X-Ray Vision Beneath the Seafloor
Yesterday we deployed one of the Langseth’s long hydrophone array cables and began the second phase of our survey. We looked forward to this with much anticipation. It’s outside work and at times requires some physical exertion, which we will not have much of on this expedition. Most of the time our job is to be inside the main science lab, closely monitoring the recordings that come in from all of the instrumentation that is running continuously as we traverse the ocean.
Up to now we have been sending soundings to the 47 ocean bottom seismometers that the Oceanus deployed early last week. The multi-channel seismic data we are acquiring in this next part of our study provide x-ray like images of remarkable resolution of horizons and faults in the sediments and crust beneath the seafloor. To construct these images we are towing one very long (over 8 kilometers!) streamer cable behind the Langseth containing 636 listening devices, or hydrophones. Each hydrophone records the return echos from all of our soundings. By adding the signals from each of these records, we are able to enhance reflections and see very fine-scale structures.
We began our first survey line near the Endeavour Ridge, part of the volcanic Juan de Fuca ridge that lies hidden beneath the ocean only 400 to 500 kilometers offshore. At this ridge, the Juan de Fuca plate is continuously replenished with the eruption and intrusion of magmas from the earth’s mantle. Now we are transiting away from the ridge imaging continuously as we go. When we reach the easternmost end of our line where the plate begins to dive under North America, we will have imaged the deep structure across an entire continuous transect of an oceanic plate for the first time!
One of the aims of our study is to understand how the Juan de Fuca plate changes as it ages and moves slowly toward the trench. Starting at birth and driven by heat from molten magma that lies under the Juan de Fuca ridge, seawater circulates through and reacts with the oceanic crust, altering its composition and structure. In this way seawater becomes incorporated into the oceanic plate. This process continues on as the plate ages in ways that are not well understood. Then when the plate dives back into the mantle beneath North America, this water is released and contributes to many subduction phenomena, including the properties of the fault interface where the great earthquakes occur and the formation of the magmas that periodically erupt at the Cascade volcanoes of the Pacific Northwest.