The Great Unconformity

You might think that this is related to someone sitting in the most uncomfortable position ever, or about a very awkward moment and you would be completely wrong. You would have to pay attention to the fact that there is an “n” and not an “m” in there…

The first time I heard about the Great Unconformity was a few days back in the Nature podcast. An unconformity in this context refers to the contact surface between younger and older rocks in the geological record where a discontinuity is present. So, the Great Unconformity refers to the large gaps left in the planet’s rock record (ahem… nothing to do with music…) where young sedimentary rocks sit on top of much older metamorphic rock. For example, in the Grand Canyon, a layer of sandstone dating back to 500 million years ago sits on top of a 1.7-billion-year-old metamorphic rock layer. There are similar unconformities around the world.

Blacktail Canyon and The Great Unconformity - ...

Blacktail Canyon and The Great Unconformity - Grand Canyon (Photo credit: Al_HikesAZ)

Why is this so interesting you ask? Well, among other things, these gaps  leave a limited record precisely when life was advancing very quickly. 500 million years ago or so, new forms of multicellular life forms appeared, something that is come to be known as the Cambrian explosion. In the article referred to during the Nature podcast, researchers from the University of Wisconsin and Pomona College link changes in ancient ocean chemistry to this remarkable transformation of life. One important change is that of biomineralisation, by which organisms started using minerals, such as calcium carbonate, to build structures such as shells and skeletons. The formation of the Great Unconformity “may have been an environmental trigger for the evolution and mineralisation and the ‘Cambrian explosion'”, the researchers say.

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