Off the coast of New Jersey, the continental shelf becomes a rocky and violent terrain etched with deep canyons and scoured by fresh landslides. Now, geologists believe that an underground layer of highly pressurized fluid may be the source of the turmoil. The water in this layer could burst out suddenly, triggering submarine landslides and creating tidal waves that would endanger the Jersey shore.
Such unstable situations are created when rivers dump layers of dense sediment on top of silt. The silt is squeezed so hard that it becomes fluid; oil drillers often strike such pockets of mud or silt that gush forth. But these trapped layers were not expected on the Atlantic seaboard, because the Hudson and other rivers don't dump as much sediment as delta-forming rivers such as the Mississippi. So it came as a surprise when geologist Peter Flemings of Pennsylvania State University, University Park, discovered unusually porous rock in a 1997 core taken by the JOIDES Resolution, the world's only drilling ship devoted to science.
Flemings and graduate student Brandon Dugan used a mathematical model to calculate the stability of the trapped fluid layer. They estimate that the silty layer started out with about 60% water by volume. As the sediment piled up more than a kilometer thick, the solid particles compressed, but the water maintained its volume--and the pressure built up. "Imagine laying a water balloon flat and burying it with dirt," Flemings says. The water balloon would tend to squirt out wherever the layer above it is thinnest. In the case of the Hudson River sediments, that spot is about 160 kilometers offshore, where the continental shelf comes to an end. The researchers estimate that the fluid layer is oozing out to sea at a rate of 7 millimeters per year, they report in the 14 July issue of Science.
Another possibility, however, could give residents of the Jersey shore something to worry about. A blow-out of high-pressure fluid could conceivably trigger a massive landslide; and that could cause a tidal wave, according to recent work by Neil Driscoll of the Woods Hole Oceanographic Institution in Massachusetts. Geologists say it is unknown at present how common such giant landslides might be. As Peter Rona, a marine geologist at Rutgers University in New Brunswick, New Jersey, says, "the continental margin is dynamic, it can affect people, and we should be watching it."