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
Superfast Spread Crust Mission to Hole 1256D
http://www.somethingawful.com/articles.php?a=3743
Signs of the Apocalypse for April, 2006
Apocalypse Rating: (5 out of 7)
Drilling Our Way to Satan
Meddlesome scientists claimed to be pursuing knowledge rather than the chthonian layer
of the Unlord when they drilled nearly one and a half kilometers beneath the ocean floor. What
they found down there was a "fossilized magma chamber" instead of the Beast they were
secretly seeking. I'm not sure how a rock can become fossilized, but what I am sure of is that
drilling deeper than oil is dangerous. I would have told people not to even drill as deep as oil if
at all possible, but I wasn't around when they first started doing that.
Whether or not deep oil wells have unleashed Satan on our world is open to debate by the
wisest scholars of the Weekly World News, but this deep drilling for no good reason has got
to stop. We can't just rove around all willy-nilly and drop a drill miles underground wherever we
please. Even if the devil is deeper than our drills could ever reach there's got to be something
bad down there like Morloks or CHUDs or something. Having those cats out and about is not
going to slow down our careening rollercoaster ride towards the Final Battle……..
Superfast Spread Crust Mission to Hole 1256D
Thanks to Captains, Crews, and
Happy Science Parties of
ODP Leg 206 and IODP Expeditions 309 & 312
13th December, 2005, Hole 1256D, Gabbro!
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
Conductive Cooling and Sinking
Age-depth relation: Mass balance
The mass in column x1 is equal to the mass
in column x2 => “balanced”
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
Plate Subduction
Volcanic Arcs
Element recycling
Distillation of the mantle
Growth of continents
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)