A 2005 research cruise in the remote Pacific found something surprising: nothing. The scientific team aboard the research vessel Melville, mapping and drilling in the central South Pacific seafloor, traced out a region of bare rock that's bigger than Alaska. It had none of the mud, clay, ooze or manganese nodules that cover the rest of the deepest sea. This was not freshly made rock either, but oceanic crustal basalt that was 34 to 85 million years old. The finding was published in the October 2006 Geology, and Science News also took note. It goes to show that there are places on Earth where nothing happens for a very long time.
If sedimentary rocks were to be laid down today on that ancient lava seafloor in the Pacific Bare Zone, the result would be strange: a sequence of rock with a large gap of up to 85 million years in the middle of it. Such gaps do occur—geologists are used to them and classify them as unconformities.
The concept of an unconformity arises from two of the oldest principles of geology, first stated in 1669 by Nicholas Steno:
- Layers of sedimentary rock (strata) are originally laid down flat, parallel to the Earth's surface. That's the law of original horizontality.
- Younger strata always overlie older strata, except where the rocks have been overturned. That's the law of superposition.
So in an ideal sequence of rocks, all the strata would stack up like the pages in a book in a conformable relationship. Where they don't, the plane between the mismatched strata—representing some sort of gap—is an unconformity. There are four main kinds of unconformity. I've drawn sketches of each, showing rocks of Pennsylvanian age overlain by rocks of Triassic age. If you know your geologic time scale, you'll be asking, "where's the Permian?" The answer may be very different in each case.
The Angular Unconformity
The most famous and obvious kind of unconformity is the angular unconformity. Rocks below the unconformity are tilted and sheared off, and rocks above it are level. The angular unconformity tells a clear story:
- First a set of rocks was laid down.
- Then these rocks were tilted, then eroded down to a level surface.
- Then a younger set of rocks was laid down on top.
In the 1780s when James Hutton studied the dramatic angular unconformity at Siccar Point in Scotland—called today Hutton's Unconformity—it staggered him to realize how much time such a thing must represent. No student of rocks had ever contemplated millions of years before. Hutton's insight gave us deep time, and the corollary knowledge that even the slowest, most imperceptible geologic processes can produce all the features found in the rock record.
The Disconformity and Paraconformity
Now take away the second step: strata are laid down, then a period of erosion happens (or a hiatus, a period of nondeposition as with the Pacific Bare Zone), then more strata are laid down. The result is a disconformity or parallel unconformity. All the strata line up, but there is still a clear discontinuity in the sequence—maybe a soil layer developed on top of the older rocks, or a rugged surface where they were eroded.
If the discontinuity is not visible, it is called a paraconformity. These are harder to detect, as you might imagine. A sandstone in which trilobite fossils suddenly give way to oyster fossils would be a clear example. Creationists tend to latch onto these as proof that geology is mistaken, but geologists see them as evidence that geology is interesting.
British geologists have a slightly different concept of unconformities that is based purely on structure. To them, only the angular unconformity and the nonconformity, discussed next, are true unconformities. They consider the disconformity and paraconformity to be nonsequences. And there's something to be said for that because the strata in these cases are indeed conformable. The American geologist would argue that they are unconformable in terms of time.
Here is a better match to the situation in the Pacific Bare Zone. There is a body of rock that is not sedimentary, upon which strata are laid down. Because we aren't comparing two bodies of strata, the notion of them being conformable doesn't apply. This kind of junction between two different major rock types is a nonconformity.
A nonconformity might mean a lot, or not much. For instance, the spectacular nonconformity at Red Rocks Park, in Colorado, represents a gap of 1400 million years. There a body of gneiss 1700 million years old is overlain by conglomerate, made of sediment eroded from that gneiss, that is 300 million years old. We have almost no idea of what happened in the eons between.
But then consider fresh oceanic crust created at a spreading ridge that is soon covered by sediment settling down from the seawater above. Or a lava flow that goes into a lake and is soon covered with mud from local streams. (Examples of both types can be seen in my California Subduction tour, at stop 1 and stop 30 respectively.) In these cases, the underlying rock and the sediment are basically the same age and the nonconformity is trivial.
But the Pacific Bare Zone shows another way of creating a significant nonconformity. We can now imagine a hiatus of 85 million years. Looking at an outcrop with that degree of nonconformity, the stories we come up with to explain it must now include that possibility.