Activity 3: Ocean-Floor Topography and Crustal Creation
Early static models of the ocean basins suggested that the sea floor was uniformly flat, with the deepest regions in the center. We now know that it has a very complex bathymetry in which the centers of the oceans are shallow regions above very large mountain systems. The depth to the ridge crests is uniformly 3 km, but the topographic profiles across ridge systems vary considerably. Some ridges, such as the Mid-Atlantic Ridge, are steep sided, whereas others, like the East Pacific Rise, are much broader and flatter.
Ridges are the site of magma intrusion and formation of new oceanic crust, so they are much hotter than the normal ocean basins. Because they are hot, midocean ridges are also less dense and more buoyant than cold older crust and will float higher on the asthenosphere. In addition, the common temperature of new crust worldwide means it has the same buoyancy worldwide, and all midocean ridges occur at the same depth, 3 km (Fig. 1).
oceanic crust sinking diagram

Fig. 1: Ocean-floor bathymetry.


As crust moves away from the ridge, it cools, becomes denser, and sinks deeper into the asthenosphere. Thus ridges are topographic highs, and the oceans become deeper farther away from the ridge. The amount oceanic crust sinks and ocean depth are related to lithosphere age. For crust younger than 80 my, the depth-to-age plot defines a curve that is related to the square root of time. Measurements of ridge systems worldwide have shown that the amount the lithosphere sinks is described by


where z is the distance the lithosphere sinks (in kilometers), t is the age of the crust (in millions of years), and c is a constant with a value of 0.3 km/my0.5.
By 80 my, the oceanic crust attains a thermal balance between heat lost by cooling and heat gained from the underlying mantle.  Ocean floor older than 80 my has a constant depth of about 6,000 m. Because the deepest parts of the ocean basins, oceanic trenches, are much deeper than 6 km and are found near the margins of the oceans, they must owe their great depth to processes very different from those which form the abyssal plains of the ocean floor.
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