Plate Tectonics Basic Concepts

1. Plate TectonicsBasic Concepts • Earth’s lithosphere is composed of rigid plates • “Float” upon a hot, plastic layer of the upper mantle • Convection cells in mantle best current theory for driving force of plate movement • 7 major plates (pp. 54-55, fig. 2.19) • Numerous smaller ones

2. Earth’s Tectonic Plates

3. Basic Concepts(continued) • Plates move relative to each other in three ways • Divergent – tensional stress at mid-ocean ridges (fig. 2.21) • Convergent – compressional stress at subduction zones (fig. 2.22) • Transform – shear stress at strike-slip faults or mid-ocean ridges (fig. 2.245) • New oceanic crust is formed at divergent plate boundaries • ~6-11km thick • Mostly gabbros and basalts – ophiolite suites • Convergent plate boundaries can involve (fig. 2.22) • Oceanic to oceanic subduction (mostly Pacific) • Oceanic to continental subduction (Western S.A.) • Continent to continent collision (Himalayas) • Most seismic activity, volcanism, and mountain building takes place along plate boundaries • Pacific “Rim of Fire” • The centers of plates are mostly geologically stable

4. 3 Types of Plate Boundaries • Divergent • tensional • Convergent • compressional • Transform • shear

5. 3 Types of Plate Boundaries

6. Pangaea (all land)Alfred Wegener’s proposed “supercontinent”

8. Fossils as evidence for continental drift

9. Mountain Ranges as evidence of continental drift

10. Magnetic reversals along mid-ocean ridges

11. Tracking Plate Motion • Relative measurements • Two moving plates with no fixed reference point • Absolute measurements • Calculation of speed and direction with respect to a fixed point • Hot spots • Hawaii – tracks motion as Islands get younger to the SE (fig. 19.29) • San Francisco volcanic field (?) – volcanic rocks generally get younger to the NE • Magnetic Field reversals (p.52-53, figs. 2.15,16 and 17) • Track Earth’s magnetic field reversals through time in symmetrical bands moving away from and parallel to mid-ocean ridges

12. Rifting and the Origin of Ocean Basins • Intercontinental upwelling of a convection cell causes crust to thin – tensional stress- and break creating rift zone • East African Rift Zone • Red Sea • Gulf of California • Three arm rift zone • Two continue to form mid-ocean ridge system • One fails to form aulacogen • Rio-Grande, Mississippi valley • Gulf of Aden

13. Rifting in Africa

14. Mid-Ocean Ridges • Production of new oceanic crust • Upwelling of ultra-mafic magma along linear or sinuous ridge systems • Hotter at ridge spreading centers • > cooling at > distance from rift • Topographic high along spreading centers • Ophiolite Suite – • Siliceous or carbonate sediments • Pillow basalts • Gabbros • Ultra-mafic peridotites • Alteration through heated seawater penetration along cracks and fissures

15. Mid-Atlantic Ridge Complex The Mid-Atlantic Ridge complex is responsible for the volcanic islands of Iceland. A contrasting land of ice and fire, Iceland is composed of some of the youngest oceanic crust surfacing the Earth today, and certainly one of easiest areas geologists can study how new oceanic crust is formed in situ.

16. Mid-Atlantic Ridge Complex Aerial view of the area around Thingvellir, Iceland, showing a fissure zone (in shadow) that is an on-land exposure of the Mid-Atlantic Ridge. Right of the fissure, the North American Plate is pulling westward away from the Eurasian Plate (left of fissure). (Photograph by Oddur Sigurdsson, National Energy Authority, Iceland.)

17. Mid-Atlantic Ridge Complex A subaqueous image of a “smoker vent” along the Mid-Atlantic ridge. Seawater seeps into cracks and fissures and is heated by the molten rock, or magma, that lies beneath the Earth's crust. As the water is heated, it rises and seeks a path back out into the ocean through an opening in the seafloor. Its temperature may be as high as 750°F (400°C).

18. Oceanic Crust Age(all pre-K has been destroyed)

19. Subduction Zones • Oceanic Trenches • Deep, narrow depressions as subducting plate bends downward • Marianas Trench (>36,000ft deep) • Accretionary Wedges • Sediments and pieces of oceanic plate plastered against continental plate • California Coast ranges • Highly deformed sandy and shaly matix with blueschist facies interbedded • Volcanic Arcs • Typically on overriding plate • Partial melting of descending plate results in silicic volcanism on overriding plate • Benioff zone – concentration of seismic events tracing a subducting plate • Fore- and back-arc basins • Sediment traps in shallow basins in front and behind island arc systems

20. 3 Types of Subduction Scenarios

21. Convergent Boundaries • Collision zones • Form large complex mountain chains • Alps, Himalayas, Appalachians, Urals • Suture Zone – where the ocean once existed between continents • New theory that partial continental subduction occurs

22. Cenozoic tectonic settingof the SW U.S.

23. Transform Boundaries • Offsets perpendicular to ridge complexes • Rocks moving in same direction at distance from ridge complexes • San Andreas Fault system (p.65, fig. 2.25) • Zig-zag ocean ridge system • Overridden by subduction zone • Transform boundary extends as subduction zone moves beneath continental plate • Additional motion is taken up in lateral movement

24. San Andreas Fault Zone • The San Andreas fault zone, which is about 1,300 km long and in places tens of kilometers wide, slices through two thirds of the length of California. Along it, the Pacific Plate has been grinding horizontally past the North American Plate for 10 million years, at an average rate of about 5 cm/yr. Land on the west side of the fault zone (on the PP) is moving in NW direction relative to the land on the east side of the fault zone (on the NAP).

25. San Andreas Fault • The Blanco, Mendocino, Murray, and Molokai fracture zones are some of the many fracture zones (transform faults) that scar the ocean floor and offset ridges (see text). The San Andreas is one of the few transform faults exposed on land.

26. Basin and Range Province

27. Oceanic-Continent Subduction • The convergence of the Nazca and South American Plates has deformed and pushed up limestone strata to form towering peaks of the Andes, as seen here in Peru. (Photograph by George Ericksen, USGS.)

28. Continental Collision

29. Theoretical Driving Forces for Plate Motion • Ridge Push • From active mid-ocean ridges • Rising and extruding magma pushes plates apart • Compressional forces at ridges not seen • Slab Pull • Colder, denser plate subducting pulls plates apart • Tensional forces at ridge complexes • Plate Sliding • Cooling plate “slides” downslope from elevated hot ridge complex • Tensional forces at ridges • Convection cells • Thermal differences create upwelling and downwelling cells that provide the energy for plate movement

30. Convection cells as a driving force for plate movement

31. Continents • Continental Shield • Oldest rocks • Exposed crystalline and metamorphic rocks • Canadian Shield • Continental Platform • Extends around shield • Covered with younger sedimentary and igneous rocks • Craton • Above combined • Tectonically stable over vast period of time (1billion years or more)