Some evidence for the cause of geology’s Great Unconformity
Ars Technica said:Believe it or not, the geology at the bottom of the Grand Canyon is extraordinarily common. There, layers of sedimentary rock lie flat atop angled layers of significantly more ancient metamorphic rock. The gap there is enormous—if Earth’s rocks constitute a book of the planet’s history, there are about a billion pages missing. The story only picks up again around 540 million years ago in the Cambrian period, with an evolutionary explosion of complex life just as remarkable as the sudden change in the rock.
This gap can be found all around the world, and has picked up the name the Great Unconformity. Cambrian sedimentary rocks rarely rest on anything other than much older metamorphic or igneous rock, implying that whatever rock formed in the intervening time was scrubbed away by something. This erasure of a chunk of geologic history has long been an enticing mystery for geologists.
A period of intensive global erosion doesn’t seem sufficient to fully explain the pattern of change in the rock. An alternative, that the formation of new rock suddenly accelerated beginning in the Cambrian, doesn’t quite fit the evidence, either. So what gives?
To dig into this, a team led by the University of California, Berkeley’s C. Brenhin Keller turned to a database of almost 30,000 zircon crystals. Zircons are most commonly found in the igneous rocks of volcanic arcs along tectonic plate subduction zones, where one plate is sent diving beneath the other (think of the Pacific Ring of Fire). If a huge amount of continental rock was eroded away, it would have ended up in the ocean, where it could hitch a ride into the tectonic recycler at these subduction zones—possibly leaving a chemical mark in the magma fueling volcanoes.
To look for that chemical mark, the researchers analyzed an isotope of the element hafnium. This isotope is produced by the (very slow) radioactive decay of element-you-also-forgot-existed lutetium, meaning it is slowly accumulating in the Earth’s mantle. But this is not happening in the Earth’s crust, which means crustal rocks are a little light in the hafnium isotope department.
So what can hafnium tell us? Imagine you’re cooking down a stew, aiming for a slightly thicker and saltier broth. At some point, you worry you’ve gone too far, so you add a little water back in. If you took out a spoonful every few minutes and set them aside in a sort of stew timeline, you could figure out just by taste where in that sequence you added the water.
The idea here is similar. If a lot of continental sediment—containing less of that interesting hafnium isotope—was being eroded and recycled back into the zone where mantle rock is melted, you ought to see a sudden drop in the hafnium numbers in zircons produced by the volcanoes above.
And that’s exactly what the researchers found. The zircons in the database span nearly the entire history of the Earth, and by far the most noticeable wiggle lines up neatly with the Great Unconformity. When they ran the numbers to see how much erosion would be required to explain a wiggle of that size, they found that it would be something in the neighborhood of 3 kilometers (or 2 miles) of rock shaved off all the world’s continents and dumped on the ocean floor.
Erosion alone can't explain all the details of this episode, and you need something that affects the entire globe. Is there anything else that can wipe a few kilometers of rock off the Earth's face? The authors propose that three periods of epic cold snaps in the 180 million years leading up to the start of the Cambrian—sometimes referred to as “Snowball Earth” periods—could be the key.
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