Explain types of Stress and Strain with suitable examples ?
Answers
There are 5 main concepts with which students struggle when thinking about stress and strain:
rocks deform,
stress causes strain and strain results in structures,
different physical conditions create different structures,
inferring stress from faults, and
the relationship between analogs and reality.
Rocks deform
Many students have a difficult time realizing that rocks can bend or break. They also may have difficulty imagining the forces necessary to fold or fault rocks or comprehending that the seemingly constant Earth can change dramatically over time. This is especially true of students who live in tectonically stable areas. If students are to understand the basics of stress and strain, they must overcome this barrier since it will be difficult to examine the causes and conditions of deformation if students cannot comprehend deformation. It is often helpful to have students create analog models of the structures present in rock photos or hand samples.
Here is a picture of a structure known as boudinage (named after the French word for blood sausage - note the sausage-like structure). Can you make a similar structure using your Silly Putty®? 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.
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. 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.
Stress causes strain, strain results in structures
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. As a result, the differences between stress, strain and structures formed during strain become key concepts.
Show Stress, strain, structure - What's the difference?
After 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. Students can experiment with types of stress and rates of strain necessary to make analogs break or bend. Alternatively, they can use structures in the analog to infer stresses and strain rates after the "structure" is created. See Rocks deformabove for an example of having students create boudins.
Stress, strain and structure start with the same three letters, yet mean very different things. These words are also used differently in geology than in common usage in English, which can cause confusion. However, here are some tricks that I use to remember:
Stress is the same as pressure. When you are under pressure, you are stressed!
Stress can happen with out strain, but strain cannot happen without stress.
Look at this rock I am squeezing in my hand.
Is it stressed? (Yes, it is under pressure.)
Is it strained? (No, it hasn't changed shape.)
Now look at this rock with a fold in it.
Is it under stress? (No, it is not under pressure).
Is it straining? (No, it isn't currently changing shape.)
Does it have structure? (Yes, there is a fold.)
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. Analogs, however, are difficult to scale appropriately (both in time and space) to the gigantic scale on which geologic structures form. Students may still have difficulty understanding the tremendous scale of forces needed to bend or break rock and the long time scales involved to generate structures. Make sure that you make it clear to your students that these pitfalls exist. More detailed ideas for analogs are available at Teaching Structural Geology analog materials web page.
Once students have mastered the connections among stress, strain and structure, I develop a 3 x 2 table of different structures that form under differing stress and strain conditions. I then proceed to fill out the table with students' help.
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