Revised 8 / 06 (Monroe 6th ed.)
Including...
Continental Drift: Introduction
For additional information, refer to...
GeoTours background information on Tectonics (READ THIS FOR SURE!)
AskGeoMan section on Global Tectonics
GeoMan's Grants Pass High School Geology Home Page
GPHS class notes on Continental Drift
GPHS class notes on Plate Tectonics: an overview
GPHS class notes on Divergent Plate Boundaries
GPHS class notes on Convergent Plate Boundaries
Really only 2 earth processes
Construction and destruction
Destruction is surface weathering, erosion, etc.
Construction is tectonics
Earthquakes, volcanoes, mountain building in general
Revolution in the Earth Sciences
Similar to revolutions in the other sciences
Biology - Darwin in mid-1800's
Natural Selection and Evolution
Physics - Einstein in early-1900's
Non-Newtonian mechanics
Basic change in the way geologists view and interpret internal earth processes
A "whole earth" type of model
No so during the early days of formal geological thought (1700's & 1800's)
Concerned with the continents - not the oceans
Continents seems static - no lateral movement
Some vertical movement was recognized
Isostasy
Mountain building
Possibly some marginal accretion, but basically stable
Geologists worked in a vacuum (when there's only 3 in all of Europe and North America...)
Little or no interdisciplinary discussion
Tough to make advances in an academic vacuum
Enter Plate Tectonics
Changed all of the above
That's what revolutions do!
Resulted largely from study of the ocean basins
Mapping after WWII
Military and communications - more on this later
Active planet - powered by immense internal forces
Produce a constantly changing surface
Very slow rate, overall (see Strickler's 2nd Law of GeoFantasy)
Theory states that the earth's crust is broken into a relatively small number of sections, or plates
Move around in response to internal, or external, forces which we don't fully understand
There's a mouthful!
Most surface features related to plate margin interactions
Differential plate motions cause the continental land masses to "drift" relative to each other
Constantly changing land/sea inter-relationships
Drift has affected all aspects of the surface of the earth
Including biological evolution - will discuss in detail later
Theory unified geologic thought
Ties together nearly all facets of Earth Science
Must be true! 99.67% of all geologists believe it
Only the most recent theory
May 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
So far it has stood the test of time
And what a test it has been
Gained prominence in the early 1970's
Caused a rush to re-interpret the earth in light of the new model
Some very exciting times
Not really a new idea
Proposed many times in the past
Usually based on similarities of coastlines - Africa/South America
Little or no back-up evidence
1620 - Sir Francis Bacon
Commented on Atlantic similarities
Couldn't be by accident alone
Extremely perceptive for the time!
1658 - François Placet
Agreed with Bacon
"Separated in the Biblical flood"
mid-1800's - Antonio Snider-Pelligrini
Similar fossil plants in coal beds of Europe and North America
Reconstructed a 'supercontinent'
1908 - Taylor & Baker
Correlation of global mountain chains
All these early ideas were pretty casual
Based on landforms - not the ocean floor
Sparked little interest
Ran counter to existing dogma
Continents permanent and fixed
Any suggestion of movement was "geological fantasy"
DIGRESS TO: GeoFantasy
Alfred Wegener (Monroe; fig. 2-2, p. 35)
Instructor of Meteorology
Noted trans-Atlantic similarities
Seems everyone started here
Read article on fossil correlations in 1911 (Snider-Pelligrini?)
Sparked his obsession
1915 - published his theory with supporting evidence
"The Origin of Continents and Oceans"
Basically said that the continents had been joined into a supercontinent he called "Pangaea" (Greek: all land)
Split in the Jurassic
Initial response less than overwhelming
Developed into a full fledged battle by 1924
R.T. Chamberlain (American) - "Can we call geology a science when there exists such differences of opinion on fundamental matters as to make it possible for such a theory as this to run wild."
Baily Willis (American, 1944) - Wegener's theory is a fairy tale and should be ignored due to its deleterious effect on students
In Wegener's defense it must be noted that he took a truly multidisciplinary approach to the matter
A very easy target
Just a weatherman - not an "expert" in any of the fields he was leaning on for support
Lots of religious fervor
Pleas for "open-mindedness"
Guardians of the truth against idiotic speculation
Easy for opponents to pick holes in specifics
Regional vs. local again!
Wegener would have loved the 1st Law of GeoFantasy
DIGRESS TO: Arm-wavers (global/regional thinkers) are always easy targets
J. Tuzo Wilson - early regional work on Plate Tectonics
Carl Sagan - saved by a pretty face and TV
In any event, his opponents severely took him to task!
Pointed to the lack of a mechanism (describe)
Wegener had actually proposed two possible mechanisms
Lunar/solar drag
Centrifugal pull due to rotational velocity
Both "proven" to be inadequate
Sufficient to significantly discount Wegener's work
Wegener stoutly defended his work
Disappeared in Greenland in 1930
Wegener did have supporters
Alexander DuToit - South African
Tried to establish a sound data base
Geological studies in Africa and South America
Proposed 2 supercontinents
Gondwana - southern hemisphere
Laurasia - northern hemisphere
Separated by the Tethys Sea
Arthur Holmes - Scottish
Accepted Wegener's ideas completely
Looked for a mechanism
Whole-mantle vs. shallow-mantle
The plate tectonic model begins with an acceptance of continental drift
Once accepted the rest follows
Fit of Coastlines (Monroe; fig. 2-3, p. 36)
One of the earliest arguments
Easy to see
Problems with early reconstructions (no surprise here!)
Trying to represent a 3-D planet on 2-D maps
Match of existing coastlines wasn't perfect
Carey (1955)
Joined at the 6560' isobath
Edge of continental shelf
Much better match
Bullard (1965)
Computer-generated best fit
3000' isobath
Stratigraphic & structural similarities (Monroe; fig. 2-4, p. 37)
If they fit so good, some geologic/geomorphic features should extend across
The torn newspaper/puzzle concept
A close look at regional & local features provided many matches
SE Brazil and SW Africa
Nearly identical stratigraphy - almost perfect match
Coal beds and regional stratigraphy of the Appalachians and Greenland/England/Scotland
Paleoclimatology
Primarily related to glacial features
Evidence for extensive Permian/Pennsylvanian glaciation in the southern hemisphere
Many locations currently near the equator
Uniformitarianism suggests that this is a problem
"Direction of flow" indicators suggest that the glaciers moved onto land from what is now ocean
This is also a problem - all current glaciers move the other way
Reconstruction of Pangaea clarifies the problem (Monroe; fig. 2-5, p. 38)
Climate-sensitive sedimentary rocks
Sed rocks form at the surface and can be influenced by surface conditions
Fossils (Monroe; fig. 2-6, p. 39)
Assumed free migration of land animals if continents were joined
Within limits set by environmental needs - restricted only by habitat needs and natural boundaries
Attempt to trace common lines of descent up to the split
Then divergence of species
Ex. - Australian marsupials
Difficult to assess this data - very spotty (isn't paleontology always spotty!): Expand
Didn't do much to convert unbelievers
Recent work tends to support drift, but still essentially circumstantial
Paleomagnetism (Monroe; fig, 2-7, pg. 40)
A big deal in the post-war 40's and 50's
"Black Box" geology - geophysics
DIGRESS TO: the tendency of magnetic minerals to orient themselves relative to magnetic north upon crystallization
Stanley K. Runcorn (British)
Measured fossil magnetism in many rocks - preCambrian to recent
Calculated the average north pole region for each age
This is a complex topic
Runcorn basically found that:
Younger rock tend to align best with the current magnetic polar position
Increasing divergence with age
Also, each continent had its own North Pole
"Apparent Polar Wandering Curve" (Monroe; fig, 2-8, pg. 41)
Since the magnetic poles probably don't move about very much, then the land masses must have moved
Correlation of North America & Europe indicate they were joined until the Late Triassic
Runcorn published in 1962
Still lots of skepticism at this time
Many were swayed by Runcorn's basically incontestable data
Good example of the difference between the impact of hard empirical data and inferred evidence based on assumptions and common sense
The last (and possibly most convincing) clues came later from the seafloor
Government finally began to care about the topography of the sea floor after WWII
Defense and communications
Hide our missile subs and track the Soviet's
Mapping of seafloor indicated that there is a range of mountains running N-S down the middle of the Atlantic ocean
(Monroe; fig. 2-10, pg. 43)
Part of a larger, continuous chain which circles the planet
All volcanic in origin - basalt
As mentioned above, magnetic minerals align to magnetic north at time of crystallization (Monroe; fig, 2-9, pg. 42)
Oceanic basalt has lots of iron
Analysis of magnetic signatures of aligned magnetite grains
Identified linear magnetic anomalies
Also, evidence shows that north and south regularly switch poles!
Bi-laterally symmetrical bands of magnetic minerals oriented to north and south (Monroe; fig. 2-11, pg. 44)
Indicate that the sea floor is spreading apart, with new basalt magma rising to fill the gap
Generation of new oceanic crust, which then moves away from the volcanic axis...
...and goes where? More on this later
Some scientists began to put it all together, ask questions such as:
IF: Oceanic ridges/rises represent spreading centers
THEN: Age of basaltic oceanic crust should become older with distance from a ridge
Glomar Challenger - deep sea drilling ship
Actually CIA spy boat with drilling as its cover
Looking for sunk Soviet subs
Sent to test hypothesis
Drilled 8 sites in the south Atlantic
Cored thru sediments and into basaltic crust
All relatively young (<200 m.y.) (Monroe; fig. 2-12, pg. 45)
Results support drift
2 cm per year (at that location) (Monroe; fig. 2-14, pg. 46)
Several interesting observations/interpretations:
Ocean basins extremely youthful features
Oldest oceanic crust found is <200 million years old (Monroe; fig. 2-12, pg. 45)
Continental rocks up to 4.1 billion years old
Floating (and growing) through time in a 'sea' of constantly renewing oceanic crust
But what has happened to all the oceanic crust formed before 200 MYA?
There must have been some
Where did it go?
It was clear by the mid 1960's that the beginnings of a new global tectonic framework were in place
Hard to argue that the continents were fixed in place
Had drifted quite a bit
Needed a new theory which would account for drift, as well as internal process
Mountain building: commonly on continental margins
Magmatic activity: associated with mountain building
Metamorphism: associated with both
They all work together to build the earth
Video: Earth Revealed - The Birth of a Theory
The theory which ties them all together
Exciting time for geology
Much of the theory based on continuing observations of the sea floor and continental margins
The initial determination was that, as the continents spread apart, new oceanic crust is formed at the mid-oceanic ridges
But what happens to it then?
Somewhere it has to be consumed
If not, the earth would have to keep getting bigger to accommodate the additional crust
Originally proposed, but generally discounted
Earth would have had to start out real small if spreading rates historically approached today's rates
Harry Hess in the early 1960's
Proposed that old oceanic crust is consumed at island arcs
Linear chains of volcanoes
Commonly arc-shaped
Located sea-ward of continents
ex. Japan, Aleutians
Can be on-shore
ex. Andes, Cascades
In any event, long chains of extremely active volcanoes
Also lots of earthquake activity
Commonly composed of intermediate lava and pyroclastics
DIGRESS TO: mafic vs. felsic (compositional differences) & pyroclastics
In addition, researchers found deep trenches in the ocean floor immediately sea-ward from the island arcs
Deepest portions of the oceans
Marianas Trench: -35,810'
Incredibly active seismic areas
Deep earthquakes
45° angle
Clear that they were in some manner associated with the volcanic arcs
Hugo Benioff: describe & define Benioff Zone
Anyway, putting all this together, we end up with the model of which is referred to as plate tectonics
Attempts to explain global tectonic patterns and effects
Refer to world seafloor map
Describe general aspects of the theory
New crust generated at spreading centers
Consumed at trenches
Island arcs are surface expression of the subduction process
More on that later
The outer portion of the earth's interior is fairly complex
Not a simple crust/mantle situation
Lithosphere
High strength, brittle material
Approximately 60 miles thick
Includes crustal rocks
Asthenosphere
Low strength, ductile material
DIGRESS TO: brittle vs. ductile
Down to 150 miles
"Low velocity layer"
Global feature
Mesosphere
The bulk of the mantle material
"Stronger" than the Asthenosphere
DIGRESS TO: "Strong/weak" adjectives & how they relate to seismology
Pacific "Ring of Fire"
Chain of active earthquakes/volcanoes around the Pacific Ocean
Relatively narrow, linear zones of activity
Related to plate margin interactions
Plate margins are where most tectonic processes occur
Individual plates are moving around relative to each other
Due to these varying velocities, there are interactions where two or more plates are in contact
Three possible motions to the interaction (Table 2-1; pg. 47)
DIGRESS TO: faults
Represent breaks in the crust along which movement has occurred
Three kinds (normal, reverse, strike-slip)
Zone of Divergence (Monroe; fig. 2-15, pg. 48) (Monroe; fig. 2-16, pg. 49)
Spreading centers - location of the formation of new crustal material
Tensional features, therefore predominantly normal faulting
Crustal wounds - scab over with new basalt (blood of the earth)
Newly differentiated mantle material
Shallow earthquakes
Down to 12 miles
Also fewer quakes than at Benioff zones
Less slippage area?
Area of high heat flow
Hot rocks expand
Commonly form long, relatively narrow topographic highs
In addition, the location of a linear, near surface magma chamber will help cause doming of the overlying basaltic crust
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
Lots of volcanism
Pillow basalt
Indication of submarine volcanism
Other interesting aspects to spreading centers
Black smokers
High temperature vents
Support exotic life forms
Tube worms, etc.
Some speculation that life originated in similar environments
New and different life forms
DIGRESS TO: Sulfide deposits
Ophiolites (Monroe; fig. 2-19, p. 55)
Describe in detail
Refer to the Josephine Ophiolite GeoTour
Zone of Lateral Movement (Monroe; fig. 2-10, p. 43)
Transform faults
Strike-slip faults
Bisect and offset the spreading ridges (Monroe; fig. 2-20, p. 56)
San Andreas fault (Monroe; fig. 2-21, p. 57)
Zone of collision between two plates
Reverse faulting and crustal shortening
Thrust fault - special case of a reverse fault (angle <20°)
There are 3 main types... (Monroe; fig. 2-18, p. 53)
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 place plunges beneath the overriding plate
Collects sediments
These, too, are subducted
Remelting at depth generates magma which forces its way to the surface
Volcanic arc chains
Can be on land (Andes, Cascades (Monroe; fig. 2-21, p. 57))
Commonly mid-range composition (Andes = andesite)
Oceanic vs. oceanic plates
Volcanic arcs usually at sea
Results in "island arcs"
Japan, Aleutians
Continent vs. continent collisions
Starts with ocean to ocean
Himalaya Mountains, Urals, Atlas
Other features and/or problems
We said that there are 7 major plates
Failed to mention the many "microplates" which occur
Mediterranean region - very complex
Open and close often in "recent" history
Triple junctions (Monroe; fig. 2-16, p. 49)
Only two remain active - third commonly fails
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 (Monroe; fig. 2-22, p. 58)
Columbia River basalt (?)
How they form is a problem
What causes the plates to move?
Always been the weak point of the theory
The rallying cry of the non-believers
Unimaginable forces!
Lots of possibilities have been proposed
Lunar drag
Centrifugal pull due to rotational velocity
These first two were Wegener's original proposal
Demonstrated to be too small
Giant catfish in the center of the earth
Gravitational sliding
Away from the topographic highs of the spreading centers
Expansion of the earth
Possible mechanism to start it off
Lithosphere cracks
New material rises to fill the void
Density differences between descending lithosphere and mantle
Drags plate behind
Pulls open at the ridges
Slab-pull, ridge-push (Monroe; fig. 2-25, p. 61)
Mantle Convection (Monroe; fig. 2-24, p. 60)
Currently popular theory
Most probably a combination of several of the above
As well as others not imagined yet!
Video: Plate Dynamics
GeoTours background information on Tectonics
AskGeoMan section on Global Tectonics
GeoMan's High School Geology Home Page
GPHS class notes on Continental Drift
GPHS class notes on Plate Tectonics: an overview
GPHS class notes on Divergent Plate Boundaries
GPHS class notes on Convergent Plate Boundaries
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