Results from Prior NSF Support

Award Number OCE-11965
Amount $304,958
Period 08/15/98-11/30/00
Title Active Seismic Imaging of Axial Volcano, PI's William Menke & Maya Tolstoy

The region of Axial Volcano, Juan de Fuca Ridge region provides an excellent opportunity to study the interplay between active "hot spot" and "mid-ocean ridge" magmatic systems. Important questions include how the two magma systems are fed; their magma and heat budgets; the degree of interconnectedness (or interaction) between them; their relationship to seismicity and geodetic strains; the role of each in plate-tectonic spreading and and crustal formation; and their effect on the geochemistry (e.g. mixing, fractionation) of erupted basalts. Information on the physical layout of the magma systems is critical to the study of each of these issues. The purpose of this research was to investigate these questions through the tomographic imaging of the region using seismic data from an active seismic airgun-to-obs experiment. The experiment was remarkably successful, both in the sense that voluminous high-quality data were obtained, and in the sense that very clear signials associated with magma were detected in that data. The key elements of the new three-dimensional compressional velocity model of the Axial and Coaxial magma systems are (West, 2001):

1. A Very Large Axial Magma Chamber. At a depth of 2.25 to 3.5 km beneath Axial caldera lies an 8 by 12 km magma chamber containing 10-20% melt (West 2001). At depths of 4-5 km beneath the sea floor there is evidence of additional melt, in lower concentrations (a few percent) but spread over a larger area. Residence times of a few hundred to a few thousand years are implied (West et al. 2001).
2. A smaller Coaxial Magma Chamber, unconnected with the one at Axial. The magma chamber is loacted at the "Source Site" of the 1993 eruption (Menke et al., 2001). It is at least 6 cubic km in volume and contains at least 0.6 cubic km of melt, enough to supply at least several eruptions of size equal to the one in 1993.
3. Several other small low velocity zones are possibly outlier magma chambers from Axial. Two other low-velocity zones occur in the shallow crust near Axial volcano, one about 10 km north of the caldera on the North Rift, and the other about 10 km south of the caldera but displaced to the west of the South Rift (West 2001). They appear unconnected to the main Axial magma chamber and might possibly represent small accumulations of melt left over from past lateral diking events.
4. Strong thickening of the crust beneath Axial volcano. The crust thickens from about 6 km far from Axial to 8 km near Axial to 11 km beneath the summit (West 2001).

Publications:

1. Menke-W, Shallow crustal magma chamber beneath the axial high of the Coaxial Segment of Juan de Fuca Ridge at the "Source Site" of the 1993 eruption, submitted to Geology, 2001.
2. West-M, The deep structure of Axial Volcano, Ph.D. Thesis, Columbia University, 2001.
3. West-M; Menke-W; M-Tolstoy; S-Webb; R Sohn, Magma reservior beneath Axial volvano, Juan de Fuca Ridge is far larger than eruption size; submitted to Nature, 2001.

Data and other products This project collected new data, which is freely available on-line at http://www.ldeo.columbia.edu/user/menke/AX/. Some software, including the tomograhy code, that was written for the project is available at http://www.ldeo.columbia.edu/user/menke/software/.