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Images: The Big MELT

An ocean bottom seismometer on the Pacific Ocean floor measures the velocity of seismic waves traveling through the mantle and oceanic crust. These data allowed scientists to probe deep within the portion of the earth that generates new ocean floor.

A WHOI ocean bottom seismometer (OBS) is deployed from R/V Melville (Scripps Institution of Oceanography) at the start of the MELT Experiment in November 1995. Seismometers within the yellow spheres record ground motion generated by seismic waves traveling through the mantle and the oceanic crust. In May 1996, an anchor on the OBS was released and the instrument floated to the surface, where it was recovered.

More than 50 ocean bottom seismometers (white triangles) were deployed across and along the East Pacific Rise off the west coast of Mexico during the MELT Experiment to probe the deep structure beneath the mid-ocean ridge. Arrows show the motions of the Pacific and Nazca plates, which are spreading in opposite directions from the ridge crest, shown by shallower depths (red)

Initial results from the MELT Experiment are shown in this schematic cross-section of the upper mantle beneath the East Pacific Rise. The melting region below the mid-ocean ridge extends over a broad area several hundred kilometers wide. The region is asymmetrical, with a wider zone west of the ridge than east of it. The melting region also extends far deeper than many scientists have previously theorized: to depths of 150 to 200 kilometers beneath the ridge, although the greatest concentration of melt occurs above 100 kilometers.

To measure the thickness of oceanic crust near the East Pacific Rise, WHOI scientists measured the velocity of airgun-generated seismic waves that travel through the crust, refract off rock layers, and are recorded by ocean bottom seismometers. A single seisimic wave from a single airgun shot consists of several phases, or rays, depending on the number of layers it penetrates. Pg rays (blue) are refracted within the crust and provide information on the speed of waves traveling through it. PmP rays (orange) reflect off the Moho (the mantle-crust transition boundary) and provide data on crustal thickness. Pn rays (green) are refracted in the upper mantle and offer data on the speed of waves traveling through the mantle. Only one of every 10 rays is plotted.