Dr. Spahr Webb
Earth Institute Contact: Dr. Spahr Webb
Locations: Pacific Ocean
Observations of fresh lava flows and the injection of biogenic particles into the water column at ~9°50’N along the East Pacific Rise (EPR) show that magmatic events can trigger hydrothermal and biologic activity. Hence, a central objective for this RIDGE2000 Integrated Study Site is to detect, characterize, and monitor such events. Presently deployed sensors, such as temperature probes and ocean bottom seismometers, are concentrated within a few kilometers of the eruption site at 9°50’N. However, future magmatic events may extend beyond, or be located outside, that small region. Seismic signals related to the initiation and propagation of magmatic intrusion and extrusion on the EPR may be too small to be detected with the Autonomous Hydrophone Array. A linear array of simple, reliable pressure sensors may be the best way to monitor magma movements along the 9°-10°N ridge segment. The pressure sensor is a proven technology that has detected small (cm) to large (m) vertical motions on mid-ocean ridges. They are also relatively inexpensive and can record data for many years without service. This proposal outlines a plan to build 20 ocean bottom pressure recorders and deploy them along the 9°-10°N ridge segment for about 4 years. Observations on land and simple modeling suggest that dike opening would elevate the seafloor while sill deflation would cause subsidence. Vertical displacements of tens of centimeters are expected for either dike intrusion or magma sill deflation along the EPR. A combination of dike opening and sill deflation might lead to smaller, but likely detectable vertical deflections. The fast spreading EPR should offer the shortest time interval between magmatic events, and therefore the greatest chance of “catching” one in a limited time interval. In support of the field program, two ancillary modeling efforts are proposed. The first relates to interpreting measured vertical seafloor displacements. Numerical models of coupled dike intrusion and sill empting will be carried out with a rheologic structure appropriate to the EPR. The second set of models will examine the feasibility of using repeat microbathymetric surveys to constrain surface deformation patterns across a section of the plate boundary. Intellectual Merit Beyond the importance of event detection, this experiment may allow discrimination between models for magma delivery along the fast-spreading EPR. In the “distributed supply” model, melt is supplied nearly vertically along the ridge axis. The fairly uniform crustal thickness detected along the EPR is often interpreted to reflect such a distributed supply. In the “central supply” model, more melt is provided at segment centers than near segment ends. In that case, the uniform crustal thickness is taken to reflect efficient transport of magma from the segment center to the segment ends. If the uniform supply model is correct, then fairly uniform vertical deflections would be expected along the part of a segment cut by a new dike. If the central supply model is correct, then significant subsidence would be expected near the segment center.
Simply detecting all the magmatic events that occur along this segment and constraining their lateral extent would significantly enhance the value of other RIDGE2000 projects (seismic, hydrothermal, and biological). Accordingly, results from this experiment would be immediately released to the RIDGE2000 community. On a more general note, this project will contribute to the emerging and challenging field of marine geodesy. Indeed, most of the world’s plate boundaries are submerged beneath the oceans and very little is known on how these accommodate plate motion on a monthly timescale. This project will also provide 20 ocean bottom pressure recorders that could be used in later projects. It will involve the participation of a graduate student, and support two technicians and a staff associate.
National Science Foundation