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.

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.

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

ophiolites

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