Phys 1417.01
Summer II, 2001
PLATE TECTONICS

Crust of the Earth

All of the solid earth above Moho - crust
Two distinct types of crust
Continental Crust
Characteristic
Oceanic Crust
35-50 km
Thickness
5-12 km
Granite (light)
Composition
Basalt (dark)
to 3.8 billion B.P.
Geological age
to 150,000,000 B.P.
2.7
Density
3.0
Ave. +1,000 m
Elevation
Ave. - 3,000 m
No
Remelted and destroyed
Yes
Yes
Deformed
No
33%
Current distribution
67%
Origin of continental crust
    Partial remelting of original oceanic crust

Crustal Structures - Faults

 Hanging wall - mass of rocks that lie above the shear plane

Normal fault -
    Hanging wall goes down in relation to footwall
    Crust is lengthened
    Rift zones
Reverse fault
    Hanging wall goes up in relation to shear plane
    Crust is shortened
    Converging (compression) zones
Thrust faults
    Very low angle shear plane
Oblique normal fault
    Rotation movement
Lateral (strike-slip) fault
    Sliding blocks, sideways movement
    Transform plate boundary

Crustal Structures - Folds

Bending of earth strata
Anticlines
    Uparching of earth strata
    Oldest material in the center
Synclines
    Downward folded rocks
    Youngest material is in the center
Monocline
    Single bend to the fold
Domes and basins
    Similar to anticlines and synclines
    Elliptical to roughly circular in shape

"Continental Drift"

Sir Francis Bacon - 1620 - first maps of the Atlantic Ocean
    Noticed parallelism of opposite shores
Alfred Wegener - 1915
    "The Origin of Oceans and Continents"
    Super-Continenent of Pangea breaks apart 200,000,000 B.P.
    Ocean filling gaps
    Wegener's terminology
        Pangea - original supercontinent
        Panthalassa - large ocean surrounding supercontinent
        Laurasia - Europe and North America joined together
        Gondwanaland - southern hemisphere portion of supercontinent


Wegener's Evidence

Paleontological - fossils
    South America and Africa
       Mesosaurus - freshwater reptile in stratigraphy in both areas
       Glossopteris - plant leaf in both South America and Africa
Rock Types and Structures
    Match - Africa and South America
        Wegener matched at shorelines, later Bullard gets better match at edge of
        continental shelf

    Virginia and Morocco
    Rocks and structure match
        Faults, folds, deformation - Appalachians with Scottish highlands

Paleoclimatic evidence
    Gondwanaland glaciations
    Permian evaporite deposits

Later Evidence

Tectonics
    Association of
        Active mountain ranges - edge of continents
        Earthquakes and volcanic zones
        Island arcs
        Deep ocean trench
    Mid-ocean ridge
 
 

 Magnetic Studies

Geomagnetic field
    Outer core - magnetic field generated
    Polarity of a magnetic -N. & S. poles
All magnetics have N magnetic pole, S magnetic pole
Pattern of reversals - mid-ocean ridge
    Polarity reversals recorded in volcanic rocks
    As magma extrudes on to ocean floor, records the polarity of that time

Oldest  Youngest  Oldest
Trench     Ridge    Trench
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Each side of the ridge duplicate
    Reversals recorded
*Sea floor spreading
Paleomagnetic reversals apply to ocean rocks

Apparent polar wandering - proof that the continents have moved
    North Pole appears to have changed position
    Rocks from N. America and Eurasia show different North Poles at times in the past
        Point to different poles
            Impossible!
*North Pole hasn't moved, continents have changed position

Ocean Floor Sediments

Mid-Ocean Ridge to Deep Ocean Trench
    Youngest sediments at ridge, get progressively older as move towards trench
    Thinnest sediments at ridge, thickest sediments at margins of trench

PLATE BOUNDARIES

Geologically active regions
Three classes of plate boundaries
    1.    Converging
    2.    Diverging or rift
    3.    Transforms
Converging Plate Boundaries

1.    Converging - Ocean to Ocean plate

Subduction, volcanic island arc (andesite - conversion of oceanic crust), Benioff zone (shallow-
intermediate-deep focus earthquakes), deep ocean trench
Unstable geologically
Examples - West Indies, Japan, Aleutian Islands, Philippine Islands, Indonesia, Central America
    (Lake Nicaragua)
 
 

Converging plate boundary composite volcanoes, Fuego and Acatenango, as seen from
the flanks of the Volcan de Agua, near Antigua Guatemala
August, 2000, Nikon FE2, 200 mm Nikon AIS f4 lens.

First stage in conversion of oceanic crust to continental crust


Benioff Seismic Zone
Proof of Subduction Plate

Benioff zone outlines subducting plate
Deep focus earthquake is occurring at depth which should be plastic
    Evidence of solid plate
 
 

Volcanic caldera of Lake Aitilan, the volcano has subsided inward on its magma chamber,
near Panajachel, Guatemala, June, 1974.






2.    Converging Plate Boundary - Continentent to Ocean

Volcanic alpine chain, subduction, deep ocean trench, Benioff seismic zone
Examples - Andes Mtns., West coast of Mexico, Cascades - Pacific Northwest of United States

3.    Converging - Continent to Continent











Suturing, shallow focus earthquakes, granitic mountain range
Himalayan Mtns. - India/Australian plate collides with Asian plate - creates Himalayan Mtns.
    Destructive earthquakes in China - 2,000 miles from plate boundary

Diverging (Rifting) Plates

1.    Continent-Continent Rifting (Diverging)

Rift Valley, block faulted mountain ranges, basaltic volcanic activity, shallow focus earthquakes
East African Rift Valley
    Lake Region
    Extends to Jordan River Valley
        Sinai Peninsula - Asia and Africa Rift apart
        Dead Sea - between Israel/Jordan - 300 m below sea level
            Lowest point on continental crust
    Djibouti - Red Sea trickles into Africa
*First stage in the development of an ocean basin

2.    Ocean - Ocean Divergence (Rifting)

Sea floor spreading, pillow basalts, shallow focus earthquakes, oceanic ridge
Growth of an ocean basin
Red Sea - early stage
Atlantic Ocean - advanced
    Iceland
Mid-Ocean Ridge present in all ocean basins
    East Pacific Rise

Transform Fault

Sliding plates
One plate grinds past another - friction
Shallow focus earthquakes
Only contact without volcanic activity
San Andreas Fault, Anatolian Fault, Motagua Fault
 
 

Convection - Heat Flow

Heat (convection) cells in mantle/asthenosphere
    Lithosphere rests on asthenosphere
Rising heat cells - plates separate
Sinking heat cells - plates pushed down into mantle

Measured heat flow
    Highest at ridge
    Lowest at trench
Radiogenic heat vs. deep heat
    Radiogenic heat from uranium in continental crust (granite) - 30% measured value
        Low with oceanic crust
        Not involved in moving plates
    Deep heat - from convection, highest under ocean crust

Thermal Plume - Mantle Hot Spots
Area of concentrated heat flow from deep mantle
Hawaii - not on plate boundary
    Intraplate volcanic activity
    High temperature basaltic lavas
    Pacific plate passing over thermal plume
    New Hawaiian Island Loihi

Other thermal plumes
    Afar Triangle - numerous thermal plumes intra African plate
    Yellowstone
    Pacific Plate contains numerous thermal plumes
        Reunion Islands - similar to Hawaii

Rate of Plate Movement

San Andreas Fault - 5.5 cm/yr
Mid-Atlantic Ridge
    Iceland - 1.8 cm/yr; South Atlantic (Ascension Island) - 3.9 cm/yr
East Pacific Rise - off South America
    Most rapid movement - 17.1 cm/yr

Gravity Anomalies

Negative anomalies (less than average pull of gravity) over margins of deep ocean trench
    Less dense material
Negative anomalies also over large mountain ranges - less dense material
Positive anomalies - stronger than normal pull of gravity
    Mid-Ocean Ridge - dense rocks of mantle closer to surface

Lost Continents and Alien Terranes

Micro-continent
    Seychelles Bank in Indian Ocean
        Granitic crust "floating" over oceanic crust
        Broke away from Africa, has not subducted
        Beaches of Seychelles Is. - pink sand - indicative of granite
Allochtonous terranes - continental crust of non-local origins
    Indicated by magnetic studies
    Terranes  present in the Cordillera of Western North America
    Micro-continents have collided with the North American continents
        Non-oceanic in origin, do not subduct
        Added on to continent

Cycles of Plate Tectonics

Numerous cycles of breakup and collision have preceded Wegener's Pangea
Late Precambrian - continents together in one land-mass
    Break apart during Cambrian and Ordovician, come back together Devonian through
    Permian - reassemble Pangea
        Form Appalachian Mtns.
Cycles of breakup and collision have influence on biological evolution
Breakup/rifting - continents separate
    Milder climate, separation of forms - genetic drift
        Diversity of species
Collisions - continents reassembled
    More extreme climate - land masses together
    Species brought together - competition
    Continents reassembled - times of extinction

Link to: