What is a geologic structure ?
A geometric feature in rock whose shape, form, and distribution can be described.
It is the branch of geology that deals with:
- Form, arrangement and internal architecture of rocks
- Description, representation, and analysis of structures from the small to moderate scale
- Reconstruction of the motions of rocks
What is structural geology ?
It is the branch of geology that studies the 3D geometry from micro to macro scale of rocks to explain the deformation processes the rocks experienced since their origination.
It introduces the physical side of Geological Sciences and emphasizes:
- Geometry (shape, orientation, position, size, etc.)
- Motion (beginning and ending positions and paths of particles and bodies—deformation or change in geometry)
- Mechanics (explanations of why the geometry and motion are asthey are)
Includes lots of observations from the field (but also some from the laboratory and the computermodelling)
Teaches you not only facts, but also skills and techniques that are necessary in advanced classes and central to geologic practice.
Structural geology provides information about the conditions during regional deformation using structures.
What are the job opportunities for a structural geologist ?
- Oil and gas industry (Field exploration)
- Mining (Field exploration)
- Underground Water resources (Field exploration)
- Engineering (Construction, Industry, Hazard)
- Academics & Research in various subjects (Earthquakes, Classical
Geology, Planetary Sciences, NASA).
What do we study in structural geology?
Structural geology studies the strain, which is the end product of deformation in
extremely heterogeneous materials.
We infer the stress that causes strain; we never observe stress while it is
Strain ——> Shortening or lengthening (extension)
Stress —–> Compression or Tension
We measure attitude of planes and lines.
Attitude: the orientation of a plane or line
Global:A scale covering almost the entire world
Regional or Provincial:Roughly definable; generally corresponds to a physiographic province. Taurus Mountains, Himalayan Plato.
Macroscopic or Map Scale:Larger than an area one can see from a particular point on the ground.
Mesoscopic:An area visible from a particular point on the ground (outcrop to hand sample)
Microscopic: Visible with the help of an optical microscope.
Submicroscopic:Visible the with help of an advanced microscopic device like TEM (Transmission electron microscopy) or SEM (scanning electron microscope)
Penetrative:Characterizes the entire body of the rock
Non-penetrative:Does not characterize the entire body of the rock (ex. a part of the body)
Are structures that develop during the formation of the rock. (Ex. Bedding, ripple mark or cross bedding in sedimentary rocks). Primary structures represent the local conditions of the environment within which the rock forms (Davis & Reynolds, 1996).
Are structures that develop in sedimentary or igneous rocks after lithification, and in metamorphic rocks during or after their formation. Fundamental secondary structures are joints and shear fractures; faults, folds, cleavage, foliations, lineations, shear zones (Davis & Reynolds, 1996).
Important Terms in Measurement
- Trend: The direction of a horizontal line specified by its bearing or azimuth.
- Bearing: The horizontal angle measured east or west from the true north or south.
- Azimuth: The horizontal angle measured clockwise from the true north.
- Strike:the trend of a horizontal line on an inclined plane. It is marked by the line of the intersection with a horizontal plane. (Davis & Reynolds, 1996).
- Inclination: Theverticalangle, measureddownwardfromthehorizontaltoa slopingplaneorline.
Is a local separation or discontinuity plane in a geologic formation, such as joints or faults.
A joint is a separation in rock where the amount of separation is not greater than the displacement associated with the opening of the fracture.
Distinct fracture surfaces along which rocks have been offset by movement parallel to the fracture surface.
Folds are planar surfaces that are curved or bent due to external forces.
Foliation is any penetrative planar fabric or layering in a rock.
(Marshak & Mitra, 1988)
Is a prefered linear alignment of elements in rocks. (Davis & Reynolds,
Is a general term for a relatively narrow zone with subparallel boundaries in which rocks are more highly deformed than rocks adjacent to the zone. (Marshak &
Symbology in structural geology
Some common symbols whose usage is well established: