Why is the Earth Missing a Billion Years of Rocks?
"A mystery lies deep within the Grand Canyon: one billion years' worth of rocks have disappeared," Space.com reported last week. The BBC explains: Today geologists know that the youngest of the hard, crystalline rocks are 1.7 billion years old, whereas the oldest in the sandstone layer were formed 550 million years ago. This means there's more than a billion-year-gap in the geological record. To this day, no one knows what happened to the rocks in between. While the missing rock is particularly obvious in the Grand Canyon, the phenomenon is ubiquitous. "It's one of these features that pretty much occurs under a lot of people's feet, when they don't even realise it," says Stephen Marshak, professor emeritus in the Department of Geology at the University of Illinois. He explains that in the centre of any continent, whether you're in the United States, Siberia or Europe, if you drill down far enough you'll hit the two layers of rock involved in this mysterious geological anomaly.... [F]inding out what happened during, and led to, the missing billion years is no trivial matter. There are two reasons for this. The first is that it just so happens to have occurred immediately before another inexplicable event — the sudden proliferation in the diversity of life on Earth 541 million years ago. The Cambrian explosion refers to an era when the oceans suddenly shifted from hosting a scattering of weird and unfamiliar creatures — such as triffid-like leaf-shaped animals and giant steamrollered ovals which continue to defy all efforts to categorise them — to an abundance of life, with many of the major taxonomic groups around today. It happened in the space of just 13-25 million years — an evolutionary twinkling of an eye... The second is that it's thought Earth underwent radical climate change during the lost years — possibly turning into a giant ball of ice, with an almost entirely frozen surface. Very little is currently known about how this "snowball Earth" formed, or how life managed to cling on. They share the three good theories. First, "snowball" — the earth develops a global ice sheet, with the speedy glaciers wearing away surface rocks. The second theory is that it was all lost during the erosion of the supercontinent Rodinia. And theory #3 is: confusion. The BBC cites new research that "suggests that the epic interruption in the geological record was not a single, discrete phenomenon — but instead is actually at least two mini-gaps, which look like one big one because they occurred at around the same time." Even the missing rocks on the two sides of America's Grand Canyon "may instead have vanished in several separate events over the course of several hundred million years."
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