Ocean Basins

Ocean Basins

This is a bathymetric map of the ocean floor created by measurements of gravity anomalies taken from satellites. For more images like this one, check out NOAA's web site. (where I got this image)

Outline Notes:

3.2 Ocean Basins

General:

The ocean bottoms on average are flat. Oceanic crust is of uniform thickness, ~3-10 km, avg 5 km, and of uniform composition, predominantly basaltic. (continental crust, ~35 km thickness and granitic)
Flat areas include abyssal planes and the ridges- mid-ocean ridges average 1 to 2 degree slopes.
Major features of the ocean bottom include ridges, transform faults, fracture zones, oceanic plateaus, aseismic ridges, and island/continental arc-trench systems.
Gravity measurements over the ocean indicate that generally the free-air anomaly is nearly zero. Therefore, ocean basins are in isostatic equilibrium. Elevation differences reflect differences in the density or thickness of the underlying crust and/or mantle.
Definition 1: Gravity Anomaly- the difference between the observed gravity and the expected gravity at that location based on a model for the earth's gravity field.
Definition 2: Free-Air Correction/Anomaly- In comparing gravity anomalies measured at different locations, it is necessary to correct them to some common datum- sea level. After the free-air/altitude correction is made, any remaining difference or anomaly is called the free-air anomaly.

Seismic-velocity-layer models (P-wave velocities, Vp) of typical oceanic crust and other oceanic features have been created and are based on lithologic interpretations which come from direct sampling and from comparison with continental exposures thought to represent old oceanic crust. See figure below.

With oceanic crust, seismic velocities are characteristic for each type of oceanic feature. Layer 1 is the surface of the crust. Layer 2 is below layer 1 and so on.

Layer 1, Vp of 3 to 5 km/s. Interpreted as unconsolidated sediment of pelagic, hemipelagic, or turbiditic origin.
Layer 2, Vp from 5 to 6 km/s. Subdivided into 2A, 2B, and 2C. Predominantly submarine basaltic extrusive and shallow intrusive rocks. 2A- low velocity, increasing with depth. 2B- relatively constant velocity. 2C- rapid increase of velocity with depth.
Layer 3, Vp from 6 to 7.5 km/s. Subdivided into 3A and 3B. Composed of mafic-ultramafic plutonic rocks and/or serpentinized mantle peridotite. The subdivisions possibly represent varying quantities of olivine in plutonic rocks.

Next Page, Oceanic Plate Margins (Ocean Basins Continued)