Creation, Aging and Recycling of the Ocean Floor How is the ocean floor made? Why does the ocean floor get deeper with age? How are volcanic arcs made and why are they so dangerous?
Formation of Ocean Lithosphere Mid-ocean ridges-- global circuit of plate divergence-- youngest ocean floor-- shallowest ocean floor Architecture of oceanic plates-- seismic profiling-- ocean drilling-- fossil slivers of ocean floor
Ocean Drilling • International program(20 nations; began 1968) • 2-month cruises • Deepest hole: 2 kmBut, new vessel will drillto >6km!
Fast vs. Slow Spreading Fast Spreading Ridge (10-20 cm/yr) Slow Spreading Ridge (1-5 cm/yr)
Mid-Ocean Ridges Uniform process of mantle melting, lava freezing- magma chambers- effects of spreading rate- segmentation
o 10 cm Mineralized Volcanic Breccia (1027-1029 mbsf) o
Plate Aging involves heat loss through Conductive Cooling Relation: Depth increases with the square root of age
A New View on Plate Cooling Adam & Vidal, Science, April 2, 2010
Hydrothermal Circulation • Natural consequence of porous, hot rock and seawater • Chemical exchange • Fluid venting -- black smokers • Chemosynthetic ecosystems • Crack sealing and sedimentation
Plate Aging and Recycling Slab sinking -- denser than asthenosphere
Mt St Helens Water, from slab, promotes mantle melting and leads to explosive danger of arc volcanoes
Toba (71,000 B.C.) • 3000 times as big as Mt St Helens • - Sulfuric acid and dust blocked sunlight, reducing temperatures 5°C for several years • - May have reduced human population to <5000!