DEFORMATION OF THE EARTH’S CRUST AND MOUNTAIN BUILDING Chapter 10

 

                        DEFORMATION (AND GEOLOGIC STRUCTURES)

 

A.     Mechanics of  Deformation

1.      Terminology p.219-220

a.      Stress—force per unit area (directional)  See Fig. 10.3, p. 220

i.                    Tension = Tensile stress ---stretching

ii.                   Compression = Compressive stress---squeezing

iii.                Shear Stress: 

                                                    b.  Strain---deformation in material as a response to stress

                                    NOTE:  ignore “Strike and Dip” portion of p. 221

                                    NOTE:  the next three items (#2-#4) are covered in more detail in class than in the text

2.   Types of behavior of solid materials See page 220  

b.      Elastic behavior:  Stress results in deformation accompanied by a build up of strain energy; when the stress is removed, the strain energy is suddenly released and the material “snaps” back to its original (pre-deformation) size and shape

c.       Plastic = Ductile Behavior:  material readily deforms under stress, but accumulates no strain energy; when stress is removed, the deformation remains.

d.      Materials that show elastic behavior are considered “Brittle”.  Brittle failure occurs when the stress exceeds the material’s ability to deform.  Strain energy is released when the material fails (breaks)

3.    Behavior of rocks under stress

e.      At the earth’s surface, rocks are brittle, however, we see highly deformed sedimentary and metamorphic in nature. 

f.        Under pressure, stress and elevated temperatures, over long periods of time, many normally brittle rocks can act as ductile or plastic solids

g.      Rocks such as shale and rock salt are particularly subject to ductile flow under pressure

4.    Causes of Stresses in the Earth’s Crust—these come in all sizes from very small to extremely large; small stresses cause slight deformation and large stresses       (plate collisions, for example) cause large scale deformation

h.      Earth Tides (small scale)

i.        Removal of overburden (small scale—causes fractures)

j.        Tectonic activity---complete range from small force fields to (extremely large) plate collisions

                        B.  Types of Geologic Structures ----Note: fractures, faults and folds come in all sizes from microscopic to enormous

1.      Fractures---breaks in rock; called “JOINTS” see p. 224-226

a.      Random fractures (joints) ---random orientation

b.      Systematic joints (Joint Set)---breaks occur in preferential orientations

2.      Fault----occurs when differential movement occurs along opposite sides of a fracture pp. 226-229

a.      Terms

i.                    Fault  plane

ii.                  Hanging Wall/ Foot Wall see Fig. 10.11-a,  p. 227

b.   Type of Faults See Fig 10.12, p. 228; ignore “oblique-slip fault”

i.                    Normal Fault

ii.                  Reverse Fault

iii.                Strike-Slip = Lateral Fault

iv.                Thrust Fault = special type of low angle reverse fault associated with fold belt mountain building

3.      Folds----See pp. 221-225  (Ignore “domes” and “basins”)

a.            Terms

i.                    Know:  “axial plane” and “limbs” of a fold

ii.                  Anticline:  limbs dip away from the axial plane

iii.                Syncline:  limbs dip toward the axial plane

iv.                Know:  “plunging fold” 

b.            Classification of Folds (used for both anticlines and synclines) See Fig. 10.7 (more detail in class than in text)

i.                    Symmetric Fold = axial plane vertical

ii.                  Asymmetric Fold = axial plane slightly tilted

iii.                Overturned Fold = axial plane tilted such that beds “lie over themselves”

iv.                Recumbent Fold = axial plane horizontal.

 

MOUNTAIN BUILDING

 

A.  Terminology  page 232

      1. Mountain:  area of land about 1000 ft higher that surrounding land that has a restricted (small) summit area

      2. Mountain Range:  linear (elongate) series of related mountains of same age and origin:  Example:  Cascades

      3. Mountain System:  complex linear (elongate) zone of deformation and crustal thickening; consists of a number of mountain ranges.  Ex:  Appalachians

 

B.  Types of Mountains  see page 232; Also read pp. 233-237 (but don’t be too concerned as this is much more detailed than covered in class)

1.      Volcanic Mountains—may occur as isolated peaks or as part of a mountain range (a group of closely space and related mountains).  Associated with either divergent or convergent plates, but not neutral boundaries.   Example:  the CASCADES result from the subduction of the small Juan de Fuca Plate beneath the N. American Plate.  The MID-ATLANTIC RIDGE is at the divergent boundary where the N. American and Eurasian Plates are separating.

2.      Fold-Belt Mountain Systems----Large complex mountain systems commonly associated with convergent plates involving one leading edge of oceanic (that is subducted) and the other leading of continental crust.  When the subduction can not longer continue, the extensive compression lead to uplift and formation of major mountain systems such as the ALPS, APPLACHIANS or the URALS for example

3.      Fault-Block Mountains----Ranges forms by normal faulting---may be associated with Rifting (as in E. Africa) or other plate relationships (such as occurred in much of the western U.S.) Example:  Grand Tetons, Wyoming or Sangre de Cristo Range in Colorado.

                 

C.    Isostasy (previously note with regards to glaciers)—after the formation of a major mountain system is completed, there continues be continuous weathering and erosion coupled with isostatic rebound that may persists for 10’s or 100’s of millions of years; hence, major mountain systems persist for long periods of geologic time.  See 239-241.   See Figures 10.21 (p 240) and 10.22 (p241).