Structural geology is that part of geology that looks at shape
arrangement
and interrelation of rock units and the forces behind them.
Tectonic forces at work
1) STRESS - force that tends to change the shape or size of a body
and stress could be compressive, tensional, or shear. Here strain is
to direction of force.
2) STRAIN- the resultant change from stress
In response to the application of stress, rock could be deformed (strained) in 3 different ways.
. Plastic strain happens when the rock does not return to its original size or shape - ductile
. Elastic strain happens when the rock recovers to its original shape/size after stress is released
. Fracture happens when
a rock breaks
or cracks - brittle
Stress is usually slow, but could be a sudden event, as during
earthquakes.
A fault is a fracture (crack) where there has been movement
before.
Structures as a record of the geologic past
Structures such as dikes, sills, stocks, and batholiths give us some
idea
about past igneous activity.
STRAIN
Geologic Maps and Field methods
- Aerial view (not always possible)
- Geologic cross section is "a vertical representation of a portion of the earth"
- Formations are geologic mappeable geologic units.
Strike is the "compass direction
of a line formed by the intersection
of an inclined plane with a horizontal plane" (the direction of the
edge
of the tilted bed)
Angle of dip is measured from
a horizontal plane to the bedding
plane
Dip direction is the angle of
direction upward or downward from the strike
Folds are bends in layered bed rock
Geometry of folds are:
-Axis
-Hinge
-Plane
-Limb
-Anticline
-Syncline
Note: Synclines may not be related to valleys
Plunging fold - hinge line and
axis
are not horizontal
Structural Domes and Structural Basins
basin and dome are usually large > 100 km. across
Interpreting Folds
Open folds
Isoclinal folds
Overturned folds
Recumbent folds
Fractures in Rock occur in brittle rock or when force
is exerted too fast for the rock to accommodate the bending. To
accommodate
the strain, the rock breaks. If there is no movement, we call the
fracture
a JOINT and if there is movement, we call it a FAULT.
Joints can be formed as hexagonal Columnar joints formed from
cooling
of lava.
Exfoliation (sheet jointing) expansion
Joint set -
Could have several sets of joints
Characteristic of faults
strike of fault, plane, and dip of fault were already given earlier
in this chapter.
hanging wall
foot wall
foot wall block
1. Normal fault
2. Reverse fault
3. Thrust fault
4a. Left-lateral fault - displacement to the left
4b. Strike-slip Faults
5. Right-lateral fault
Unconformities- represent a break in the geologic record.
3 Types
1. Disconformity
2. Angular unconformity
3. Nonconformity
