites around North America and the world were discovered to have missing rock dating from just before 550 million years ago. Before that time, in a lot of places around the world, up to a billion years of rock (and history) are missing.
So what happened to it? The short answer is that we still don't really know, but we do have several theories. One proposed in 2019 is that the gap was caused by "snowball Earth", proponents of which believe the Earth's surface froze from pole to pole. According to this theory, the missing rock can be explained by glacier growth and then retraction, scouring at the rock and taking it into the seas.
However, since that study was published, other studies have challenged it. A 2020 study looked at rock at Pikes Peak, Colorado. Here, older rock made its way to the surface, before being heavily eroded beginning around 717 million years ago. This means it was likely caused by processes other than Snowball Earth.
Instead, the team believes that the unconformities seen around the world, though often from the same time, were not made by a common cause.
“We’re left with a feature that looks similar across the world when, in fact, there may have been multiple great unconformities, plural,” lead author of the 2020 study, Dr Rebecca Flowers said at the time. “We may need to change our language if we want to think about the Great Unconformity as being more complicated, forming at different times in different locations and for different reasons.”
The Great Unconformity which took place in North America, the team suggested, was likely caused by the breakup of the supercontinent Rodinia 700-800 million years ago, bringing older rock to the surface while the newer rock got swept into the ocean.
While a reasonable explanation, it's still a slightly depressing thought that we have lost so much information about what happened to our planet, or parts of it, during all those millions of years.
“There’s more than a billion years that’s gone,” said Barra Peak, co-author of a 2021 study on this puzzling gap in Earth's history, in a statement. “It’s also a billion years during an interesting part of Earth’s history where the planet is transitioning from an older setting to the modern Earth we know today.”