title: Derivation of the eruption history of the prehistoric Ring Creek lava flow, Southern British Columbia
Creators: name: Bruno, Samuel
Publisher: name: Western Washington University
Year: 2011

description: <p>The Ring Creek lava flow is a 10,000 year old, post-glacial dacitic lava flow that originated from Opal Cone, a small cinder cone on the south east flank of Mt. Garibaldi, British Columbia. Disequilibrium texture of amphibole combined with clotting in plagioclase suggests that the Ring Creek magma stalled beneath Opal Cone for at least 440 days before the eruption of the Ring Creek lava flow. Use of an igneous plagioclaseliquid thermo barometer and hygrometer indicate shallow storage conditions of 818 - 868o C, 2.4 - 3.6 kbar, and 0.2 - 2.0 wt% H2O. Surge in plagioclase growth, evidenced by complex zonation and clotting, in conjunction with amphibole reaction, indicates substantial degassing prior to the emplacement of the Ring Creek lava flow. A possible reason for stalling of the Ring Creek magma, as well as eruption mechanism for Opal Cone, is glacial ice. Glacial studies suggest the Mamquam River Valley, which the Ring Creek lava flow currently occupies, was completely deglaciated just prior to the emplacement of the Ring Creek lava flow in approximately 900 years or less. Deglaciation of the Mamquam River Valley corresponds to a deduction of at least 4.5 MPa of overburdening pressure, which could have initiated a positive feedback loop of degassing and upwelling of the Ring Creek magma. This scenario is supported by progressive amphibole reaction observed in samples associated with three morphologically distinct (yet chemically identical) flow units which include a torta (or potentially a tuya), a lava flow underlying the Ring Creek lava flow, and the Ring Creek lava flow. Because of the prehistoric nature of the Ring Creek lava flow and the lack of ideal phases such as magnetite and stable amphibole, multiple assumptions were made to use mathematical and chemical models. The igneous plagioclase-liquid thermo barometer and hygrometer shows poor correlation (R2) when calculating temperature (0.55), pressure (0.012), and water content (0.4) of hydrous dacite. Estimation of heat loss parameters used to calculate extrusion rate, eruption duration, and emplacement duration required the assumption of exact consistency between the Ring Creek and analogue lavas, which is not likely. Other mathematical models used to calculate eruption duration and flow velocity make simplifying assumptions of constant slope of underlying topography, flow velocity, morphology, and vent radius. In an effort to compensate for the high degree of uncertainty of applied models, several approaches were used to create a most likely scenario for the evolution of the Ring Creek magma and emplacement of the Ring Creek lava flow. Using flow morphology and the Gratz number, emplacement of the Ring Creek lava flow was estimated to range from 1 - 20 years. From this estimate, an average flow front velocity of 1x10-4 m/s is estimated assuming constant flow front advance. Flow length, measured in the field and using ArcGIS, and flow velocity were used to calculate lava core cooling rate of 3.5x10-3 C/hr. Viscosity estimates of 1010 - 1011 Pas were calculated using morphology and flow velocity as inputs for the Jeffrey's equation. Viscosity, core cooling rate, and flow front velocity estimates suggest that the Ring Creek lava flow was well insulated by thick lava armor, which would have accommodated slow core cooling and prolong advancement. Insulation by lava armor is supported by estimates of 65 m for the combined thickness of flow plug and lava armor.</p>