What is Structural Geology, and Why
Study of how the lithosphere is bent, broken, and deformed during plate
Structural geology is important
The locations of earthquakes.
The formation of
The tectonic history of the
How to safely building structures
such as bridges and dams.
How to locate natural resources like
oil and gold.
Mechanical Behavior of the
Stress = Force/Area
Strain = A change in shape or
volume in response to a stress.
Stress is the cause, strain is
are three types of stress:
1) Compressive stress –
Forces that squeeze or push toward one another from opposite directions (cause shortening or
2) Tensional stress –
Forces that pull
away from one another in opposite directions (cause stretching or extension).
3) Shear stress – Forces
that are offset from one another and operate in opposite directions (cause a shear strain).
The behavior of rock during
stress depends on:
1) The type of rock.
2) The type and amount of stress.
3) The temperature.
4) The pressure.
5) The rate at which the stress is
applied (think about Silly PuttyÔ).
geologists do “rock squeezing” experiments to examine how rocks respond to
The three types of
rock behavior are:
3) Ductile/plastic behavior
Elastic Behavior – A lightly-deformed material will recover its original shape after the stress
is removed. Elastic behavior occurs at very low stresses.
Every material has an elastic limit, the maximum amount of stress a material can feel and still
recover to its original shape.
The behavior of rock upon reaching the elastic limit depends mostly
on the temperature and pressure.
– Near the Earth’s surface (low T and P), rocks exhibit brittle behavior. When a stress exceeds the elastic limit,
fracture (faults and joints form).
– At depth in the Earth (high T and P), rocks exhibit plastic behavior.
When a stress exceeds the elastic limit, rocks will bend and flow (folds and foliations form).
The Development of Folds by
Rocks that are deeply seated within the Earth exhibit plastic behavior during stress.
Fold- A bend or wavelike
feature in rocks formed by compressive stresses.
There are two main types of folds:
1) Anticline- An upward arching
Wikipedia Commons photo.
2) Syncline- A downward arching
Rainbow Basin Syncline near Barstow, California. Mark A. Wilson (Department of
Geology, The College of Wooster)
Synclines and anticlines often occur adjacent to one another as part of a larger
A syncline-anticline pair, Wikipedia Commons Image.
Warning! Anticlines do not necessarily
form hills and synclines do not necessarily form valleys on the surface of the
beveled flat at the Earth’s surface due to erosion.
How would a syncline look after erosion? Would the oldest or youngest
layers appear in the center of an anticline?
How would an anticline look after erosion, and which layer (oldest or youngest)
would appear at the center?
Other Vocabulary Related to
Axial plane- A plane that
bisects a fold
Fold axis- A line marking the
intersection of the axial plane with the folded layers (also called the hinge of
Limbs- The portions of a fold on
each side of the fold axis.
Symmetrical fold- A fold in which the axial plane is vertical.
Asymmetrical fold- A fold in which the axial plane is inclined.
Overturned fold- A fold in which one of the limbs has overturned bedding.
Recumbent fold- A fold in which
limbs are overturned to the extent that the limbs are essentially
Plunging Fold- Not every fold has a fold axis that is parallel to Earth’s
surface. The fold axes may plunge into the earth.
plunging anticline will have the oldest layer in
the center and will "V" in the direction of the plunge.
plunging syncline will have the youngest later in
the center and will "V" in the direction opposite of the plunge.
Measuring Strike and Dip
The law of original horizontality
says that sedimentary rocks are deposited in horizontal layers. However, stress
may cause the layers to become tilted (or folded) such that they are no longer
James D. Dana, New Text-book of
Geology (New York: Ivison, Blakeman & Company, 1883)
Geologists measure geologic
structures and plot their orientations on maps in order to learn about the
stresses that caused them (thereby unraveling the tectonic history).
The orientation of a planar surface like a bedding plane
can be fully defined using strike and dip.
Strike- The compass direction of a line marking the intersection of an inclined plane
with a horizontal plane (i.e., the Earth’s surface).
Dip- The maximum angle between the inclined plane and the horizontal plane. Dip is
measured perpendicular to strike. Dip has both an angle and a compass direction.
The above images are from James D. Dana, New Text-book of
Geology (New York: Ivison, Blakeman & Company, 1883)
Anticlines and synclines can be
detected by looking at the strike and dip of layering. The limbs of a syncline dip
towards the fold axis, whereas the limbs of an anticline dip away from the fold axis.
The Development of Joints and Faults by
Rocks at Earth's surface are brittle and tend to fracture, or break, due to stress.
- A planar feature (a type of fracture) along which there has been no movement of one side
of the fracture relative to the other side. As conduits for fluid flow, joints may be sites of ore deposition and
Primary joint- A joint that forms as a result of non-tectonic stresses,
Tectonic joint- A joint that forms by tectonic stresses.
Columnar jointing- A type of primary jointing
that forms when basalt cools from solidification temperatures.
Columnar basalts at Devils Postpile, California, © Bruce Molnia, Terra
Sheet jointing- A type of primary
jointing that forms by the unloading of rock (also known as exfoliation).
Sheet jointing in Granodiorite. © Marli Miller,
University of Oregon.
Joint set- A set of parallel joints
(often indicative of tectonic stresses).
Joint-controlled streams in central New York
flow along two joint sets that are oriented at right angles to each other.
Copyright © Bruce Molnia, Terra Photographics.
A planar feature in which there has been movement
of rock, called displacement, along the plane. The displacements may range
from centimeters to hundreds of kilometers.
Fault breccia- Some faults are characterized by ground up rock fragments
that may be cemented together to form a fault breccia.
Slickenside- Rocks moving against each other along a fault plane may develop
smooth surfaces and striations called slickensides.
The three main types of faults (classified
by how the rocks on each side
move relative to each other):
Fault- Movement of the rocks is
parallel to the dip of the fault plane.
Hanging wall block-
The part of the fault
above the fault plane.
The part of the fault below
the fault plane
Normal fault- A type
fault in which the hanging wall moves down relative to footwall. Normal
faults are the products of tensional stress.
Tension in the Earth’s crust leads to normal faulting and a
and graben terrain (the Basin & Range topography of the southwestern US).
-Horst- A block of land that is bounded by normal faults and moves upward relative
to adjacent blocks.
-Graben- A block of land that is bounded by normal faults and moves down
relative to adjacent blocks.
Reverse fault- A type of dip fault in which the
hanging wall moves up relative to footwall. Reverse faults are the result
of compressional stress. A low angle reverse fault is also known as
a thrust fault (imagine a subduction zone).
2) Strike-Slip Fault-
A type of fault in which rock moves horizontally along the fault plane.
Looking across the
fault, a feature like a stream has been displaced to the right. The San Andreas Fault in California is a
right-lateral strike-slip fault.
-Left-lateral Fault -
Looking across the fault,
a feature like a stream has been displaced to the left.
3) Oblique Fault- A type
of fault in which movement is neither
pure dip fault nor pure strike-slip fault. In other words, this type of
fault has components of both dip and stike-slip faults.