The other day I sat down to a summer breakfast of fresh yogurt with a large dollop of homemade blueberry jam. The dollop was large so please don’t think I am on a health kick, far from it!
Anyway as I mixed the jam into the yogurt, I was reminded of the many times I have used this analogy to explain the stages of tectonic deformation and the origin of such things as tectonic laminations, boudins and even augens. Out came the phone and as I mixed, I snapped away.
So, we have two materials – jam and yogurt – with different colors and viscosity (rheologies) which we mix slowly. The mixing is the tectonic bit – folding, faulting and generally stirring up the rock layers. To start with the two materials are quite distinct, but as we mix fine strings of jam get drawn out and incorporated into the yogurt. These strings are attenuated folds and form what is referred to as tectonic laminations. You might draw out and stretch a lump of chewing gum thinner and thinner until it breaks. The same is true of the jam, the spoon folds it over and as you mix the fold is stretch out till it breaks. The more you mix (deform) the finer the laminations become. Thicker ‘blobs’ become shaped like sausages (boudins) and form distinct streamlined shapes. In Photo C you can still see some of the folds but they are getting very attenuated. Keep mixing the color begins to become more uniform, the tectonic laminations become so fine you can barely see them. In time you get a complete uniform mix of the two materials. When mixing sediments we refer to this as diamicton, a sediment with a mix of grain sizes and properties.
Now unlike rocks you can eat yogurt and it is very tasty!
Professor Sandcastle 