Themes of the Day:

• The Seafloor Spreading Hypothesis (continued)
• The Wilson Cycle - Evolution of Ocean Basins
• Hot Spots
• Subduction Zones & Continental Collisions

The Seafloor Spreading Hypothesis

• Harry Hess (Princeton) proposes Seafloor Spreading Hypothesis - new crust produced at mid-ocean ridges (mantle upwelling) and consumed at deep-sea trenches (mantle downwelling) - based on Mantle Convection in late 1950's
• Vine & Matthews and Morley independently link seafloor magnetic stripes with Seafloor Spreading Hypothesis (1963)
  • Seafloor Magnetic Stripes (e.g., Mid-Atlantic Ridge S. of Iceland) discovered in 1950's
  • Calibration of Magnetic Polarity Reversal Time Scale
  • New oceanic crust records Earth's magnetic field when it forms
  • Spreading seafloor moves away from the ridge as newer crust is formed in a conveyor belt fashion
  • Oceanic crust acts as a "tape recorder" of Earth's magnetic polarity reversals
• Implications: oceanic crust spreads symmetrically, crust gets older as it moves away from the ridge, new crustal material is constantly being added at the ridge - rates of plate motions can be determined based on well calibrated polarity reversal time scale and mapped oceanic stripes
• Age of oceanic crust (eventually confirmed seafloor spreading hypothesis)
• Age of oceanic crust vs. depth to seafloor - cooling of crust as it ages and moves away from the ridge
• Cross section of a Mid-Ocean Ridge

The Wilson Cycle - Evolution of Ocean Basins

• Act I: Opening of an Ocean Basin
  • Scene 1: Birth - Continental Rifting - e.g., East African Rift Zone, Basin & Range Province of Western USA & Mexico
  • Scene 2: Youth - Narrow Ocean Basin - e.g., Red Sea, Gulf of Aden, Gulf of California (Sea of Cortez)
  • Scene 3: Adolescence - Maturing Ocean Basin - e.g., Atlantic Ocean, Indian Ocean, Arctic Ocean
• Act II: Closing of an Ocean Basin
  • Scene 1: Maturity - Subduction at Basin Margins - e.g., Pacific Ocean
  • Scene 2: Old Age - Subduction Dominates, Basin Narrows - e.g., Mediterranean Sea
  • Scene 3: Death - Continental Collision, Closing of Ocean Basin - e.g., Tethys Ocean (Alpine-Himalayan Mountain Belt)
• Passive vs. Active Continental Margins

Hot Spots

• Localized upwelling of the Mantle (similar to divergent plate boundaries) - may or may not be located at a plate boundary
• Plate moves over hotspot like cloth through a sewing machine (hotspot = needle, plate = cloth)
• Hotspot at a plate boundary - e.g., Iceland
• Hotspot in the interior of a plate - e.g., Hawaii, Tahiti, (Yellowstone), etc.

Convergent Plate Boundaries: Subduction Zones & Continental Collisions

• Oceanic lithosphere recycled back into the mantle
• When oceanic and oceanic crust collide it is often the older, colder plate that subducts - e.g., Marianas Trench, Izu-Bonin Volcanic Arc, Western Aleutian Islands
• When continental and oceanic crust collide oceanic crust always subducts beneath continental crust - e.g., West Coast of South America, Pacific NW, Alaska Peninsula
• Subduction Zone features:
  • Deep-sea Trenches
  • Earthquakes - Wadati-Benioff Zone
  • Accretionary Wedge
  • Volcanic Arc
  • Forearc/Backarc Basins
• When continental and continental crust collide neither subducts - suture formed - e.g., India-Asia Collision, Himalayan-Alpine mountain chain