Analogs, however, are difficult to scale appropriately (both in time and space) to the gigantic scale on which geologic structures form. In addition to Silly Putty ® and Play-Doh ®, wooden blocks with layers drawn in or a compression/squeeze tank filled with layered sand or breakfast cereals also model structural features well. Does it have structure? (Yes, there is a fold.).Is it straining? (No, it isn't currently changing shape.).Is it under stress? (No, it is not under pressure).Is it strained? (No, it hasn't changed shape.).Is it stressed? (Yes, it is under pressure.).Look at this rock I am squeezing in my hand. Stress can happen with out strain, but strain cannot happen without stress.When you are under pressure, you are stressed! However, here are some tricks that I use to remember: These words are also used differently in geology than in common usage in English, which can cause confusion. Stress, strain and structure start with the same three letters, yet mean very different things. See Rocks deformabove for an example of having students create boudins. Alternatively, they can use structures in the analog to infer stresses and strain rates after the "structure" is created. Students can experiment with types of stress and rates of strain necessary to make analogs break or bend. Geologists use these features to identify the type of stress a rock experienced, as well as the conditions of stress it suffered (or enjoyed, depending on your point of view).Īfter showing pictures of deformed rocks, giving students the opportunity to create their own "structures" with Play-Doh ®, Silly Putty ® or other geological analog material helps them to understand concepts that underlie stress and strain and allows them to investigate relationships among stress, strain and deformational structures. Structures in geology are deformation features that result from permanent (brittle or ductile) strain.Ductile deformation is also called plastic deformation. Strain can be elastic, brittle, or ductile. Rocks only strain when placed under stress. Strain is a change in shape or size resulting from applied forces (deformation).Stress can cause strain, if it is sufficient to overcome the strength of the object that is under stress. There are three types of stress: compression, tension, and shear. It has the same units as pressure, but also has a direction (i.e., it is a vector, just like a force). Stress is a force acting on a rock per unit area.As a result, the differences between stress, strain and structures formed during strain become key concepts. Many geologists consider it important for introductory students to understand that visible structures are a record of the stress and physical conditions in the Earth. Stress causes strain, strain results in structures There are several good collections of these types of images such as the AGI Earthscience World Image Bank, Martin Miller's collection, or the National Geophysical Data Center Faults slide set. In order to show students that rocks deform, pictures and hand samples of real faulted and folded rocks at a variety of scales can be used. What kind of rate of strain is necessary to produce something similar (do you get the same result if you pull it apart quickly or more slowly)? Does it work better if your Silly Putty ® is warm or cold? Do you think the same result would happen with PlayDoh ® or cookie dough? Does the amount of Silly Putty ® affect how easily you can reproduce the structure? All of these conditions can be translated to rocks - rate at which it is strained, temperature, type of material, scale - and affect the types of structures that show up in the rock record.