Global Plate Tectonics n Plate Tectonics = Sea Floor Spreading + Continental drift n These two concepts came from separate sets of observations n The Plate Tectonic theory: n combined these ideas n answered “all” our questions n provided incredible predictive POWER!

Global Plate Tectonics (Cont.) n To understand the Theory of Plate Tectonics we will consider n Lithospheric plates n Seismic activity / plate boundaries n The types of plate boundaries n The opening and closing of ocean basins

Global Plate Tectonics (Cont.) n Proposes that the earth’s surface is composed of a series of lithospheric plates the edges of which are defined by seismic activity n The plates consist of some combination of continental and oceanic crust n The bottom boundaries extend downward through the entire lithosphere

Global Plate Tectonics (Cont.) n The rigid and brittle lithospheric plates overlie the molten and plastic asthenosphere

Lithospheric plates n Lithosphere = rigid (won’t bend) n Note that it can contain Either continental or oceanic crust n OR BOTH! n Asthenosphere = pliable (will bend or even flow)

Global Plate Tectonics (Cont.) n Nearly all seismic activity is associated with plate boundaries. n Seismic activity gives rise to earthquakes and volcanic activity, most of which occur in the Pacific Ocean. n Plate boundaries and seismic activity: n co-occur n exist on continuous lines

Seismic activity These occur together!

Global Plate Tectonics (Cont.) n Three flavors of plate boundaries: n 1) Midocean ridges n Sites of tension, plates diverge n Addition of new crust to trailing plate edge

Global Plate Tectonics (Cont.) n Midocean ridges are: n also called “spreading centers” n site of the generation of new crust / lithosphere

Global Plate Tectonics (Cont.) n Midocean ridges occur in the middle of the ocean. n start off as a rift in a continent n the ocean forms on both sides of the ridge

Global Plate Tectonics (Cont.) n Three flavors of plate boundaries: n 2) Subduction zones n Sites of compression, plates converge n Destruction of old lithosphere/crust

Global Plate Tectonics (Cont.) n Subduction zones are regions where oceanic crust /lithosphere is being destroyed n These are zones of intense seismic activity

Global Plate Tectonics (Cont.) n The most significant feature of subduction is the volcanic activity that goes with it

Global Plate Tectonics (Cont.) n The Pacific Ocean is surrounded by a “Ring of Fire” n both volcanoes and earthquakes n all subduction-related n Because this seafloor is lost forever, the Pacific Ocean is shrinking

Global Plate Tectonics (Cont.) n Subduction is ocurring under: n Japan n Western South America n Central America n Alaska n New Zealand n Caribbean n Northwest USA! Mt. St. Helens Mt. Ranier

Global Plate Tectonics (Cont.) n Three flavors of plate boundaries: n 3) Transform faults n Sites of lateral movement n Lithosphere/Crust is neither created or destroyed n Conservative plate boundary

Sea-Floor Spreading (Cont.) n Transform Faults: n occur where plates are sliding past each other n California: San Andreas Fault n No volcanoes n In most cases n Some exceptions n LOT of earthquakes

Global Plate Tectonics (Cont.) n The amount of old lithosphere destroyed by subduction zones equals the amount of new lithosphere created by ridge systems

Plate Tectonics n Overview of Processes

Global Plate Tectonics (Cont.) n Assuming the the size of the Earth isn’t changing, n the net change in plate volume is zero n the total RATE of production = the total RATE of destruction n The locations of the zones of sea-floor formation and destruction implies that n the Atlantic Ocean is gradually expanding, n the Pacific Ocean is gradually decreasing in size. n However, the rates are not identical because the production of new crust at some ridge systems in the Pacific Ocean counteracts the destructive processes

n One idea of what is going on:

Global Plate Tectonics (Cont.) n Notice that most earthquakes occur on active plate boundaries n Some occur where new boundaries are forming (rifting) n Others occur where ancient continents have collided (Alps, Himalyas, Rockies, New Madrid………)

Global Plate Tectonics (Cont.) n Based upon the Theory of Plate Tectonics ocean basins open and close. n The Wilson Cycle describes the stages in ocean basin development. Early rifting:Early rifting: underwaterunderwater produces “pillow lava”produces “pillow lava”

What causes the plates to move? n Thermal convection n Produces slow moving currents in the underlying asthenosphere which exert drag on the lithospheric plates setting them in motion

Plate Tectonic Forces n These forces require an enormous amount of energy n they may be combined on some plates

Summary n Plate Tectonics is a wonderful theory n unifying n understanding n predicitive n Three kinds of plate boundaries: n constructive (spreading center) n destructive (subduction zone) n conservative (transform fault) n Plates are defined by their boundaries n seismicity n NOT the kind of crust! n Next we’ll look at: n details of each kind of boundary n predictive nature of the theory n applications