#varve

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

The Varve & The Gluons @ Walabi’s, July 25-26, 1980

The Varve and The Gluons at Walabi’s in Denver, CO, July 25-26, 1980.  This was one of the last gigs that The Varve played before moving to San Francisco. Courtesy: Carolyn Crampton Courtesy: Carolyn Crampton Courtesy: Carolyn Crampton  

An earthquake, recorded...

The layers in this rock are called varves, and form when rhythmic conditions control sedimentation. An example would be the bottom of a mountain lake that gets covered in ice during the winter. The coarser grained layers might be brought by streams flowing into the lake during spring thaw and through the summer, carrying the winter's worth of erosion load after its cycles of freeze thaw and grinding ice. The finer grained darker sediments would then slowly settle out of suspension when the turbulence of the streams or wind caused waves are quashed by father frost and the lake settles into quiescence.

The name comes from the Swedish word for 'in layers', and while it technically means annual sedimentary rhythms, the use has widened somewhat to include any rocks displaying seasonal cyclicity, or even sometimes daily. They only form in fresh water, as too great a salt content coagulates the particles of clay together, destroying the sedimentary record. Varved lake sediments are very useful in reconstructing past climates and record some of the shortest time intervals in the geological record. Chemical and isotopic analysis of the sediments allows us to tell past temperatures and other information about long gone weather patterns.

This example is even more exciting, recording a quake near the Dead Sea in Israel, that shook the layer deposited in that particular year and deformed it in a manner reminiscent of the large scale fractal version of this process that occurs during mountain building events as rocks are baked and squished towards gneiss. The Dead Sea is a graben, where a large chunk of rock has dropped down in between two bounding faults (see http://on.fb.me/1Kj8ies). Since Arabia is separating from Africa at the Red Sea and bumping into Asia, powerful tectonic forces are being redistributed across the region. As a result of all this excitement quakes are fairly frequent in the region.

Loz