Nicaragua is Wet Underneath

A new seismic study of a rock slab deep underneath Nicaragua shows that the slab has the highest concentration of water of any comparable slab associated with volcanoes. Just as radar can be used to tell you about landforms and vegetation at the surface, so seismic waves can tell you about the lay of the land 150 km down. Geoffrey Abers and Terry Plank, scientists from Boston University, and their collaborator from UCSB, Bradley Hacker, observed that seismic waves at depths of 100-150 km beneath a string of Nicaraguan volcanoes traveled as if the rock slab down there were acting like a waveguide. From the wave speeds, the researchers deduced that the water content of the slab was about 5%, some 2 to 3 times greater than for other subducted slabs. Since water subducted along with oceanic crust sometimes returns to the surface along with lava, one can check the elevated water content finding. Indeed, the fluid concentration of Nicaraguan lavas is quite high. Abers says that the Nicaraguan slab, and another very "wet" slab he has studied near Guam, are quite steep (the angle of subduction in the Nicaraguan case is about 70 degrees), which he believes makes the slab a better conduit for fluids. (Geophysical Research Letters, 1 April 2003.)