- Structural geology: branch of geology concerned with the shapes, arrangement, and
interrelationships of bedrock units and the forces that cause them.
- Stress- a force unit per area
- Strain- the change in size, shape or both in response to stress.
- Compressive stress- something being pushed together or squeezed from opposite directions.
It’s common along convergent plate boundaries and typically results in rocks being deformed in
a shortening strain.
- Tensional stress- caused by forces pulling away from each other. Results in the stretching or
extension of material.
- Shear stress- produced when stresses act parallel to a plane. Results in shear strain parallel to
the direction of the stresses, occurs on active moving faults.
- Elastic- if something recovers from a stress or strain and returns to its original state then it is
elastic. Ex: an elastic band
- Elastic limit- rocks are elastic until they reach their elastic limit, at which point they will deform
permanently. Ex: breaking an elastic band.
- Ductile- a rock that bends under stress and does not return to its original state after the stress is
gone. Ex: pizza dough
- Brittle- rock that will fracture at stresses higher than its elastic limit. Rocks near the earth
surface in low temperature/pressure climates.
- Geologic map- uses standardized symbols and patterns to represent rock types and geologic
structures. Most often produced from a field map for a given area.
- Strike- compass direction of a line formed by the intersection of an inclined plane with a
- Angle of dip- no real definition, figure 11.7 shows an example..
- Direction of dip- compass direction in which the angle of dip is measured.
- Geologic cross section- represents a vertical slice through a portion of Earth.
- Folds- bends or wavelike features in layered rock.
- Anticline- upward arching fold
- Hinge line- basically the middle of a fold.
- Syncline- downward arching counterpart of an anticline
- Limb- shared by an anticline and a syncline.
- Axial plane- an imaginary plane that connects all the hinge lines of a fold.
- Plunging folds- folds where the hinge lines are not horizontal.
- Structural dome- when the beds dip away from a central point.
- Structural basin- when the beds are dipping towards the central point.
- Open folds- have limbs that dip gently. Usually, the more open the fold is, the less it has been
- Isoclinal fold- limbs are parallel to one another. Implies more stress/strain has occurred. - Overturned fold- when the axial plane is inclined so much that the fold limbs dip the same
- Recumbent fold- are overturned to such an extent that the limbs are almost horizontal.
- Joint- when there is no displacement or movement in a fracture/crack in bedrock it is called a
- Joint set- can be defined when joints are basically parallel to one another.
- Dip slip fault- movement is parallel to the dip of the fault surface.
- Strike slip fault- indicates horizontal motion parallel to the dip of the fault surface.
- Oblique slip fault- has components of the 2 previously mentioned types of faults.
- Hanging wall- side of the fault above the inclined fault surface.
- Footwall- side of the fault below the fault surface.
- Normal fault- hanging wall block goes down in relation to the footwall block.
- Reverse fault- hanging wall block goes up, footwall block goes down.
- Thrust fault- dip of the fault plane is at a low angle or even flat.
- Strike slip fault- movement/slip is mostly horizontal in relation to the strike
- Right lateral fault- if you look across the fault and see the displaced stream to the right.
- Left lateral fault- you look across the fault and the displacement is to the left.
Chapter Notes (a lot of this chapter is based upon the diagrams and pictures from the textbook. It’s also
boring as hell, warning you now.. But ya, I tried to cite the more important diagrams that explain things
visually. Hope it works!)
This chapter focuses on explaining how rocks respond to tectonic forces and how geoscientists study the
resulting geologic structures or architecture of the earth’s crust. We will examine faults, which are
related to understanding earthquakes. Faulting and folding is also important in understanding how
mountains and continents have evolved (ties into chapter 20). Structural geology is another thing that
will be looked at; basically it is the architecture of the earth.
What causes rocks to bend or break?
Stress and strain in the earth’s lithosphere.
- Tectonic forces move and deform parts of the lithosphere, mostly along plate margins.
- Rocks are deformed by stress. It is difficult to measure stress on rocks that are buried. So we
observe past stress forces when rock bodies get exposed from uplift or erosion.
- Strain is the change in shap