Plate tectonics

• Final topics:

– Development of theory
– Implications

Continental Drift

• Large scale movement of continents

– Originally postulated in 16th and 17th century

• 19th century: Gondwanaland
• 1915: Pangaea “all lands”: Alfred Wegener

– Evidence:

• Fit of continents
• Similarity of rocks
 

• Rock, fossil, climate evidence

– Rock and structural trends cross Atlantic
– Similar types of vertebrates and plants in S. America and Africa

• “land bridges”

– Permian glacial deposits in S. Am., Africa, India, and Australia

• Problems

– No driving force to cause continents to drift

• 1928, Arthur Holmes

– Proposed convection of lava in mantle moves crust
– At the time there was no evidence for convection

• World war II

– Extensive mapping of seafloor
– Maps show mid-ocean ridges

• Harry Hess (Princeton) Robert Dietz (Scripps)

– Proposed idea of “sea-floor spreading”
– mid-ocean ridge create new crust

Plate Tectonics

• Plate tectonics developed from sea-floor spreading and continental drift
• Ideas:

– Plates slide over partially molten, weak asthenosphere
– Continents carried passively
– Continental drift is a consequence of sea-floor spreading
– Move as distinct unit with little deformation

Three plate boundaries

• Divergent boundaries

– Seafloor plate separation: new seafloor
– Continental plate separation

• African Rift valley
• Red Sea
• Gulf of California
 

• Convergent boundary: Three types

– Volcanic islands, “island arcs”
– Ocean-ocean convergence (Mariana)
– Ocean-continent convergence (S. America)
– Continent-continent convergence (Himalayas)

• Transform boundaries

– Breaks in spreading zones
– Strike slip motion, but opposite of expected direction from ridge crests

Rates

• Magnetometers developed to find submarines in WW II
• Also found symmetrical magnetic patterns around mid-ocean ridges
• Cause:

– Record of earth’s magnetic field
– Ocean crust move away symmetrically from ridges

• Time of magnetic reversals known from lava flows
• Correlate reversals to seafloor stripes
• Know distance from ridge to stripes
• Velocity = distance/time

• Isochrons

– Lines of map showing equal age
– Parallel to magnetic lineations
– Seafloor becomes progressively older away from ridge crest
– Wider spacing reflects faster spreading

• Deep sea drilling: provides corroborating evidence

– Drilled through sediment and dated sediment at basalt sediment interface
– Found ages decrease away from ridge crest
– Found ages symmetric on either side of crest
– Found ages correlate with the magnetic ages

• Velocities

– Relative velocity: one plate compared with the other
– Absolute velocity: change in position of one plate
• Done with hotspot traces

• Fast plates

– Large amount of subduction
– Slab pull driving force

• Slow plates

– Many continents
– Drag of deep roots

• Geometry of plate motion

– Plates are rigid: points on one plate stay equal distances from each other
– Points on different plates move relative to each other

• Direction of motion

– Transform faults parallel plate motion
– Isochrons reveal past position of plates

Rock Assemblages

• Divergent boundaries and ophiolite suites

– Layers of rocks
– Peridotite, Gabbro, Basalt (pillow and dikes), Deep sea sediment
– Identical to oceanic crust

• Evolution of continental margins

– Rifting sequence
– Thinning crust
– Form continental margins with separation

• Ocean-Ocean convergence

– Subduction leads to melting
– Water release from downgoing slab
– More buoyant magma rises forms volcanoes
– Sediment scraped off of subducted plate
– Form accretionary prisms & forearc basins
 

• Ocean Continent collision

– Magma more silicic
– Material shed from mountains form melange (mixture)
 

• Continent-continent collision

– Continental crust will not subduct: its too buoyant
– Uplift occurs (Himalayas)
– Form suture zone (Urals)

• Continental dynamics

– Continents are created by gradual addition of other continental material at edges
– Center of North America 3.5 to 2 by old
– Appalachians 1 to 0.5 by old

• Microplate terranes

– “displaced”, “exotic”, “suspect” terranes
– Rocks that are assembled along the edge of continents
– Boundaries between rocks show they are very different ages and origins
– May be small continents, seamounts, island arcs

• Appalachians composed of several “microplates”

– Florida basement similar to Africa
– >100 microplates make up western North America

Continental Reconstruction

• Much evidence allows reconstruction of location of continents

– Magnetic rocks
– Location of mountains
– Rock types and fossils

• Pangaea – super continent (250 mybp)
• Rodinia – earlier supercontinent (750 mybp)
• No seafloor left from time between Rodinia and Pangaea
• Reconstruction of continents between Rodinia and Pangaea difficult
 
 

• Pangaea breakup

– Seafloor still present, possible to track motion of continents
– Pangaea surrounded by single ocean Panthalassa
– Opening of Atlantic about 200 mybp
 

• 140 mybp: Drift for 60 my

– Atlantic partially open
– Laurasia (northern hemisphere) separated from Gondwana (southern)
– Separated by Tethys sea
– S. Atlantic opens

• 65 mybp: 165 my of drift

– S. Atlantic wider
– Tethys closed to form Mediterranean
– Madagascar split from Africa

• Now

– East Pacific rise: not symmetric
– Most of the old pacific plate has been subducted
– Implications for mantle rock chemistry/composition

 

• Implications

– Economic: mineral and fossil fuels
– Paleontology and evolution
– Ocean currents and climate
– Geologic problems: mountain building

Driving Forces

• ???
• Ridge push and slab pull
• Shallow mantle convection
• Whole mantle convection
• Rising limbs of hot spots
• Combination of all?