Welcome back from summer break! Please join us for the September 15, 2016, meeting of the Potomac Geophysical Society (PGS) at 7:00 p.m. at Crowne Plaza – Tysons Corner hotel, 1960 Chain Bridge Road, 22102. This location is within one-half mile of the Tysons Corner Metro station, near I-495, and has free parking available. Our private meeting room is located in the back of the Tuscan Grille restaurant on the second floor of the hotel. The optional dinner cost will be discounted to $30 for members in good standing (have paid dues) and students, and $40 for non-members, and is inclusive of iced tea, coffee, tax and gratuity. Members and guests may attend the presentation after dinner for no charge; we estimate that the presentation will begin at 8:15 p.m. For attendees who arrive early, the social will be held in O’Malley’s Pub on the first floor of the Crowne Plaza hotel. Drinks may also be purchased in the private meeting room on a cash basis.
Social: 6:00-7:00 p.m. O’Malley’s Pub, first floor Crowne Plaza
Dinner: 7:00-8:15 p.m. Tuscan Grille, second floor Crowne Plaza
Meeting & Presentation: 8:15-9:30 p.m. Tuscan Grille, second floor Crowne Plaza
Dr. Michael Ryan, The Magma Physics Project, Hilo, Hawai’i
“Rock-, mineral-, and melt-physics and magma neutral buoyancy in Hawai’i”
Compressional and shear wave velocities in the mafic and ultra-mafic rocks that make up Hawaiian volcanoes and their crustal and upper mantle underpinnings have been combined with additional petrophysical properties to understand the mechanical rationale for the existence and long-term evolution of subcaldera magma reservoirs in active volcanic centers such as Kilauea and Mauna Loa. These additional properties include single crystal elastic constants, high temperature elastic moduli, thermal expansion coefficients, the pressure-dependence of crack and joint closure, and the P-T dependence of in-situ rock, melt and magma densities. When the data is combined with long-term geodetic, seismic, geologic and 3-D modelling data, compelling and very durable existence and evolutionary criteria have been discovered for these magma reservoirs. Regions of neutral buoyancy are produced by the crossover in the in-situ densities of magmatic fluids and the rocks surrounding subcaldera magma reservoirs and rift systems. Beneath this region, magma parcels ascend driven by positive buoyancy forces, whereas above it, they decend under the influence of negative buoyancy. Within Mauna Loa and Kilauea, the region of neutral buoyancy is coincident with the location of the subcaldera magma reservoir. Based on laboratory and field measurements of Vp and Vs, the compression of the rock column beneath these volcanoes is divisable into two fields: an upper field of fracture, macropore, micropore and mineral compression (0-9 km depth) and a deeper field of mineral compression only (9 km and deeper). This compression—or contraction—profile, is inherently non-linear: the upper (non-linear) portion reflecting the greater bulk compressibility of rock porosity+aqueous fluids, and the lower (linear) portion reflecting aggregate mineral compressibilities. Velocity-density systematics connect the rock data with the melt and magma data—revealing the in-situ density crossover in the 2-7 km depth range for both these volcanoes: the region of neutral buoyancy. The region of neutral buoyancy combined with the contraction profile that induces it thus provides for the long-term stability (the existence) of subcaldera magma reservoirs and their rift systems. As Hawaiian volcanoes age and evolve, they carry their contraction profile and region of neutral buoyancy upward with them. Thus the evolutionary progression from seamount (Loihi) to subaerial immature shield (Kilauea) to mature volcano (Mauna Loa), is one characterized by the progressive elevation of the summit reservoir complex and rift zones—a process that leaves beneath a wake of high velocity mafic and ultramafic rocks within the core region of the shield. For Hawaiian rift systems, the lateral magma injection process follows the horizon of neutral buoyancy, where countryrock and magma densities are equal. This is the region of preferred dike formation and of magma residence and mixing with the accompanying migrating swarms of microseismicity in the 2-4 km depth interval. Beyond Hawaii, the neutral buoyancy systematics also apply to the Earth’s mid-ocean ridge system, to Icelandic central volcanoes, and to island arc and continental arc volcanoes, among others.
Michael Ryan has 42 years experience in physical volcanology. He has worked on active volcanoes in Hawai’i (Kilauea, Mauna Loa), the high Cascades (Mt. St. Helens), Iceland (Krafla, Askja) and Japan (Sakurajima). He has studied the interiors of ancient dissected volcanoes across the U.S., Iceland, and Japan. He has a B. Sci. and M. Sci. in geology from Michigan State University, and a Ph. D. in geochemistry & mineralogy from the Pennsylvania State University with parallel work in engineering mechanics, ceramic science, and petroleum & natural gas engineering. His post-doctoral work was in the Dept. of Mineral Sciences of the Smithsonian Institution of Washington, D.C. He has taught graduate courses in geology and geophysics at the University of Hawai’i-Manoa where he also conducted research on active volcanism. He conducted research for 30 years with the U.S. Geological Survey and is now affiliated with the Magma Physics Project in Hilo, Hawai’i. His emphasis is on the pathways and processes that regulate the migration of magma from Earth’s mantle through the crust and into the interiors of active volcanoes.