Strike-slip faults
Nearly vertical fault with horizontal motion
Bisect and offset the spreading ridges
San Andreas fault - good example
Active planet - powered by immense internal forces
Produce a constantly changing surface, but at a very slow overall rate
Strickler's 2nd Law of GeoFantasy: There are no catastrophic processes, only catastrophic results
Plate boundaries
Two (or more) plates interact along huge linear zones of faulting
Tectonic activity is common here
The amount of energy involved here is immense
Spreading centers vs. subduction zones
The outer portion of the earth is fairly complex
Not a simple crust/mantle situation like was originally assumed
Lithosphere: the outermost layer
Includes the crustal plates
High strength, brittle material beneath them
Approximately 60 miles thick
Asthenosphere: below the Lithosphere
Low strength, ductile material extending down to 150 miles
Where 2 plates are moving away from each other
Deep rifts are opened through the crust
Allow magma from the upper mantle to rise to the surface and cool
Attempt to "heal the wound"
Newly differentiated mantle material
Basaltic flows, pillows, and breccias
Basalt: dark, fine-grain, high density igneous rock
The "Blood of the Earth"
Moves like conveyor belt to a subduction zone where it is destroyed
Therefore, oceanic crust is geologically young (<200 million)
Ophiolites:A section of seafloor
A true miracle of preservation: Should be subducted
Describe basic stratigraphy
Sediments / Basalt / Sheeted Dikes / Gabbro / Ultramafics
Volcanic features of divergent boundaries
Basalt is both fluid and dry (low volatile content)
Therefore, non-explosive
Characterized by fluid eruptions and shield volcanoes
Videodisc movies & stills
Go to rock display
Crustal stress
The movement of large areas of the crust vertically & horizontally
Immense stresses at an extremely slow rate (see Strickler's 2nd Law of GeoFantasy)
Different sections of the crust are moving at different velocities
Therefore they interact at their edges - plate margins
All this up and down involves distorting the crust
Several things can happen
Break - fractures & joints
Break & slip - faults
Fold
What happens depends on
Rock type
Temperature
Type and magnitude of the force (stress)
When cold rock moves, it can break
Must be relatively cold and brittle
Called fractures, or joints
Faults: A fracture where the sides have moved relative to each other
Usually a planar surface
Hangingwall vs. footwall
Several types of faults, based on the relative sense of motion
Normal faults - hanging wall down
Common at spreading centers
Usually the result of tension and crustal lengthening
Horst & Graben (see fig. 8-13; pg. 171)
Actually Valley Building, not Mountain Building
Tensional features - stretching of the crust
Therefore predominantly normal faulting
Shallow earthquakes: Down to 12 miles
Area of high heat flow: Hot rocks expand
Commonly form long, relatively narrow topographic highs
Example: Mid-Atlantic Rise (or Ridge)
Actual spreading takes place near the crest of the rise
As plate moves away from spreading center
Rock cools and subsides to a lower elevation
Results in the Guyots described by Hess
Hydrothermal vents (black and white smokers)
High temperature vents which support exotic life forms
Tube worms, etc.
Some speculation that life originated in similar environments
Also lead to formation of massive sulfide ore deposits
Gold, copper, zinc
Triple junctions
EXAMPLE: super ball in drawer, cracks in pavement
Only two remain active - third commonly fails
Saudi Arabia vs. Africa; Baffin Bay
Zone of Lateral Movement
Strike-slip faults
Nearly vertical fault with horizontal motion
Bisect and offset the spreading ridges
San Andreas fault - good example
You are GeoManiac number since April 1, 1997