Midocean Ridges

At Midocean Ridges:

1) plates spread apart

Tensional (stretching) stresses are apparent at the midocean ridges because the newly forming ocean crust is broken by a series of normal faults. Normal fault scarps have been observed from deep-sea submersibles along the midocean ridges. First-motion studies of earthquakes at midocean ridges indicate movement on normal faults. So the crust is being pulled apart at the midocean ridges (not forced apart).

2) new crust/lithosphere forms

Stretching causes thinning of the crust. This allows deeper, hotter mantle rocks to rise up and approach the surface. These rocks are hot enough that they would melt if they were at the Earth's surface, but the great pressure from the weight of overlying rock prevents the rocks from changing from the solid to the liquid state. But as these hot mantle rocks rise, the pressure decreases (less mass overhead) to the point where they are able to partially melt. This is called decompression melting. The magma slowly migrates toward the surface and intrudes into fractures forming at the midocean ridge. Some of the magma cools and crystallizes beneath the surface forming coarse-grained intrusive rocks. Some injects into fractures in the crust that constantly form as the crust is continuously stretching. Some of it crystallizes in the fractures to form fine-grained dikes. Some of the magma passes upward through the fractures to form sheet flows and pillow lavas on the seafloor.

The thickness of the oceanic crust is nearly constant at around 7 km except right at the ridge where it is forming. In contrast, the lithosphere is very thin at the ridge, not much thicker than the crust itself. But the lithosphere thickens away from the ridge. The boundary between the lithosphere and the underlying weak, partially molten asthenosphere is a thermal boundary. The crust and upper mantle are hot at the midocean ridge because of the heat carried with mantle as it rises under the ridge. As the crust/lithosphere spreads away from the ridge, it cools. So, at the ridge, the zone of partial melting is at a shallow depth beneath the crust, and away from the ridge, the depth to the top of the zone of partial melting (the top of the asthenosphere) is deeper because the rocks have cooled. Mature (cooled) oceanic lithosphere has a thickness on the order of 80-100 km.

3) the ocean crust is almost entirely mafic: extrusive basalts and intrusive gabbros

The partial melting of the ultramafic mantle rocks generates mafic magma. The resulting ocean crust rocks are coarse-grained intrusive gabbros and fine-grained basaltic dikes, pillow basalts, and basaltic sheet flows. This is true for all midocean ridges and ocean crust indicating that only process has been forming ocean crust, at least for the the last couple hundereds of millions of years. (This is unlike the heterogeneous nature of igneous rocks formed on the continents indicating various mechanisms of magma generation and modification.)

4) newly formed lithosphere is hot and buoyant

At the ridge, new crust forms by igneous intrusion and extrusion. It is obviously hot. The mantle beneath the ridge is also hotter than the surrounding mantle because it is rising up from deeper depths (where temperatures are higher) to replace the spreading crust/lithosphere. So the entire crust/lithosphere/asthenosphere beneath the midocean ridge is hotter than the cooling lithosphere and "normal-temperature" mantle at distance away from the ridge. Since hot rocks are in a more expanded state and then contract as they cool (as they spread away from the ridge), the midocean ridges stand up high above the surrounding seafloor. The seafloor depth increases with distance away from the midocean ridges. The midocean ridges lie about 2.5 km below sea level. The depths increase quickly at first and then more gradually to 4 km to 5 km to 6 km below sea level.

Layered Structure of Oceanic Crust

From studies of ophiolites (fragmented sheets of ocean crust obducted onto land), from seismic studies of the ocean crust, and drilling in the ocean crust we have found that the ocean crust has a characteristic series of layers. Seismic velocities increase from ~2 km/s in the sedimentary layers, ~4-6 km/s in the extrusives and dikes, ~7 km/s in the gabbros, to ~8 km/s in the ultramafics.

oceanic sediments	seismic layer 1	deposited on the seafloor	(0-1 km)
pillow basalts and sheet flows	seismic layer 2a	extrusive igneous rocks	(~0.5 km)
sheeted dikes	seismic layer 2b	shallow intrusive igneous rocks, intruded into crack	(~1 km)
gabbro layered gabbro near bottom	seismic layer 3	intrusive igneous rocks	(5-6 km)
--------------------------------------	seismic moho	separating mafic & ultramafic rx
layered peridotite		ultramfic mineral assemblage (settled from magma chamber?)	(~1 km)
--------------------------------------	petrologic moho	(the true crust-mantle boundary)
peridotite		normal upper mantle rocks

Magma Chambers?
Models of oceanic crust formation based on ophiolites assumed that the gabbros and cummulate layers of the layered gabbroic and peridotite beneath the isotropic gabbros had formed in a large magma chamber setting. But seismic studies and other geophysical measurements (electrical resistivity) indicate only a very small magma chamber (>50% melt, <50% crystals), perhaps 1 km wide and only 100 m thick, at fast spreading ridges (see below) and no magma chamber beneath slow spreading ridges. Low seismic velocities below the ridges do indicate larger areas of probable "mush zones" (mostly crystals with some melt) and transitional zones (solid with melt along some grain boundaries).

Ridge Morphology and Spreading Rate
The morphology (shape) of midocean ridges depends on the spreading rate. At slow-spreading ridges (1-5 cm/yr separation rate), such as along the Mid-Atlantic Ridge, there is a deep axial valley 30-50 km wide and 1.5 - 3 km deep. The ridge generally has rough topography. At fast-spreading ridges (>9 cm/yr), such as portions of the East Pacific Rise, there is no axial rift valley and the topography is smoother. Small ridges running parallel to the axis of the MOR are fault blocks produced by steep normal faults. Some surveys have shown that at slow-spreading ridges the normal faults predominantly dip inward toward the axial valley whereas at fast-spreading ridges the faults dip in both directions.

Hydrothermal Alteration of Ocean Crust
Seawater that saturates the fractures near the midocean ridge are heated by the presence of magma and hot newly-formed rocks. The hot saltwater and hot basaltic rocks exchange ions. The result is the hydrotherm alteration of ocean crust rocks, principally near fractures and conduits for water circulation. The resulting altered crustal rocks contain a large number of hydrous minerals, those containing OH or H₂O groups in the chemical formula.

Black Smokers and Vent Communities
Water that has circulated through the ocean crust at the ridge returns to the surface at hydrothermal vents. The waters are well above 100 °C but do not boil because of the pressure beneath 2.5 km of ocean. It is now a metal-rich brine The temperature of the ocean bottom water is near 4 °C. When the hot brine mixes with the cold seawater the metals precipitate forming a cloud in the water (white smokers, brown smokers, black smokers). The precipitates build chimneys meters tall. Chemosynthetic bacteria use the energy in the chemical bonds of sulfur compounds in the "smoke" to fix energy non-photosynthetically. They form the base of a food chain for vent communities that includes unique tube worms, clams, blind shrimp and others that live on the energy from the Earth's interior rather than from the sun.