A stiff new layer in the mantle

July Editorial      

 

Washington under water the strange case of the sinking Capitol

Washington is going under. For once the reports of the decline of the capital of the USA are not political rhetoric but cold, hard geological fact. In 100 years time Washington DC and the land around it will be six inches lower than today.

This may not sound like a lot, but much of the city sits on reclaimed swampland was never very high to begin with. Add to this the higher tides expected as a result of global warming and Washington is well on its way to becoming the Venice of the eastern American seaboard. Nor will the sinking stop at six inches. It is part of a long-term geological process that will continue for millennia.

The problem is a geological phenomenon called 'forebulge collapse'. This is also a consequence of global warming, but warming which began over 20,000 years ago. At that time the northern part of New York state was covered by an ice sheet over a mile high. So thick and heavy was this ice sheet that it pressed the rock beneath into the mantle, causing the land mass further south to rise as a consequence.

To use the colourful analogy of Ben DeJong who conducted the research as a doctoral student at the University of Vermont's Rubenstein School of Environment and Natural Resources, 'Its a bit like sitting on one side of a water bed filled with very thick honey - then the other side goes up. But when you stand, the bulge comes down again.' The 'standing up' part of the event happened when the world got warmer and the ice-sheet retreated. Without the weight on the glaciers on the underlying rock, the pressure of the magma lessened. Without that pressure from below, the land around Washington DC is subsiding back to its natural level.

In a study supported by the US Geological Survey and published on 27 July 2015 in the journal GSA Today the authors report that the tide-line of Cheasapeake Bay was at one time 3-5 meters higher than it is today. Now tide gauges show that the sea around Washington DC is coming back higher and faster than elsewhere on the eastern seaboard, putting monuments, wildlife refuges and military bases at risk.

The 'bullet-proof' conclusions of the study were based on seventy boreholes drilled on the eastern shore of Chesapeake Bay which examined the underlying geology of the area. A study of rock, sediment and other material produced a clear picture of the moving shoreline. This study ruled out other phenomena associated with lowering landmass, such as the extraction of groundwater; demonstrating conclusively that the subsidence is linked to forebulge collapse.

It may be a long way down. The authors point out, 'The presence of near-shore MIS 3 deposits near present sea level suggests an alternative sea-level history for the region, one that implies forebulge uplift of at least 40 m since the time of deposition.' (MIS stands for 'Marine Isotope Stage'. This is a standard geological dating technique which works back from MIS 1which is the present day.) If Washington has been lifted 40 meters higher than it would otherwise be, the implication is that once the forebulge has completely gone, Washington and the surrounding coastline will be deep under water.

Fortunately the process will take thousands of years to complete, and by then even the famously deadlocked US Congress may have found time to act. What is certain is that with modern technology and understanding of geology, the process is unstoppable and irreversible.

The geology of the region has in fact always been affected by the interaction of glaciers with the underlying rock. The same glaciers that created the original magma forebulge also created Chesapeake bay when the glaciers melted and flooded what had originally been a system of river valleys. Though supported on a bed of crystalline rock, most of the surface of this part of the Atlantic coastal plain is made of up sand, clay and gravel deposited by these primaeval river systems. The maximum elevation in the region is about 300 feet.

Under these circumstances, a subsidence of even six inches is, as the paper's authors comment,'a very big deal'.

The full paper in GSA Today can be found here - http://www.chesapeakebay.net/discover/bayecosystem/baygeology

 

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