Fig. 3.3.1: Topographic map of the seafloor in the central Atlantic calculated using satellite data (ETOPO1, 2008). The elevation profile across the Atlantic shows the deep ocean plains and the central ridge (after Heezen, Tharp & Ewing, 1959). (Meschede, unpubl., 2021)

The bathymetric map of the central Atlantic shows the topography of the seafloor with all underwater ridges and mountains, the seamounts (ETOPO1, 2008). The map shows the so-called predicted seafloor topography. It is not a directly observed morphology but rather a seafloor topography calculated from satellite measurement data (Fig. 3.3.1). The resolution is only very rough, but kilometer-sized structures like those across the entire width of the Atlantic can be seen very well. A topographic cross-section through the central Atlantic (after Heezen, Tharp & Ewing, 1959) shows that there are abyssal plains on both sides of the Mid-Atlantic Ridge. The difference in height between the ridge and the deep sea level is at least two and a half thousand meters. This height difference is associated with the increasing age of the oceanic lithosphere – and this is a consequence of the gradual thickening of the lithospheric mantle.

Fig. 3.3.2: Age of the oceanic crust in relation to its depth. (Meschede, unpubl, 2021; data from Parsons & Sclater, 1977)

In Fig. 3.3.2 the ages of samples from the three major oceans are plotted against the depth from which they were taken. There is a clear correlation between age and depth. When an age of around 80 million years is reached, a distinct flattening of the curve can be seen, indicating that the amount of  lithospheric mantle growing from below decreases.

Fig. 3.3.3: Changes in the average density of the oceanic lithosphere (oceanic crust + lithospheric mantle). (modified after Meschede, 2016)

The average density of oceanic lithosphere increases with age (Fig. 3.3.3). At the beginning, the lithosphere consists only of oceanic crust, i.e. the density of the newly formed lithosphere, which does not yet contain any lithospheric mantle, is 3.0 g/cm³. As the thickness of the lithospheric mantle increases, the average density of the lithosphere, i.e. the sum of the oceanic crust and the lithospheric mantle, increases. The lithospheric mantle gradually becomes the determining factor for the density of the entire lithosphere.

Due to the isostatic compensation, the increasingly heavier lithosphere sinks deeper into the asthenosphere and abyssal plains are formed on both sides of the spreading center.

At a certain point in time, an interesting effect comes into play: when the lithosphere reaches an age of around 100 to 150 million years, it becomes heavier than the asthenosphere underneath. The difference is not large, it is 3.3 g/cm³ for the lithosphere versus 3.25 g/cm³ for the asthenosphere. However, heavier material comes to lie on top of lighter material. This density inversion causes the stratification to become unstable and can cause the oceanic lithosphere to sink into the upper mantle, creating a subduction zone.