May 14. Chert

Earlier this month, we talked about the Rhynie Chert, in Scotland – a hot spring or geyser deposit that preserves a remarkable array of Devonian plants and animals. 

Chert, very fine grained silica, the same as the mineral quartz, is found in lots of places around the world. The Rhynie Chert is special because it was deposited by geysers. Chert forms in other ways, too. Often it’s found as thin discontinuous layers in sedimentary rocks, or as nodules that might range from a few millimeters to maybe as much as a meter, but most nodules are smaller.

Devonian chert (dark bands) from Pennsylvania (photo by Jstuby, public domain)

Since silica, in the form of quartz, is so common in the earth’s crust, it shouldn’t be too much of a surprise that silica is a common material that’s dissolved in water. When that water percolates through rocks or sediments, the silica can precipitate – sometimes into an open space, say one that’s been dissolved in limestone, or sometimes it might precipitate on something that serves as a nucleus, like a fossil. All of this can be part of the process called diagenesis, the change from loose sediment to solid rock, or it might happen later, with the chert precipitating in pore spaces in the solid rock.

Some layers of chert probably develop from concentrations of silica in the sedimentary environment. For example, sponge spicules, which we talked about back in February, are mostly silica. If you had a whole lot of sponges living in an area, when they die their siliceous spicules might accumulate enough to actually make a sediment that is mostly silica. That could lithify into a discontinuous bed of chert.

Likewise, microscopic animals called radiolarians make siliceous shells, and enough of them could also turn into a layer of silica-rich sediment that might become chert.

As with the Rhynie Chert, most chert is extremely fine grained, and often replaces things on a molecular scale. The rich Devonian fossil beds at the Falls of the Ohio, which we talked about on May 5, are mostly in limestones, but there are also interbedded chert layers that contain a wealth of microscopic plant and animal fossils.

There are also thick bedded cherts, especially in the Permian of the western United States. We’ll talk about them in August.

—Richard I. Gibson

Devonian chert (dark bands) from Pennsylvania, Public domain photo by Jstuby via Wikipedia.