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Creation, Aging and Recycling of Ocean Floor

According to plate tectonics, there is no beginning, no end to the opening and closing of ocean basins -- all is driven by convective flow of the mantle which continues as long as radioactive decay produces heat inside the planet. So, the record of seafloor spreading that produced the present distribution of continents and oceans is just the latest chapter in the dynamic history of the Earth's surface. This important conclusion has been illustrated by a concept called the Wilson cycle:

Oceans come and go, but continents remain -- in fact they grow slowly with time as a result of plate collisions.

The creation, aging and destruction of the ocean floor is an on-going, continuous process that governs many fundamental aspects of the physics and chemistry of our planet: (1) It produced the oceans and atmosphere very early in Earth history, and (2) now buffers the composition of seawater and the atmosphere. (3) It controls the composition, distribution and size of continents. And (4) recycling may be important in regulating certain aspects of Earth's climate.

We have a simple picture of ocean crust "architecture" from several sources of information:

- seismic profiling

- ocean drilling

- ophiolites (fossil slices of the ocean floor thrust onto continents)

We see a remarkably uniform process of formation:

- upwelling of warm mantle

- partial melting to produce even less dense magmas

- rising to sub-crustal magma chambers

- cooling, crystallization and eruption

Feeder dikes and plutonic rocks, together with the basalts, make up a 6-7 km thick section of crust that has formed from melted rock. The remainder of the mantle cools slowly by heat conducted outward and is welded onto the base of the crust, traveling with it from spreading center to trench. Magnetic anomalies acquired as the rocks cool through about 500°C (very near the eruption site)

What happens when the hot rock comes in contact with seawater at spreading ridges?? -- black smokers and hydrothermal activity:

The discovery of hot springs on the ocean floor during the 1970s was one of the most exciting events in the history of oceanography -- and much of the discovery and earliest research happened here at OSU! The most spectacular of these are the "black smokers" where hot water gushes out of the vents in the sea-floor at temperatures up to 350°C, forming a dense black plume made up of minute particles of metal sulfides. At lower vent temperatures (30-330°C) the "smoke" is dominated by white particles of barium sulfate, giving rise to the term "white smokers". Less spectacular but more wide-spread are the lower temperature warm water vents where water emerges at about 10-20°C above surrounding bottom-water temperatures of 2-3°C.

These sources of warm water supply nutrients to unusual ecosystems in which the primary production that supports the local food web is not photosynthesis, but chemosynthetic bacteria, which derive energy from the oxidation of sulfides from the vent waters.

Once the ocean crust has been formed by crystallizing molten rock at the spreading ridges (at average depths of 2500m), hydrothermal circulation of seawater through the hot rocks begins. The entire ridge system (50,000 km long) is affected. The rate of circulation is sufficient for every drop of the 1.4 billion (109) km3 of ocean water to circulate through the ocean crust every few million years. This results in major chemical exchanges of elements between seawater and hot basalt. Consequently, the ocean crust determines much of the chemical composition of seawater, and for some elements it is a more important source than rivers. The formation of concentrations of metal sulfide which accompanies discharge of hydrothermal fluids into the ocean is one of the principal mechanisms of economic ore deposits (particularly Cu, Pb, Zn).

Metalliferous sediments (high concentrations of Fe, Mn, Ag, Cr, Cu, Pb, Zn) occur near spreading ridges; these are deposited as a result of hydrothermal venting. Studies of ophiolites confirm that large volumes of seawater can penetrate more than 5 km into oceanic crust. Hydrothermal systems occur on land (e.g., Yellowstone, Iceland) and all along the spreading ridges, wherever there is a high thermal gradient and a network of fractures (permeability) that allows fluids to move through the solid rock to extract heat and exchange chemically.

Lifespan of hydrothermal vents? Probably about 103 years, but variable. Spacing of vents may be due to intermittent magma supply, and "plumbing".

Conductive heat flow accounts for plate cooling and increasing depth, such that

depth ~ (age)1/2

Sedimentation -- up to several km thick, increasing with distance (age) from spreading ridges, but trenches are usually not filled (young)

Plate Collision -- the return of ocean lithosphere back to the mantle. Subduction at convergent margins - reversing the cooling process

volcanic arcs, forearcs, accretionary prisms


elemental recycling -- H2O and CO2 returned to the atmosphere; climate effects

Pacific NW -- Cascadia Arc subduction zone

Earthquake (the "big one") and volcanic hazards (Mt St Helens)

What drives plate tectonics??

Ridge push -- Elevation of ridges relative to abyssal plains causes plates to slide "downhill";

Slab pull -- the density of old plates causes them to "fall" back into the mantle, dragging the rest of the plate;

Resistive forces -- transform fault friction, plates over mantle

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Last update: March 25, 2000