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Overview of Geologic Structures Part 1: Rock Deformation, Stress and Strain
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Overview of Geologic Structures Part 1: Rock Deformation, Stress and Strain

Geologic Time and Structures

In this section, we learn about the four eons of geologic time and explore how rocks on Earth can change their shape, thereby creating dramatic landforms over the millennia that field geologists love to study today. We also get a basic understanding of the mechanical properties of geologic materials and how these mechanical properties can change under different pressure-temperature conditions.

Deformation of Rocks

  • Understanding how rocks deform requires a basic understanding of the mechanical properties of geologic materials and how these mechanical properties can change under different pressure-temperature conditions.
  • Rocks in the shallow crust are deformed differently than deeper ones, thereby forming different structures than rocks in the deep crust.
  • The term viscosity relates to a material’s resistance to flow, and it is a very important concept in structural geology.

Stress and Strain

  • Stress describes the quantity of forces that cause deformation and is expressed in terms of force per unit area or Pascals.
  • There are three normal components of stress: tensile stress, compressive stress, and shear stress.
  • If an object experiences sufficient stress to cause deformation, whether it is stretched, squashed or sheared, it undergoes strain.
  • The relationship between stress and strain is represented by Young's modulus which is dependent on the physical properties of a material.

Competence

  • The relative tendency of a rock to strain in response to stress is called its competence which can also be defined as the relative viscosity of a rock.
  • Incompetent rocks are composed of soft weak minerals like clay while competent rocks are composed of strong rigid minerals like garnet or feldspar.

Deformation of Rocks

  • The manner in which a rock responds to stress is controlled by the conditions that it is under, mainly temperature and pressure, but other factors like the deformation rate and presence of fluids are also important.
  • As the pressure and temperature of a rock are increased, its strength or viscosity decreases, making it more willing to deform.
  • Rocks that deform in a brittle manner break instead of bend, and this behavior is most commonly observed at shallow depths.

This transcript provides an overview of geologic time and structures. It introduces key concepts such as stress, strain, competence, and deformation mechanisms.

Elastic and Plastic Deformation

This section covers the concepts of elastic and plastic deformation, how they occur, and their differences.

Elastic Deformation

  • Elastic deformation is the bending of atomic bonds that occurs when an object is stressed.
  • When a force is exerted on an object, its atomic bonds stretch parallel to the maximum tensile force and contract parallel to the maximum compressive force.
  • If the force is increased, the material experiences more strain, but once the force is removed, it reverts back to its original shape when the atomic bonds relax.
  • However, if a force so great is exerted that the rock fails and forms a fracture, all of the accumulated stress responsible for pulling atoms apart is released causing an earthquake.

Plastic Deformation

  • Plastic deformation describes how weak materials deform under stress by breaking atomic bonds.
  • Materials that deform plastically do not revert to their original shape once stress is removed.
  • Plastic deformation occurs mainly under enormous pressures and temperatures of lower crust and mantle but can occur closer to surface if temperature or material weakness allows it.

Specific Structures

This section discusses specific structures related to stress and strain.

Subduction Zones

  • Subduction zones are known for massive earthquakes because they transport very competent material (oceanic crust) to great depths and therefore great pressures.
  • Normally materials at these depths are too weak to cause earthquakes due to high temperature causing them to bend instead of break.

Volcanoes

  • Plastic deformation can occur beneath volcanoes if either temperature is high enough or material is weak enough.

That concludes this section on stress and strain.

Video description

Now that we've briefly gone over the history of the Earth, it's time to look at some different geologic structures that span all those eons. This will require gaining an understanding of rock deformation. We need to discuss the terms stress and strain, as well as Young's modulus, which describes their relationship. From there we can describe the different types of rock deformation. Let's go! Script by Jared Matteucci Watch the whole Geology playlist: http://bit.ly/ProfDaveGeo Mathematics Tutorials: http://bit.ly/ProfDaveMath Classical Physics Tutorials: http://bit.ly/ProfDavePhysics1 General Chemistry Tutorials: http://bit.ly/ProfDaveGenChem Biology Tutorials: http://bit.ly/ProfDaveBio Microbiology Tutorials: http://bit.ly/ProfDaveMicrobio Botany Tutorials: http://bit.ly/ProfDaveBotany Zoology Tutorials: http://bit.ly/ProfDaveZoo EMAIL► ProfessorDaveExplains@gmail.com PATREON► http://patreon.com/ProfessorDaveExplains Check out "Is This Wi-Fi Organic?", my book on disarming pseudoscience! Amazon: https://amzn.to/2HtNpVH Bookshop: https://bit.ly/39cKADM Barnes and Noble: https://bit.ly/3pUjmrn Book Depository: http://bit.ly/3aOVDlT