Dynamic topography

The term dynamic topography is used in geodynamics and oceanography to refer to elevation differences caused by the flow within the Earth's mantle and the ocean water, respectively.

Geodynamics

In geodynamics, dynamic topography refers to topography generated by the motion of zones of differing degrees of buoyancy (convection) in the Earth's mantle.^[1] It is also seen as the residual topography obtained by removing the isostatic contribution from the observed topography (i.e., the topography that cannot be explained by an isostatic equilibrium of the crust or the lithosphere resting on a fluid mantle) and all observed topography due to post-glacial rebound. Elevation differences due to dynamic topography are frequently on the order of a few hundred meters to a couple of kilometers. Large scale surface features due to dynamic topography are mid-ocean ridges and oceanic trenches.^[1] Other prominent examples include areas overlying mantle plumes such as the African superswell.^[2]

The mid-ocean ridges are high due to dynamic topography because the upwelling hot material underneath them pushes them up above the surrounding seafloor. This provides an important driving force in plate tectonics called ridge push: the increased gravitational potential energy of the mid-ocean ridge due to its dynamic uplift causes it to extend and push the surrounding lithosphere away from the ridge axis. Dynamic topography and mantle density variations can explain 90% of the long-wavelength geoid after the hydrostatic ellipsoid is subtracted out.^[1]

Dynamic topography is the reason why the geoid is high over regions of low-density mantle. If the mantle were static, these low-density regions would be geoid lows. However, these low-density regions move upwards in a mobile, convecting mantle, elevating density interfaces such as the core-mantle boundary, 440 and 670 kilometer discontinuities, and the Earth's surface. Since both the density and the dynamic topography provide approximately the same magnitude of change in the geoid, the resultant geoid is a relatively small value (being the difference between large but similar numbers).^[1]

Oceanography

In oceanography, dynamic topography refers to the topography of the sea surface related to the dynamics of its own flow. In hydrostatic equilibrium, the surface of the ocean would have no topography, but due the ocean currents, its maximum dynamic topography is on the order of two meters.^{[citation needed]}

References

1. ^ ^{a b c d} Hager, B. H.; Richards, M. A. (1989). "Long-Wavelength Variations in Earth's Geoid: Physical Models and Dynamical Implications". Philosophical Transactions of the Royal Society A: Physical, Mathematical and Engineering Sciences 328 (1599): 309–327. doi:10.1098/rsta.1989.0038. 
2. ^ Lithgow-Bertelloni, Carolina; Silver, Paul G. (1998). "Dynamic topography, plate driving forces and the African superswell". Nature 395 (6699): 269–272. doi:10.1038/26212. http://www.nature.com/nature/journal/v395/n6699/abs/395269a0.html. 

• Hager, Bradford H.; Clayton, Robert W.; Richards, Mark A.; Comer, Robert P.; Dziewonski, Adam M. (1985). "Lower mantle heterogeneity, dynamic topography and the geoid". Nature 313 (6003): 541–545. Bibcode 1985Natur.313..541H. doi:10.1038/313541a0.