Including...
What are the plates moving around on?
Let's look closer at plates and plate boundaries
Continue our look at the earth's crust
Learned yesterday that the crust is broken into numerous "plates"
Will start with a closer look at the theory of Plate Tectonics
Revolution in the Earth Sciences
Theory unified geologic thought
Ties together nearly all facets of Earth Science
Only the most recent theory: improve on earlier work, but still not dogma
At least not yet, and when it is it will be useless
"The safe course leads ever downward into stagnation." Muad'dib
Basic change in the way geologists view and interpret the earth
Resulted largely from study of the ocean basins
Mapping after WWII - Military and communications
Where to hide our nuclear missile submarines
And how to track the Soviets' with passive sonar
Exciting time for geology
The spreading ridges were recognized first
As the continents spread apart, new oceanic crust is formed
Like a scab over a cut
But what happens to all this new crust?
Somewhere it has to be consumed by the earth
Or it would keep getting bigger to accommodate the additional crust
So where was the excess crust going
Additional research found deep trenches in the ocean floor immediately sea-ward from linear chains of volcanoes
Trenches are the deepest portions of the oceans (Marianas: -35,810')
Incredibly active seismic areas with deep earthquakes 45° angle
Clear that they were in some manner associated with the volcanos
These volcanic chains can be off-shore (ex. Japan, Aleutians)
Or on-shore (ex. Andes, Cascades)
In any event, long chains of extremely active volcanoes
Also with lots of earthquake activity
Proposed that old oceanic crust is consumed in these deep trenches
Hugo Benioff: describe & define Benioff Zone
Plate Tectonics attempts to explain global tectonic processes and results
Active planet - powered by immense internal forces
Produce a constantly changing surface, but at a very slow overall rate
See Strickler's 2nd Law of GeoFantasy
There are no catastrophic processes, only catastrophic events
Earthquakes and volcanos may seem catastrophic, but...
They are really the result of very slow processes
Theory states that the earth's crust is broken into a relatively small number of sections, or plates
Poorly understood forces cause these rigid crustal plates to move relative to each other
Different plates move at different velocities
Ranging from 1 to >10 centimeters per year
This may not seem like much, but, over the course of geologic time...
Most surface features related to plate margin interactions
Two (or more) plates interact along huge linear zones of faulting
DIGRESS TO: faults
Represent fractures along which movement has occurred
Three kinds (normal, reverse, strike-slip): Details next week!
Tectonic activity is common here
Earthquakes, volcanoes, and mountain building in general
Thanks to friction and Newton's First Law of Motion...
Earth tries its best to keep the plates from slipping
But, as stress increases to the breaking point...
The fault will rupture
Accumulated strain will be released in a seismic event (earthquake)
The plates will shift along the boundary
Only to lock-up again and begin the process anew
The amount of energy involved here is immense
Beyond our ability to comprehend, as well as restrict or control
All we can do is study the process, try to keep out of the way, and, if we can't, hang on tight
New crust generated at spreading centers
Older crust is consumed at trenches
Volcanic arcs are surface expression of the subduction process
More on both later
DIGRESS TO: brittle vs. ductile
The outer portion of the earth is fairly complex
Not a simple crust/mantle situation
Lithosphere: the outermost layer
Includes the crustal plates as well as high strength, brittle material beneath them
Approximately 60 miles thick
Asthenosphere: below the Lithosphere
Low strength, ductile material extending down to 150 miles
Mesosphere
The bulk of the mantle material
Plate margins are where most activity happens
Three possible motions to the interaction
Spreading centers - location of the formation of new crustal material
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
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 begins to adjust to a lower elevation
Extensively studied since the 1970's
Exciting areas for study
High heat flow with lots of volcanism
Several interesting aspects to spreading centers
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
Subduction zones where crustal rocks are consumed
Good example: Pacific "Ring of Fire"
Chains of active earthquakes/volcanoes around the Pacific Ocean
Relatively narrow, linear zones of activity
Zone of collision between two plates - results in crustal shortening
Reverse faulting
Three possible situations
1) Continental vs. oceanic plates
Oceanic plate is subducted due to density differences
Location of many shallow to deep focus quakes
Extend to depth beneath the overriding plate
A deep trench commonly occurs where the subducting plate plunges beneath the overriding plate
Collects sediments which are also subducted
Remelting generates magma which forces its way to the surface
Volcanic arc chains on-shore
EXAMPLE: Andes, Cascades
2) Oceanic vs. oceanic collision
Same basic idea as above
Also results in subduction
Volcanic chain usually off-shore
EXAMPLE: Japanese Islands, Aleutian Islands
3) Continent vs. continent collisions
Massive slabs of granitic crust collide
No subduction possible so some major mountains are pushed up
EXAMPLE: Himalaya Mountains; Ural Mountains
Strike-slip faults
Bisect and offset the spreading ridges
San Andreas fault - good example
There are 7 major plates
Need to mention the many "microplates" which occur
Mediterranean region - very complex
Open and close often in "recent" history
Triple junctions
EXAMPLE: super ball in drawer
Only two remain active - third commonly fails
Saudi Arabia vs. Africa
Baffin Bay
Back arc spreading
Between the arc and continent
Spreading centers without corresponding subduction zones
Africa is surrounded (and cut) by spreading centers!
Hot Spots and Mantle Plumes
Mid-plate volcanics
Hawaiian/Emperor chain
Columbia River basalts
How they form is a problem
Remember I said that the crust was essentially 2 types of igneous rock
Granite forms deep within the crust
Associated with regions where two plates are moving towards each other
For a variety of reasons, the majority of the earth's most violent earthquakes and volcanoes occur in these areas
Example: eastern edge of the Asian continent vs. the Pacific Ocean
Collision is between a continental plate and an oceanic plate
Heavier oceanic basalt is forced beneath the lighter continental materials along a deep trench called a "subduction zone"
Subjected to increasing heat and pressure
Can cause it to undergo metamorphic changes
In extreme conditions, completely remelt to form new magma
If this magma can find a path of weakness
Generally not a problem where two plates are grinding into each other
Erupt from the surface as lava
Form volcanic mountains (called an "island arc")
Some cools at depth forming large granitic "plutons"
These are sutured to the "continental margin" (added to craton)
Basaltic crust is created at spreading centers
Moves like a conveyor belt to a subduction zone where it is destroyed
Therefore, all oceanic crust is geologically young (<200 million years old)
Continental crust, however, is essentially just along for the ride
This lighter "scum" is basically the end product of the earth's differentiation
Granitic crust has been accumulating for 4 billion years
Edges get beaten up, blocks split and join, but it basically isn't destroyed
Always been the weak point of the theory: The rallying cry of the non-believers
Lots of possibilities have been proposed
Lunar drag
Centrifugal pull due to rotational velocity
Giant catfish (Namazu) in the center of the earth
Gravitational sliding - away from the topographic highs of the spreading centers
Density differences between descending lithosphere and mantle
Drags plate behind and pulls open at the ridges
Mantle Convection: Currently popular theory
Most probably a combination of several of the above
As well as others not imagined yet!