Ocean floor faulting explains differences in Central American lavas



[1] The steady eastward progression of the Cocos tectonic plate, which lies off the west coast of Central America, forces it under the Caribbean and North American plates, driving seismic and volcanic activity. Along the Central American coast, the chemical content of the lava produced by such volcanism varies significantly, an unusual finding given that the raw material for the magma—the Cocos plate—feeds the entire region. For instance, magma in central Nicaragua has a much higher ratio of barium to lanthanum than the magma in Costa Rica, with the concentrations of these and other trace elements serving as an indicator that distinct processes were involved in generating the different magma. To identify structural properties that vary along the Cocos plate that may explain these differences, Van Avendonk et al. conducted a 396-kilometer sweep of the plate's eastern boundary, collecting measurements of seismic velocity, i.e., how fast seismic waves are able to travel through the various rock layers. Previous research describes a complicated history of the Cocos plate. The northern half, which lies off Nicaragua, was the product of deep-ocean spreading at the East Pacific Rise (EPR). The Cocos-Nazca (CN) spreading region, along with intraplate volcanism near the Galápagos Islands, generated the southern portion of the plate. The authors found that seismic velocities within the northern portion of the plate are anomalously low. The authors suggest that as the Cocos plate is forced down, forming the Middle America Trench, the rock stretches and breaks. This high degree of faulting within the crust would allow seawater to penetrate into the mantle and form hydrated rocks called serpentinites. As the northern portion of the Cocos plate is subducted, these high serpentinite concentrations are carried deeper into the mantle, releasing their trapped water and significantly affecting the chemical composition of the magma. (Geochemistry, Geophysics, Geosystems, doi:10.1029/2011GC003592, 2011)