2012-09-10

See the Lithosphere moving up and down

The lithosphere, the uppermost resistent layer of the Earth, rests on the fluid mantle underneath and moves up (or down) in response to weight removed from its surface (or placed on it), such as ice capes, large lakes, mountain ranges, volcanos...  
Isostatic sinking (subsidence) and
rebound (uplift) occurring when an
ice sheet forms by climate cooling
and when it is removed by climate warming.
The downwarping (isostatic subsidence)
produced by the ice accumulation in a) is
fully recovered in this process (b and c). 
My first steps into geoscience dealt with this concept called isostasy, which looked somewhat simple to a recent graduate in Physics as I was back then, since it simply applies the Archimedes Principle to the Earth's lithosphere. But this idea was just emerging in the late 19th century. And still, G.K. Gilbert was there to get it and to apply it to one of its most conspicuous scenariosLake Bonneville. 

Lake Bonneville was an enormous closed lake (meaning it had no outlet) encompassing the western half of Utah during the Pleistocene. The Great Salt Lake is a small remnant. It would be among the few largest, deepest, and highest lakes today. When its level raised to 1500 m above sea level at the end of the last glaciation, 15,000 years ago, its waters found an exit through the Red Rock pass and the lake was suddenly drained. It produced one of the largest floods ever recorded: the Bonneville Flood (see this previous post). But there was another consequence to the flood: When the lake water was released, the lithosphere under the lake moved upwards to readjust its isostatic equilibrium with the viscous mantle that underlies the Earth's crust. 

Now, do you believe this story?

If you are skeptic and you feel unsatisfied by the extensive scientific literature on isostasy accumulated ever since, you can use Google Earth to see it by yourself. Pay attention to the elevation of these two preserved shorelines at two locations 1) in the margins of Lake Bonneville (left) and 2) at the center of it (right):

 Lake Outlet. Shoreline altitude: 1510 m Lake Center. Shoreline altitude: 1590 m
Just in case this is not obvious to you. The shorelines are the abrupt changes in slope produced either by the deposition of materials brought by streams when they where reaching the lake, or by the erosion of the lake waves along the coast.

Both shorelines record the maximum level of the same lake, but the one in the center of the lake is today 80 m higher. The uplift implied for the central areas of the Bonneville Lake was a lithospheric response to the unload of water during the Bonneville Flood and the desiccation of the lake thereafter.

Bonneville lake shorelines as depicted in Gilbert's USGS report,
1890. As you have seen in the maps above, Gilbert also realized
this shorelines are slightly tilted, in apparent contradiction with
the idea that they were simultaneously formed at the surface of
a single water body.   
What you just did is exactly what Gilbert did in 1890. Except he mapped everything by foot with the help of local natives and a donkey. Plus, he also performed numerical modeling (yes, in 1890, really) to test his interpretations with physical laws. He even could estimate the strength of the crustal plate sustaining the lake, a concept that would still take some 70 years to become standard: Flexural isostasy.

If geniuses would exist, then Gilbert would be one of them. His report "Lake Bonneville" (downloadwas revolutionary for two reasons: It showed the first model of flexural isostasy, and the first confirmation of isostatic vertical motions in a lake setting. But secondly, it provided geomorphological evidence for a huge flood (a débacle) that challenged the principle of uniformitarianism

I thus ow these words to Gilbert since I first hold his 130 years-old book in a basement of the ANU, in Canberra. It opened my eyes to a geological-scale tragedy: the century-long wait for the geological establishment to listen to both ideas: isostasy and megaflooding. It took them that long because those ideas were at odds with the Principles of Geology. But principles should remain just tools, not goals! 

Aerial picture of Western Spitsbergen, Scandinavia, where
deglaciation has lead to widespread isostatic uplift. 

Three large raised beaches are today at different elevations
(in meters above present sea level). The highest is the oldest in
age, >120,000 yr old.

Source: Norsk Polarinstitutt, Oslo.
Incidentally, this geological process has been used to estimate the viscosity of the material underlying the lithosphere (Bills et al., 1994). But isostasy is a concept that still challenges researchers because its ability to explain the Earth's topography is not complete: Elevation and vertical motions cannot always be perfectly explained by the isostatic equilibrium. Deviations from isostatic predictions are called dynamic topography and are of great interest to understand the dynamic forces related to mantle convective flow. But that's another story. 


More information:


G, K, Gilbert (1890). Lake Bonneville USGS Reports DOI: 10.5962/bhl.title.45550 Lake Bonneville fluctuations and global climate change. (1997). 

Oviatt, Charles G. Geology, 25 (2) DOI: 10.1130/0091-7613(1997)025

Watts, A. B. (2001). Isostasy and flexure of the lithosphere. Cambridge: Cambridge univ. press. Introduction pdf available.

Billis et al., 1994, JGR v.99 

J. E. O'Connor (1993). Hydrology, Hydraulics and Geomorphology of the Bonneville Flood GSA Special Paper, GSA, Boulder, CO, , 274 DOI: 10.1002/(SICI)1099-1417(199609/10)11:5

[update: recent review paper on isostasy]