A couple of weeks ago, I did a fun experiment with my Structural Geology class at George Mason University. We injected plaster of Paris liquid into a 2-liter soda bottle full of solidified gelatin to induce a fracture by increasing the pore pressure. These experiments show that the fractures (which geologists call "joints") are elliptical in shape, and bear distinctive structures including hackle fringes which correspond to real structures seen in real rocks. (see previous NOVA Geoblog posts on joints and their distinctive structures)

Here's a video of our first joint, made with a horizontal maximum stress:


Here's a shot of the experimental set-up for the second round, in a bottle with a vertical maximum stress:


Lousy, blurry video of the second experiment:


Low-res shot of the resulting joint surface. Note how it flares to parallel with the side of the bottle due to variations in the stress field, and also the lovely hackle fringe:


A close-up of the hackle fringe where the "joint" (white plaster of Paris) stops and the "rock" (transparent gelatin) begins:


As you can tell from the audio in the YouTube clips, we all found this pretty exciting!

For those who teach geology, this is a relatively simple experimental set-up (although I hate the smell of unsugared gelatin cooking) that is a great visual demo of the relationship between stress orientations and joint orientation, pore pressure, and joint/ vein structures. A big thumbs up from the Bentley classroom!

Labels: structure, teaching