The Earth Story

This might be the first time Youtube’s “Recommended videos” actually showed me something I wanted to see! This is a sandbox geologic model - layers of sediments with different mechanical properties are put in a box and then the bottom of the box is moved backwards and forwards, mimicking the type of forces that cause the development of faults and folds in the crust.

This video doesn’t have sound but it is really well annotated. First they apply extension, pulling the layers apart, and create normal faults. When the forces reverse, as can happen with plate tectonics (first extension then contraction), the already broken layers become the focus of compression. The faults that were once normal faults get reactivated as thrust faults, building mountains instead of a basin.

Fourth of July geology!

A classic technique used to understand how rock layers fold and fault is illustrated here. These are sand tables, a great way to see how rocks deform experimentally.

In this setup, a scientist will set up layers of sand or other material with different properties. At the bottom here they have coarse-grained bird seed which is able to flow with almost no resistance, comparable to hot rocks deep in Earth’s crust. At the top they have layers of sand and cornstarch; the cornstarch is used to simulate a layer that is strong and doesn’t fold easily. The layers are also often colored so that they can be seen after the experiment.

The layers start out flat and the whole table is either squeezed or pulled apart, leading to the formation of folds and faults. Scientists can then test how the structures and patterns change as a result of changing properties like the strength and thickness of the various layers. Note here how the strong white layer fractures while the thinner, weaker layers are folded.

In this case, the layers have been pulled apart, leading to the formation of normal faults and a basin like that found today in the East African rift. This particular experiment was done in a structural geology class and that’s Dr. Steve Gao behind it.

Whether they intended it or not when the experiment was set up, this set of normal faults and colors somehow seems incredibly appropriate for the 4th of July. Now if you’ll excuse me, I’m going to go watch some fireworks.

-JBB

Image credits: http://rocktraumacenter.wikispaces.com/Sandbox+Models

Ok I’ll admit, I do not know how to do the erupting volcano effect on these neat 3-d sand tables (the light turns the sand elevation into Topography, but the eruption, that’s pretty cool)

This is a structural geology “Sand table” experiment. Geologists can use simple experiments like this to see how complicated factors interact in the building of a mountain range. They use different materials to create strong and weak zones in their experimental box, compress them so that there’s an added force, and then compare final results to the types of faults and folds found in large mountain ranges like the Himalaya.

Tiny Thrust A really cool, small-scale example of geologic processes. This is a layered beach sand found along the California coastline near San Francisco. The different layers of sand probably have different abundances of organic material, creating alternating light and dark layers. Those layers are cut by a tiny thrust fault. A small landslide outside of the photographed area created a compressive stress pushing against the beach sand. The layers were strong enough to hold together and instead of deforming or folding actually broke, forming a small thrust fault that relieved some of the stress. The landscape on the surface then leveled out by continued movement of sand into the newly-created topography. -JBB Image credit: https://flic.kr/p/pAMN

Geologists will often use experiments like these, conducted in simulated "sand tables" to observe how sediment is moved by flowing water. Lovely and addictive to watch!