#rhythmite

Feel the Rhythm

These centimeter scale sedimentary layers in Virginia are really quite neat. They’re considered “Rhythmites” – sedimentary layers deposted in a repeating pattern. A number of processes can create rhythmites, including tides, annual floods, rainy and dry seasons, and even orbital changes that trigger changes in climate over thousands of years.

Some of the most common rhythmites on earth are found near the ends of glaciers where the conditions create extreme differences between winter and summer. During the winter, any lakes or ponds will commonly freeze over, cutting them off from access to the weather and creating a setting where photosynthetic organisms can still live. In the summer, these lakes will melt and be open to sedimentary input, and they will often get that sediment supplied by the nearby glacier that also melts a little.

Rhythmites like these, where there is likely annual banding produced once a year, receive the special name “Varves”.

These rocks are extra neat because of where they’re found. These sediments in Virginia are found in the Appalachian Mountains today. The sediments in this mountain range were originally deposited hundreds of millions of years ago, all the way back in the Precambrian. During the latter part of the Precambrian, the Proterozoic eon, there are several large glacial advances recorded in sedimentary rocks around the world. One part of the Proterozoic is nicknamed the “Cryogenian” due to the abundance of glacial deposits in that time span. This time period is the likely time when the Earth may have frozen over completely, forming an icy planet nicknamed “Snowball Earth”.

-JBB

Image credit: James St. John https://flic.kr/p/NntMeS