Transcript Lecture 5. Ocean Lithosphere: Its Formation, Aging and Recycling

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
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
Seismic Profiling (sound energy)
Ocean Drilling
International program
(20 nations; began 1968)
2-month cruises
Deepest hole: 2 km
But, new vessel will drill
to >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
Ocean Crust Layering
Superfast Spread Crust Mission to Hole 1256D
Superfast Spread Crust Mission to Hole 1256D
10 cm
o
Mineralized Volcanic
Breccia
(1027-1029 mbsf)
o
Sea floor deepens with distance from
the spreading ridges -- why?
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
And, heat loss through
Hydrothermal Circulation
Hydrothermal Circulation
Hydrothermal Circulation
• Natural consequence of porous,
hot rock and seawater
• Chemical exchange
• Fluid venting -- black smokers
• Chemosynthetic ecosystems
• Crack sealing and sedimentation
Hydrothermal Vent Sites
Hydrothermal Vents Video
The ocean floor ages and
accumulates sediments
Plate Aging and Recycling
Slab sinking -- denser than
asthenosphere
Plate Subduction and Melting
Pacific NW
Mt St Helens
Water, from slab,
promotes mantle
melting and leads to
explosive danger of
arc volcanoes
Krakatoa (1883)
Tambora (1815)
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!
Mt Hood
Broken Top
Three Sisters
Crater Lake (Mt. Mazama)
Volcanic ash (tephra)