Revised 8 / 06 (Monroe 6th ed.)
Including...
Introduction to crustal stress
Directed stress and crustal deformation
Directed stress and plate tectonics
Before we start, we need to review the following:
There are 2 main earth elevations
Above and below sea level
There are 2 main earth processes
Construction and Destruction
Over the course of geologic time these 2 are in balance
There are 2 main igneous rocks
All three of these are inter-related (DESCRIBE)
What we are studying tonight is the reality of tectonics
DEFINE: Tectonics
Tectonics: the study of earth processes which result in the creation and deformation of magma and rock
Constructional processes
My favorite part of geology (really should save to last!!)
Some are real flashy: get some videodisc frames
All involve stress at varying degrees of intensity
Mt. St. Helens - good example of stress at the surface
Volcanoes, earthquakes indicate immense stress at deeper levels
The earth is a huge rock, and it gets real hot and tight real soon with depth
Before we continue, a bit of background
Lithostatic stress vs. directed stress
Litho-static stress (Monroe; fig. 8-7, pg. 241)
The load weight of overlying rock
Equal pressure in all directions
Directed pressure (REVIEW)
Acts in a specific direction
EXAMPLE: Drop a pencil; gravity is the directed stress
Generally related to tectonic processes associated with plate motions
Results in faults, folds, and other orogenic (mountain building) processes
The 3 types of strain (Monroe; fig. 13-3, pg. 392)
Each is associated with a particular type of plate boundary
And results in different types of crustal deformation
Tension - spreading centers
Lengthens the crust
Compression - subduction zones
Crustal shortening
Shear - transform faults
DEFINE: Hooke's Law (the 1:1 relationship between stress and strain in a tectonic environment)
REVIEW: the brittle-ductile transition zone
There are several things we can all agree upon
The earth isn't flat
Erosion tries hard, but can't keep up with uplift
There is movement of large sections of the crust vertically & horizontally
Immense stresses at an extremely slow rate
Visualize India into Tibet
DIGRESS TO: Strickler's 2nd Law of GeoFantasy
The crust doesn't re-adjust to changing stress at a uniformly even rate
Different sections of the crust are moving at different velocities
(DEFINE: velocity)
Therefore they interact at their edges - plate margins
We all know that rocks are hard!
Usually takes considerable force to break one
And when it does break, it can do so forcefully
It's also clear that most of the rocks we see at the surface are broken
Cracked, shattered, tilted, bent
Some are folded without breaking
Indicate that, under proper conditions, rock isn't hard at all, but plastic
How rocks deform, and why, is the study of structural geology
Involves near surface to deep crustal processes
Complex interactions between temperature and directed stress
Most ultimately tied to plate interactions
All this differential motion results in deformation of the crust
DEFINE: Deformation
Like a sack full of pissed-off cats
Several things can happen when rock is stressed
Break - fractures & joints (Monroe; fig. 13-14, pg. 400)
Break & slip - faults (Monroe; fig. 13-16, pg. 402)
Fold - plastic deformation (Monroe; fig. 13-7, pg. 395)
What happens depends on 4 main factors
Rock type
Temperature
Magnitude of the force (stress)
Strain rate
Rock type
Different rocks have different physical (and chemical) properties
React differently to stress and strain
Fundamental differences - igneous vs. sedimentary vs. metamorphic
More subtle differences
Sandstone more brittle than limestone
CaCO3 more reactive than silica
Temperature
Directly related to depth below surface
Temperature and pressure both increase dramatically with depth
Near surface processes - low temperatures and pressures
Rocks are brittle
Rocks undergo elastic deformation (Monroe; fig. 13-4, pg. 392)
Result of directed pressure (or stress)
If elastic limit is exceeded, a rock will rupture
Works a lot like a spring (or a meter stick)
Will deform (stretch or compress) when stress is applied
Returns to original shape when stress is removed
But if overstrained, the spring (or meter stick) will break
Rocks are like this
When hit by a hammer, the rock will elastically deform (compress)
And snap back into its original shape
This snap back is why a hammer "bounces" off a rock
If hit too hard (overstrained), the rock will rupture
Indicating its elastic limit has been exceeded
Deep seated processes - high temperatures and pressures
Rocks behave as ductile (plastic) substances
Deform elastically at first, then plastically
Results in permanent deformation
May rupture if stress exceeds elastic limit
Note that this limit is directly affected by the elevated temperature and pressure
Works like modeling clay
Does not snap back into shape when the stress is removed
Evidence of deep seated deformation is only evident after uplift and erosion
Therefore, any folded rock at the surface indicates extreme uplift and erosion
Magnitude of the force - pretty obvious
Like a hammer vs. a wrecking ball
Strain rate - the rate in which the pressure is applied
The faster rock is forced to deform, the more likely it is to break instead of fold
There are actually very few cataclysmic tectonic forces in nature
See Strickler's 2nd Law of GeoFantasy
Cataclysmic results are relatively common, but it's clear that...
Low stresses over a long period can result in intense plastic deformation
DIGRESS TO: Orientation of planar features
Strike and dip (Monroe; fig. 13-5, pg. 393)
Show accepted symbols
Inclined plane
Horizontal and vertical planes
The most common type of structure (at least at and near the surface)
Try to find an outcrop without fractures
A break in the rock along which little or no movement has occurred
Result of brittle failure due to compressional and/or tensional stress
Usually come in "sets"
Caused by regional directed stress
Common to all rocks exposed at the surface
Indicates that the causes are many and varied
Tectonic activity
Mountain building (orogenics), plate interactions
Non-tectonic stresses
Shrinkage due to cooling or drying
Columnar basalt
Expansion due to release of pressure - very common at surface
Exfoliation
A joint or fracture along which noticeable movement has occurred
Single breaks Fault Plane
Complex zones of shearing Fault Zone
Can be the result of plastic deformation at depth
With no specific fault plane
Several basic types
Related to the relative sense of displacement across the structure
Dip slip vs. strike slip
Dip slip: relative motion of the hangingwall vs. footwall
DEFINE: Hangingwall and footwall (Monroe; fig. 13-15, pg. 401)
DEFINE: Relative sense of displacement
Normal Faults; Tensional stress (Monroe; fig. 13-17, pg. 404)
Hangingwall drops relative to footwall
Results in lengthening of the crust
Horst & Graben (Monroe; fig. 13-18, pg. 406)
Actually valley building, not mountain building
Reverse Faults; Compressional stress (Monroe; fig. 13-17, pg. 404)
Hangingwall goes up relative to footwall
Results in shortening of the crust
Thrust Faults; Compression (Monroe; fig. 13-17, pg. 404)
Low-angle reverse (dip < 45°)
Horizontal displacement greater than vertical displacement
(Are there low angle normal faults, and what are they called?)
Strike Slip Fault (Monroe; fig. 13-17, pg. 405)
Example: San Andreas
Near vertical dip
Right and Left Lateral
Oblique (Monroe; fig. 13-17, pg. 405)
Some combination of the above
Most faults are probably somewhat oblique
Drag folds - relatively common
Can indicate sense of relative motion
WARNING: We'll see these in G-103 during our work with geologic maps - it will pay to remember them!
"Directed compression of the crust, resulting in a semi-plastic deformation"
Immense stresses at an extremely slow rate
Usually occurs at depth in the crust
Increased heat and pressure cause rocks to bend and fold instead of break
REVIEW: the brittle-ductile transition zone
Most commonly observed in sedimentary rocks (Monroe; fig. 13-7, pg. 395)
Also evident in igneous and metamorphic rocks
DIGRESS TO: Metamorphics
In any event, folding and metamorphism go hand in hand
Both occur at depth at increased temperatures and pressures
Generally related to compression of the crust
Crustal shortening (Monroe; fig. 13-10, pg. 397)
Therefore same stress environments as those which produce reverse faults
Result of regional directed pressure: DEFINE
Lots of names (Monroe; fig. 13-11, pg. 398)
Based on relationship between the axis and the limbs of the fold
Axis of the fold (Monroe; fig. 13-8, pg. 396)
Axial plane: divides fold in half
Each half is called a "limb"
Can be defined by taking strike & dip
Commonly sub-parallel in regionally folded terrain
Axial plane cleavage (Monroe; fig. 13-8, pg. 396)
Increased temp. & pressure in axis can result in local metamorphism
Anticline (Monroe; fig. 13-10, pg. 397)
More or less well defined linear axis with symmetrical limbs
Upwarping of crust
Oldest beds in center
DIGRESS TO: old/young vs. frown/smile
DO NOT always result in a topographic high! (Monroe; fig. 13-9, pg. 396)
Usually accompanied by a...
Syncline (Monroe; fig. 13-10, pg. 397)
More or less well defined linear axis with symmetrical limbs
Downwarping of the crust
Youngest beds in center
DO NOT always result in a topographic low! (Monroe; fig. 13-9, pg. 396)
Plunging folds (Monroe; fig. 13-12, pg. 399)
Fold axis is inclined to the horizontal
WARNING: We'll see these in G-103 during our work with geologic maps - it will pay to remember them!
Dome vs. Basin (Monroe; fig. 13-13, pg. 400)
Less common than anticline/syncline
No real axis & limbs dip away (or to) the center of the structure
Lots of others
Lumpers & Splitters again
Can get extremely complicated mathematically
In any event, all tell the story of directed stress within the crust
Spreading, subduction, lateral offset
Each has a specific style of deformation due to differing types of stress
Compression of the crust
Reverse faults and folding
Intermediate to felsic magma
Where many major mountain ranges are formed
(Monroe; fig. 13-20, pg. 408)
(Monroe; fig. 13-21, pg. 409)
Extension of the crust
Normal faults, no folding
Mafic magma
Basin and Range province
Horst and Grabben block faulting (Monroe; fig. 13-18, pg. 406)
Transform faults (Monroe; fig. 13-19, pg. 407)
Lateral side-to-side shearing
Commonly offsets spreading ridges
Not associated with magma generation
Therefore no volcanics
GeoMan's Home Page | RCC Index | High School Geology Index
You are GeoManiac number since April 1, 1997