Understanding Stress and Deformation in Geology

1. This week: PS #1 assigned (due next Tuesday) Lab #1 due tomorrow at 11:00 Lectures: ch. 3- Force and Stress Lab #2: Field trip to Mt Baldy- Will collect field books at end of field trip

2. The stresses that cause deformation Understand "stress calculations” Spend some time with these calculations to convince yourself that stress on a given plane resolves itself into a single stress tensor. Stress (s) = force/unit area s = F/A

3. Stress Goals 1) Interpret the stresses responsible for deformation. 2) Describe the nature of the forces that cause the stresses. 3) Understand the relations between stress, strain and rock strength. Describing stress and force is a mathematical exercise.

4. All the deformation we see is due to Responses to Stresses 1) Folding 2) Brittle faults 3) Ductile shear zones 4) Joints

5. Stress = Force/Area Force: changes in the state of rest or motion of a body. Only a force can cause a stationary object to move or change the motion (direction and velocity) of a moving object. force = Weight is the magnitude of the force due to gravity (g) acting upon a mass. You weigh less on the moon (gravity is less) mass x acceleration, F = ma,

6. Force Units of Force: The newton (N) is the basic (SI) unit of force. 1 newton = 1 kg meter/sec2 1 newton is about the weight of one small apple (remember Sir Newton and the apple!)

7. Forces as Vectors Force is a vector - it has magnitude and direction. Vectors can be added and subtracted using vector algebra. We can evaluate vectors in order to determine whether the forces on a body are in balance. If sum of the forces=0, Then there is no change in motion (no acceleration)

8. Forces as Vectors Magnitude and direction Load Force If sum of the forces=0, Then there is no change in motion (no acceleration)

9. Forces in the Geologic World Typically we think of the Earth as at rest - in static equilibrium, or moving very slowly. When there are net forces, they cause accelerations that are usually one of 2 kinds: 1) slow ponderous motion of a tectonic plate that increases or decreases velocity over a very long time, or; 2) sudden, short lived, strong accelerations during fault slip accompanying earthquakes.

10. Two Types of Forces • Body forces, that act on the mass of a body and are independent of forces applied by adjacent material. • As geologists, there are two body forces that we really care about. What are they? • 2) Contact forces, are pushes and pulls across real or imaginary surface of contact such as faults. • Three different type of loading due to contact forces: • 1) gravitational loading - pushing on adjacent rock. • 2) thermal loading - expansion or contraction. • 3) displacement loading - push due to motion. Gravity, magnetic field

11. Stress(s) Stress is force per unit area: s= F/A

12. In class demonstration of force, stress, and deformation.

13. Units of Stress 1 pascal = 1 newton/m2 (Force/area) • 1 newton is about 0.2 pounds of force • 1 pascal is about 0.02 lb/ft2, thus for geologists, we often measure pressure in MPa • 1 MPa = 145 lb/in2 • 9.81 Pa is the pressure caused by a depth of 1mm of water • What is the pressure at the bottom of a swimming pool filled with water? • Filled with quartzite?

14. Units of Stress • Sometimes you will see stress in the units of bars or kbars • 1 bar=105 Pa • What is the pressure at the bottom of a swimming pool in kbars?

15. As geologists, we have to deal with situations when stresses act at an angle to planes (like faults and shear zones). In this case we have to resolve the stresses into components Stress as Vectors Stress is a vector - it has magnitude and direction. As geologists, we’re interested in stresses that compress things, and stresses that shear things. So, we resolve the stress vector into into it’s components

16. Stress on a plane: • Normal stress act perpendicular to the plane • Shear stress act along the plane. • Normal and shear stresses are perpendicular to one another

17. Relations between F and s Stress on a 2-D plane: Force F is applied to a plane with the trace EF. Magnitudes of vectors Fs and Fn is function of angle q What is the equation for the normal force? For the Shear force? Fn = F cos q, Fs = F sin q At what angle is the normal force the maximum? At what angle is the shear force the maximum

18. Relations between F and s Stress on a 2-D plane: Stress=Force/Area s=F/AB The magnitude of normal and shear stresses are a more complicated function of angleq, sn= Fn/Area where Fn= F cos q Area=AB/(cos q) Therefore sn =s cos2q ss = s*1/2( sin2q) At what angle is the normal stress the maximum? At what angle is the shear stress the maximum?

19. Relations between F and s sn =s cos2q ss = s*1/2( sin2q) Maximum normal stress when force is perpendicular (90°) to the plane Maximum shear stress when force 45° to plane

20. Stress ellipsoid Describing the state of stress at a point: A point represents the intersection of an infinite number of planes. The stress at this point can be represented by the stresses on all of these planes. Graphically, all the stresses on all these planes describe an ellipse. (Draw stress ellipse)

21. Stress ellipsoid Describing the state of stress at a point: Graphically, all the stresses on all these planes describe an ellipse, with one long axis (the direction of maximum stress), and one short axis (the direction of minimum stress). These two axis are perpendicular

22. Stress in 3-D Stress ellipsoid In 3-D, all the stresses on all these planes describe an ellipsoid, with one long axis (the direction of maximum stress), and one short axis (the direction of minimum stress), and one intermediate axis. In 3-dimensions, the ellipsoid is defined by three mutually perpendicular principal stresses (s1> s2 > s3).

23. Stress ellipsoid • What is important about the principal stresses (s1> s2 > s3)? • The axes are perpendicular to each other. • They do not contain shear stresses • The state of stress of any body is described by the orientation and magnitude of the principal stresses.

24. Geology sign conventions Stresses 1) Normal stress Positive or negative 2) Shear stress Positive or negative Compressive stress is + (positive) Tensional stress is – (negative) Clockwise shear stress is – (negative) Counter clockwise shear stress is + (positive)

25. Mt Baldy Mapping Field Trip Mapping Guidelines General Geology of area -stratigraphy -general structures KML of fold Hand out Map -figure out general structure, -strike and dip of north limb Axial Planar Cleavage

26. Mt Baldy Regional Stratigraphy: Colulmbia River Basalts, Basalt, basalt…

27. Mt Baldy Regional Structures: Yakima Fold Belt

28. Mt. Baldy Lidar on Google Earth

29. Mt Baldy Map Contoured Shaded Relief from lidar

30. Axial Planar Cleavage Map symbol for attitude of bedding Map symbol for attitude of cleavage

31. Lecture outline • Overview of stress • Minimum and maximum stress • Types of stress on a plane • Normal stress • Shear stress • 4. Hydrostatic state of stress • 5. Stress and the Mohr circle • 6. Mean stress • 7. Differential stress • 8. Deviatoric stress

32. Relations between F and s sn =s cos2q ss = s*1/2( sin2q) Maximum normal stress when force is perpendicular (90°) to the plane Maximum shear stress when force 45° to plane

33. Stress in 3 D: We can also think about stresses on the sides of imaginary cubic rocks: • Reference system x, y, z • Imagine a cube of rock: • Every face experiences normal and shear stress => • Each face has one component of normal stress • Each face has two components of shear stresses

34. Stress State • If the principal stress are unequal in magnitude = anisotropic stress –The greatest stress is called s1 • -The intermediate stress, s2 and minimum stress is called s3 • If the 3 principal stresses are equal in magnitude = isotropic stress and the state of stress is represented by a sphere, not an ellipsoid. • Referred to as: • Pressure, • Hydrostatic stress • Lithostatic stress • Isotropic stress s1 > s2 > s3 s1 = s2 = s3

35. Hydrostatic Stress a.k.a. Pressure Lithostatic stress Isotropic stress If we calculate stress vectors within a point of a hydrostatic stress field, we find that all of the stress vectors in all directions have the same value. Stress ellipse is a stress sphere. Equal stress magnitudes in all directions. Dive into a pool. All stresses have the same values. Hydrostatic stress = all stresses are equal in all directions. Therefore, No shear stresses!

36. Stress on a dipping plane in the Earth’s crust 2 components Normal stress & Shear stress sn = s cos2q ss = s *1/2(sin2q) Review sign conventions for normal and shear stresses

37. Stress on a plane examples…

38. We resolve stress into two components • Normal stress, sn and the component that is parallel to the plane, shear stress, ss • Normal compressive stresses tend to inhibit sliding along the plane and are considered positive if they are compressive. • 2) Shear stresses tend to promote sliding along the plane, labeled negative if its right-lateral shear and possitive if its left-lateral shear.

39. Squeeze a block of cement between two planks of wood AB, trace of fracture plane that makes an angle q with 3. The 2-D case is simple, since 2 = 3(atmospheric pressure) Important: What is angle q?

40. Maximum principal stress (s1) and minimum stress (s3) act on plane P that makes an angle q with the s3 direction. In Mohr space, we plot s1 and s3 on sn-axis These principal stress values are plotted on the sn-axes because they are the normal stresses acting on plane P. The principal stresses always have zero shear stress values (ss = 0). Mohr Stress Diagram Mohr circle: sn on x-axis ss on y-axis.

41. Mohr Stress Diagram • This give us a useful picture or diagram of the stress equations. • b) They solve stress equations on page 49 (Eqs 3.7 and eq. 3.10) • c) Plot N versus S • d) Rearrange Eqs. 3.7 and 3.10 and square them yields • [sn – ½(s1 + s2]2 + ss2 =[½(s1 – s32 )] • form (x –a)2 + y2 = r2 Important: What is angle q? In Mohr space, we use 2q!

44. Mohr Stress Diagram a) Mohr circle radius = ½(s1 – s3] that is centered on ½(s1 + s3] from the origin. b) The Mohr circle radius, ½(s1 - s2] is the maximum shear stress ss max. c) The stress difference (s1 – s3), called differential stress is indicated by sd.

45. Mohr Stress Diagram Remember, sn,p = ½(s1 + s3] + ½(s1 - s3] cos 2q ss,p = ½(s1 - s3] sin 2q Construct a circle thought points s1 and s3 with 0, the midpoint, at ½(s1 + s3) as the center with radius, ½(s1 - s3]. Now draw a line OP, so that angle POs1 is equal to 2q– confusing step, plot twice the angle q, which is the angle between the plane and s3. Remember we measure 2q from the s1 side on the sn-axis. We can read the values of sn,p along the sn-axis, and ss,p along ss-axis for our plane P.

46. Mean stress and deviatoric stress Because a body’s response to stress, we subdivide the stress into two components, mean and deviatoric stress. Mean stress = [s1 + s2 + s3]/3 or sm In 2-D, [s1 + s3]/2 Deviatoric stress is the difference between the mean stress and total stress. stotal =smean + sdev smeanis often called the hydrostatic component (s1 = s2 = s3)

47. Mohr Stress Diagram When the principal stress magnitudes change w/o differential stress, the Mohr circle moves along the sn-axis without changing ss How is this achieved? Suggest geologic examples?

48. Mohr Stress Diagram When the principal stress magnitudes change w/o differential stress, the Mohr circle moves along the sn-axis without changing ss • Change confining pressure (Pc). Increase air pressure on our clay experiment, or carry the experiment underwater. • Burial of rocks changes confining pressure. Which way along the sn-axis? • Exhumation of rock changes confining pressure. Again, in what direction along the sn-axis?

49. Various states of stress Uniaxial compression, two of the three principal stresses are zero. Hydrostatic stress, a single point on the Mohr circle that lies on the x-axis. All normal stresses are the same, and no shear stresses.

50. Various states of stress Triaxial stress, all three principal stresses are different. Biaxial stress, all three principal stresses are non-zero, but two of the principal stresses have the same value. Typical stress ellipse (plane stress).