Which Type of Stress is Shown in the Image

September 18, 2022 admin 99 Views

Quantitative concepts: trigonometry, graphing

Stress and Strain

^{by Dr. Ballad Ormand (Academy of Wisconsin – Madison) and Dr. Eric Baer (Highline Community College)}

Jump down to:
Stress, strain, & structures | Weather condition of deformation | Faults | Analogs | Teaching Examples | Resources

Essential Concepts

At that place are 5 main concepts with which students struggle when thinking about stress and strain:

rocks deform,
stress causes strain and strain results in structures,
unlike physical conditions create different structures,
inferring stress from faults, and
the relationship between analogs and reality.

Rocks deform

Many students have a difficult fourth dimension 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 tin can modify dramatically over time. This is especially truthful 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 embrace deformation. It is frequently helpful to take students create analog models of the structures present in rock photos or mitt samples.

Here is a picture of a construction known as boudinage (named later on the French word for claret sausage – note the sausage-like structure). Can y’all make a similar structure using your Silly Putty®?

What kind of rate of strain is necessary to produce something similar (do you lot go the same consequence if you lot pull it apart speedily or more slowly)? Does information technology piece of work better if your Light-headed Putty®
is warm or cold? Do you lot retrieve the aforementioned result would happen with PlayDoh®
or cookie dough? Does the corporeality of Giddy Putty®
impact how easily yous can reproduce the construction? All of these conditions can be translated to rocks – rate at which it is strained, temperature, type of material, scale – and bear on the types of structures that show upward in the stone record.

In society to show students that rocks deform, pictures and hand samples of
existent
faulted and folded rocks at a variety of scales can be used. At that place are several expert collections of these types of images such as the AGI Earthscience World Image Bank, Martin Miller’s drove, or the National Geophysical Data Middle Faults slide ready.

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 concrete conditions in the Earth. As a consequence, the differences between stress, strain and structures formed during strain go key concepts.

After showing pictures of plain-featured rocks, giving students the opportunity to create their own “structures” with Play-Doh®, Dizzy Putty®

or other geological analog material helps them to sympathize 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 tin use structures in the analog to infer stresses and strain rates after the “structure” is created. Encounter
Rocks deformin a higher place for an example of having students create boudins.

Stress, strain and structure start with the aforementioned 3 letters, all the same mean very different things. These words are also used differently in geology than in common usage in English, which can cause defoliation. Even so, here are some tricks that I use to recollect:

Stress is the aforementioned as pressure level. When you are under pressure, you are stressed!
Stress can happen with out strain, just strain cannot happen without stress.

Await at this stone I am squeezing in my manus.

Is it stressed? (Aye, it is under force per unit area.)
Is it strained? (No, it hasn’t inverse shape.)

Now expect at this stone with a fold in information technology.

Is information technology under stress? (No, it is not under pressure).
Is it straining? (No, information technology isn’t currently changing shape.)
Does it have structure? (Aye, 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 calibration appropriately (both in time and space) to the gigantic scale on which geologic structures course. 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 articulate to your students that these pitfalls exist. More detailed ideas for analogs are available at Teaching Structural Geology analog materials web page.

In one case 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 weather. I then keep to fill out the table with students’ assist.

Let’s await at what features are found under unlike stress conditions and with dissimilar styles of strain. We’ll exercise this by making a table. What are the iii types of stress? Pinch, tension, and shearing. At present, what are the ii types of permanent deformation? Ductile and brittle. Permit’s make a table that is three columns by ii rows and fill it in with appropriate structures! When we are finished, nosotros should accept 6 kinds of deformation features.

A table of stress and strain and the deformational structures

Now, see if you can make each of those with your Play-Doh®
or blocks.

Different atmospheric condition atomic number 82 to unlike deformation styles

In that location are many factors that contribute to the style of the deformation in a rock, including pressure, temperature, stone composition, presence or absence of fluids, type of stress, rate of stress, and others. However, the type of stress, the rate of stress and the temperature may be the most disquisitional factors for most introductory students.

silly putty
Lightheaded Putty®
is a material, merely like rocks, that can deform either plastically or brittlely. What controls how it will deform?

Temperature: Cold Silly Putty is easy to break, merely warm Giddy Putty is very plastic.
Strain rate: If I pull it apart quickly information technology breaks, but if I pull information technology slowly, it stretches (deforms plastically).
Type of stress: Finally, choice a strong pupil, and have him or her endeavor to break the silly putty using compressive stress. As you can encounter, this is almost impossible. Now accept a student try to interruption it using tension. This is much easier. Most materials are more easily cleaved (or otherwise deformed) in tension than in pinch; we say that they are weaker in tension, or stronger in compression.

Temperature, strain rate, and type of stress are also cardinal factors in deformation within glaciers. This provides a an opportunity to revisit these concepts later on in the term.

Relating faults to stress – hanging walls, footwalls, and different types of faults

Ane of the goals of structural geology is to relate the nature of deformation to the stress that caused it. Therefore, it is of import that students be able to distinguish between normal faults (generated by tension) and reverse faults (generated past compression).

Wooden blocks are a valuable tool for teaching about normal and contrary faults. Using 3 blocks cut on an angle, horsts and grabens can exist generated. Pull the blocks apart to create a graben; push them together to brand a horst. The advantage of using three blocks is that students tin can run into that information technology is not the orientation of the mistake that matters, but the motility on the fault. Because they tin see whether I am extending or compressing the blocks, they develop an intuitive sense of the divergence between normal and reverse faults. However, students typically still need to learn the deviation betwixt the hanging wall and footwall of a fault to be able to accurately determine whether a fault is normal or reverse and what kind of stress acquired it.

Faults are places where rocks accept been broken and offset. It is not uncommon for fluids to take flowed forth the break during deformation, leaving valuable minerals along the mistake. As a result, many mines are synthetic forth fault surfaces. Because of this, ane side of the fault is called the hanging wall (the surface from which a miner’s lantern would be hung) and ane side is called the footwall (the surface on which the miner would walk.)

Showing footwalls and hanging walls

Here’s another way to think of it: the hanging wall block is always above the mistake plane, while the foot wall block is e’er beneath the mistake airplane.
Headwall and footwall
To see this, put a point on the mistake and draw a vertical arrow pointing upward. This pointer points into the hanging wall. An pointer pointing directly down points into the footwall. Take a look at the slide that shows the fault and arrows indicating movement. Some students think the footwall looks like a pes. See how the hanging wall is resting, or hanging, on the footwall?

In one case students understand the difference between a hanging wall and a footwall, well-nigh of them take niggling trouble remembering that in a reverse fault the hanging wall moves up, indicating compression, and in a normal fault the hanging wall moves downwards, indicating extension.

Every bit your students can come across from these block models, horizontal forces tin can cause rocks to move along faults that are at an angle to the rock layering. Given that idea, your students can utilize some basic trigonometric functions to examine the human relationship between horizontal strain (the corporeality of stretching or shortening in a horizontal direction) and deportation on a fault surface (the amount of movement on the fault itself). Since this human relationship is dependent on the bending of the fault from horizontal, the angle of the mistake is a disquisitional component of how faults accommodate shortening or extension.

Diagram of Seattle Fault Models

The Seattle fault is a large reverse error that cuts beyond and underneath the Seattle, Washington, metropolitan area and its nearly 2 million residents. The Seattle fault accommodates about 1 millimeter of shortening per yr. Even so, because the error plane itself is not well exposed and or discernable in seismic profiles, nosotros don’t know what angle this mistake makes to horizontal. If the fault is low-angle, at about 25 degrees from horizontal, then to suit the 1 mm of shortening it must motion 1.1 mm/year on average. If, even so, it is at a more steep sixty degrees, it would demand to move 2 mm/year on average. Since the deportation on the error is a primary gene in determining the magnitude of an earthquake (come across convulsion page), the Seattle fault would need to motion either twice as often or have much larger quakes if it is at a steep angle.

Relating analogs to the real Globe

Torn cheese

We frequently use analogies and analog materials (Silly Putty, sand, wooden blocks, etc.) to illustrate the concepts of stress, strain and the deformation of rocks. However, students sometimes accept difficulty relating these materials and their behaviors to the Earth and existent rocks. For these students, information technology may exist useful to discuss the rates and magnitudes of deformation in the Earth and the differences between rocks and analog materials. For instance, the rocks at plate boundaries often feel a few centimeters of deformation in a yr, but the forces on them are plenty to move continents. The size and slowness of these processes are an of import concept to communicate, fifty-fifty if they are on a scale that is almost impossible to comprehend. I sometimes tell students that their fingernails are growing at near the same rate the plates are moving, to help them overcome this difficulty.

Plates move at about the same rate every bit your fingernails grow, a few centimeters per year. While this seems slow, over long periods of time it actually adds up. For instance, if you would allow your fingernails abound for 100 million years, they would exist around 4000 kilometers long!

Teaching Examples

Joints in a Cornstarch Analog

Desiccated cornstarch-h2o mixture provides an interactive introduction to joints and articulation sets. Students translate relative ages, examine intersection angles, utilise surface textures to decide propagation direction, and evaluate the function of flaws in articulation initiation.

Fracture Fundamentals: A Cheesy Analog

This action has students brand pocket-sized cuts (fracture nuclei) in processed cheese food and so utilise stresses perpendicular or parallel to the cuts to meet how fractures grow. Surprisingly (or not, depending on one’s previous thoughts on cheese), processed cheese food fractures in much the same style as homogeneous rocks.

Development of Normal Fault Systems During Progressive Deformation

This activeness is based on QuickTime movies and color digital photographs derived from sandbox experiments that produce normal faults in a variety of boundary conditions following experiments adult by Ken McClay. Students view specially edited movies to proceeds awareness of the evolution of normal error systems. They then investigate the formation and evolution of a mistake system for a item structural setting by tracing and labeling private faults on a set of photographs taken at regular intervals during an experiment. This activity helps students develop an sensation of mistake initiation, propagation, rotation, and inactivation during progressive deformation.

Analysis of Sidewalk Fractures

Using fractures in sidewalks as an analog for natural outcrops, students learn to make systematic observations, measure the orientation and location of fractures, dispense and analyze data, and consider some kinematic and dynamic questions regarding the origin and significance of fractures.

Resources

Teaching Structural Geology in the 21st century

This site contains a variety of resources for kinesthesia members who teach undergraduate structural geology. You lot will find links to activities and assignments, cyberspace and computer resources, useful manufactures and maps, presentations from the summer 2004 workshop on teaching structural geology, working groups and a discussion forum, and lots of creative ideas for teaching structural geology.

3D visualizations of structural deformation

GeoBlocks 3D, created by Steve Reynolds, contains interactive QuickTime Virtual Reality (QTVR) movies exploring the three-dimensional nature of geology, specifically geologic structures within blocks. Yous can rotate the blocks, make them partially transparent to view their internal construction, cutting through or erode them, displace faults, and more.

Structural Geology of Rocks and Regions (introductory chapter)

This textbook past Davis and Reynolds is the near widely used structural geology text according to a recent survey. The introductory chapter may be helpful for faculty in thinking nigh how to teach this section of an intro class since it examines the three basic ways structural geologists examine deformation: geometric analysis, kinematic assay and dynamic assay.

Steve Reynolds’ web page

This page contains many visualization tools and other resources, developed and gathered past Steve Reynolds, geology professor at Arizona State Academy.

Structural Geology Course Resources on the Internet

A itemize of courses with on-line resources or web-pages