Plate Tectonics

Outline

1. Background - Where the Idea Came From
   • Continental Drift - historical ideas started developing with da Vinci in 1400's although Snider-Pelegrini in 1858 is really the beginning
   • 1915 - Alfred Wegener
     • shapes of the continents
     • evidence from fossils (land dwellers that could not have swum the ocean)
     • matching rock structures (Precambrian shields can be matched across Atlantic)
     • continental glaciation (direction and extent)
     • other paleoclimate data (coal, evaporites)
     • believed that the continents plowed through the sea floor
   • Evidence was good that the continents had moved but HOW?
   • Key to puzzle came from interpreting the paleomagnetic information recorded in igneous rocksin the context of the Earth's magnetic field
   • Sea floor spreading was proposed in the early 1960's to help resolve conflicting apparent polar wandering data from different continents
     • support came from drilling and dredging the sea floor and finding only relatively young ages for the sediments and the underlying basalts

    

2. Mantle convection as the main process responsible for mountain building
   • Compare processes that built
     • sand dune
     • Mt. Everest
   • Convection as a means of transferring heat:
   • Comparison of atmospheric and mantle circulation:
     
     

     phenomenon	atmosphere	mantle
     material	air	Mg-Fe silicate rock
     driving force	bouyancy	bouyancy
     what changes density	temperature & water content	temperature & depletion
     heat source	sunlight	radioactive decay
     rate of heat transport	1000 watts/m2	0.1 watts/m2
     speed	1 m/s = 3x109 cm/year	1 cm/year
     coriollis important	yes	no
     phase change	water condenses to make rain	peridotite partially melts to make basalt
     after phase change	dry air is heavier	depleted peridotite is lighter
     effects of flow	wind makes ocean waves, sand dunes	flowing mantle moves continents
     small scale features	tornadoes	mantle plumes

     
     
     
3. How mantle convection works
   • Gross structure of Earth: inner core, outer core, mantle, crust
   • Major radioactive elements responsible for heating: K, Th, U
   • Overall pattern of circulation
   • Strong effect of temperature on stiffness (viscosity)
     • cool area near top (upper 50-100 km) is too stiff to flow
     • act as more-or-less rigid plate that is carried along by flow (sort of)
     • plate boundaries are marked by geologic faults with frequent earthquakes
   • Parts of convection cells
     • upwelling
       • lithosphere experiences divergent forces
       • rifting
       • possibly melting, if mantle is hot enough
       • formation of new crust, if melting occurs
     • downwelling
       • lithosphere experiences compression
       • mountain building
       • possible areas of subduction, if lithosphere is not too buoyant
       • destruction of crust, if melting occurs
   • idea of partial melting
     • peridotite (olivine, pyroxene, garnet, etc) gives off basalt melt and becomes enriched in olivine.
     • Fe, Ca, K are concentrated in the melt
     • partial melting in the 10-15% range
     • happens only when temperature of upwelling solid mantle crosses melting solidus
     • the hotter the mantle ...
       • the more melting occurs
       • the thicker the crust is
       • the more Mg-rich the crust is
       • the more depleted the mantle is in Fe
       • the less dense the mantle is
   • plumes as anomalous small-scale convection
     • rising mantle from very deep source (core-mantle boundary?)
     • 'anchored' with respect to deep mantle
     • large amount of melting (30%) leads to flood basalts, hot-spot island chains

     
     

4. Elements of plate tectonics
   • Ridges
     • spreading rates vary from slow (1 cm/year) to fast (20 cm/year)
     • fast spreading ridges, such as East Pacific Rise, typically smooth, and have an axial high
     • slow spreading ridges, such as the Mid-Atlantic Ridge, typically rough and have an axial rift valley
     • ridge segments are often offset by long transform faults, on which horizontal motion occurs
     • volcanism occurs along axis, creating new crust. Magma chambers are sometimes present in crust
     • high temperature (350 deg-c) geothermal systems occur in regions of recent volcanism
       • Chimneys of mineral precipitates form
       • These vents are often called black smokers
       • Exotic organisms that metabolize hydrogen sulfide live near vents
     • Extensional faulting is very common
     • crust is typically 6-10 km thick
     • age increases away from axis
       • sediments thicken away from ridge, and get older
       • depth of seafloor increases, due to thermal contraction of cooling lithosphere
       • magnetic lineations form, due to flip-flop of Earth's magnetic field
   • Transform Plate Boundaries
     • Some plate boundaries, such as the Pacific-North American Plate boundary in California consist of very large transform faults.
     • The main California fault is the San Andreas fault.
     • The motion of this plate boundary is about 50 mm/yr.
   • Rifts
     • Ocean ridges can begin when a continent rifts into several fragments
     • Volcanism can occur along the rift (e.g. Mt. Kilimanjaro in the East African Rift)
     • Example of the formation of the Atlantic.
     • Rifted continental margin site of thick sediments due to thermal subsidence.
     • Thick evaporite deposits can form during the early stages of rifting
   • Subduction Zones
     • Lithosphere can be dragged down into mantle, if it is insufficiently buoyant
       • The thicker the crust, the more buoyant (both because of crust itself and the deplete mantle beneath)
       • The younger (and hotter) the crust, the more buoyant
     • deep trench forms above the subduction zone.
     • Earthquakes occur in the cool down-going plate, down to 650 km depth
     • Water dragged down by the subducting plate can cause melting of the nearby overlying mantle.
     • Subduction zone volcanoes (typically erupt andesites and basalts, some rhyolites too)
       • The more silicic magmas, such as rhyolites, are very viscous and form very steep-sided flows.
     • Ocean sediments can collect near the subduction zone (the accretionary prism)
     • These sediments can be intensely deformed
     • Example: Cascadia subduction zone in Northwestern North America)
   • Hot Spots
     • Flood basalts, such as the Snake River Plain, erupt at the onset of the plume
     • Oceanic Plateaus, with thick (30-35 km) basaltic crust, such as Iceland, can form
     • Chains of oceanic islands, such as the Hawaiian Islands form as the plate moves across the hot spot
   • Continents
     • Centers of continents, called the shields are the oldest parts - typically 2-4 billion years
     • Continents grow by the addition of unsubductable material to their edges
     • Continental collisions can lead to substantial mountain ranges, such as Mt. Everest