Static “dynamic topography”, or what earth science would look like to Orwell…
Over the past decade, you’d think that topography had suddenly gotten hyperactive as the term “dynamic topography” became widespread, usually as a product of complex modeling of flow in the mantle. Just what is dynamic topography? Try this quiz and see what you think; answer true if you think this is an example of dynamic topography, false if not:
- Subsidence caused by a subducting slab in the upper mantle
- Subsidence caused by the remains of a slab in the lower mantle
- Uplift caused by a mantle plume
- Uplift caused by extensional thinning of mantle lithosphere
- Subsidence caused by extensional thinning of the crust
- Subsidence caused by erosion
- Subsidence caused by slow cooling of an oceanic plate
- Uplift caused by melt depletion of mantle lithosphere
- Uplift caused by igneous intrusion into the crust
Before we consider the answers seemingly most in line with current literature, consider this analogy. Imagine a slow flowing river, so slow that the surface is totally flat. Now speed the river up (you probably have to add water or tilt it) and you start to develop waves and eddies. Visit a fast flowing river and it can be quite impressive to look at a wave that just sits in place (relative to the bank) maybe 3 or 4 feet tall. Why do we get that topography on the top of the river? Because it is moving; there is a dynamic pressure. If it isn’t moving, we don’t get that relief on the river at all.
So which of our situations above are considered dynamic topography? Try 1, 2, 3, 4, and 8 (there is argument on (7)). Do these operate faster than 5, 6, or 9? Um, no, they don’t; in fact depletion of the mantle lithosphere under continents arguably is responsible for topography places like eastern Canada have held for, oh, um, maybe 2 or 3 billion years.
Pretty dynamic, that.
Some of the most dynamic topography (topographies?) on earth are in places like New Zealand and the Himalaya where deformation of the crust and intense erosion remake the landscape practically on a daily basis. Where do we hear the most about dynamic topography? Ocean basins, South Africa, Australia, and the Appalachians. Hardly hotbeds of dynamism, geologically speaking.
If you view this as GG prefers, with that analogy of the running river in mind, you might ask under what situations would the topography be different if we simply turned off all motion in the earth. (1) and (3) would clearly be different and probably (2) but arguably none of the rest. Dynamic topography would be pretty special. However, when you get to the point of including the increasing depth of seafloor with age due to cooling, you almost wonder if the better term for dynamic topography might just be, oh, let’s hunt a word down….hmmm…how about “topography”?
GG will not trouble you here with the different flavors of “dynamic topography” now in the literature (frequently with no clear definition); the point so far is that the term borders on useless and should be retired.
There is an actual earth science puzzle posed by this literature that, unfortunately, the terminology has obscured; we’ll save that for a separate post. In the meantime, feel free to suggest any other terms that seem misleading in the comments…