ISOSTASY: Another great cocktail-party word

                        Regional  isostatic compensation

Upsalla, the medieval capital of Sweden, was once a seaport but now is landlocked, many meters above the level of the sea.   From the point of view of that ancient university town, the sea has retreated markedly in just a few short centuries.  So, a conundrum: where did all the water go?

Another conundrum:  One frequently finds ancient sedimentary basins now filled with far more detritus than it would have taken to fill the original depression; say, 3000 meters of sedimentary layers in a hole known, on other geological observations, to originally have been no more than about 1000 meters deep.  How can such a thing come about?

Well, many of you geology enthusiasts will know the answer: isostatic compensation. 

Decades of measurements of gravity as a function of elevation have proven conclusively that the crust if the earth is for the most part in a kind of floating equilibrium.  The name “isostasy”, in fact, means “equal pressure”; at a suitable depth in the mantle all pressures are equal, no matter the surface topography; mountain range, ocean basin, etc.  Departures from this condition are known as isostatic anomalies and mostly occur in places where something interesting geologically is going on.  Take the case of Upsalla, for instance.  Prior to the foundation of the city the local region (the Baltic) was covered by a thick ice cap.  The weight of the glacial mound depressed the crust regionally.  When the glacier disappeared the land surface began to “rebound”; that is, gradually rise up.  To Upsallaers it must have appeared that the sea was receding but, of course, the converse was true; they were rising. 

Note that, because of the quasi viscous nature of the upper mantle*the uplift, still going on, is gradual.  In fact, it follows a sort of logarithmic law, assuring that as it nears final equilibrium the rebound rate gets smaller and smaller.  Thus, Stockholm may be safe as a seaport, but poor old Upsalla will be forever land-bound; baring, of course, another ice age!

As for the over-stuffed sedimentary basin, the answer is obvious: as stuff is washed in, the bottom sank until isostatic equilibrium was obtained.

Isostasy can introduce some minor complications into the task of working out the local geological history.  For instance, look at the diagram at the top of the page.  Because the crust has some elastic properties, when a load depresses an area partially compensating upward bulges will be generated.  Then, if the load (a glacier, say) is removed both up and down crustal displacements are generated, complicating the life of the Quaternary geological community no end!

I am tired of this subject, although I have just scratched its surface.  All good Zentnerds not familiar with this concept should go immediately to Google and read up on regional vs local compensation, as well as the models proposed by Airy and Pratt.  But: no exam, I ptomise.

·        https://frivilousessays.blogspot.com/2020/10/s-warren-ncarey-part-2.html