10. … we’ve seen that during
geological development
of an area, there can be
time intervals during
which rock layers
become tilted, but how
does this happen?
11. structural geology: study architecture of earth
reasons for study:
earth history
avoiding earth hazards
avoiding pollution
locating earth resources
energy
mineral
water
Structure & Mountain Building
12. stress
force applied to material that tends to change its
dimensions
strain
effect of stress shown by material
strength
limiting stress that a material can withstand without
failing by rupture or continuous plastic flow
Rock Behavior and Deformation
13. type of stress
amount of pressure
temperature
type of rock
length of time rock subjected to stress
response of rock to stress depends on:
Rock Behavior and Deformation
15. compressional stress
forces directed toward one another
decreases volume of material
lithostatic pressure, example of all-sided
confining pressure produced by burial
tensional stress
stretching stress that tends to increase
volume of a material
Types of Stress
16. shear stress
force parallel, but in opposite directions
results in displacement of adjacent
layers along closely spaced planes
Types of Stress
18. strain
brittle deformation
rock breaks if applied
stress is too great
rocks at or near surface
(cold, low pressure) tend
to deform by brittle
rupture
results in fracturing and
faulting (rock shows
differential movement on
either side of the fracture
surface
Rock Response to Stress
21. The Time Factor
at particular temperature and pressure, response
of rock to stress is dependent upon
type of stress
length of time over which stress applied
rapid application of stress favors brittle
deformation
slow application of stress favors plastic
deformation
22. Strength of Rocks
different types of rock respond to stress differently
different strengths for different types of applied stress
tensional strength is less than compressional strength
23. material will act
brittle when difference is large
near surface where temperature and pressure
are low
ductile when difference is small
before failure at high confining pressures and/or
high temperatures
Strength of Rocks
24. … how do we quantify
a rock layer’s
response to
application of force
from field
observations?
25. geologists use
concept of strike
and dip to
describe
orientation of
deformed rock
layers
Measuring Rock Deformation
26. strike - bearing
(direction) of
horizontal line
on rock bed or
structure
dip - angle
between
horizontal and
rock bed or
structure
Measuring Rock Deformation
34. concentric (flexural
slip) folding -
bending of surface
rock beds without
change of thickness
or volume (= elastic
deformation)
Folding and Plastic Deformation
36. flow folding - thickness
and volume of rock beds
change as plastic rocks
subjected to directed
stress at high pressures
and temperatures (=
plastic deformation)
Folding and Plastic Deformation
104. movement may be restricted to basement layers
or as growth fault, synchronous with deposition
of younger sedimentary/volcanic layers
Normal Faults
118. left-lateral - rock on opposite side of fault moves to left
Strike-slip Faults
119. transform fault is strike-slip fault that allows lateral
movement of new crust away from mid-ocean ridge
without relative position of ridge segments changing
Strike-slip Faults
120. involves both dip-slip and strike slip movement
left-laterial, reverse or right-lateral, normal
Oblique-slip Faults
122. oil and gas are formed from organic sediments and can
migrate through permeable strata
two requirements for economic plays:
$ source rocks - ‘mature’ organic rich sediment [oil and gas
window is thermal range for formation and stability
of hydrocarbons]
$ traps - permeable rocks that are ‘capped’ by impermeable
strata
Practical Aside: Oil and Gas Traps
