The Table, a flat-topped volcano in southern British Columbia: Revisited

Abstract

More than 65 years ago, W.H. Mathews published a seminal paper in American Journal of Science describing the Table, an iconic, flat-topped volcano in southern British Columbia, Canada. The Table is recognized as the type-example of a glaciovolcanic lava-dominated tuya. We use field mapping, 3-D photogrammetry, lithofacies identification and ⁴⁰Ar/³⁹Ar geochronology to re-examine the emplacement origins of this unique volcanic edifice. The Table is a steep-sided, flat-topped mass of dense, porphyritic andesite preserving a near-original outer surface of hackly fractured aphanitic material. The age of the edifice is 100 ± 12 ka. A new model for the emplacement involves dike injection and endogenous inflation within an overlying ice mass that is estimated to have been ∼250 to 350 m thick. The model assumes that the edifice expanded outwardly as an elliptical cylinder, endogenously inflating and melting an equal volume of ice. The thermal exchange between the expanding massif and the melting ice operates across a subvertical, well-drained, enveloping carapace of quench breccia. For effusion rates of 0.5 to 2 m³ s⁻¹, marginal heat fluxes are 43 to 186 W m⁻², implying low overall heat transfer coefficients (0.04 to ∼0.18 W m⁻² K⁻¹). The total volume of ice melted serves as a calorimeter and constrains the average thickness (∼3 m) and properties (thermal conductivity; 0.1 – 0.4 W m⁻¹ K⁻¹) of the breccia carapace. The heat transfer efficiency of intermediate lava-dominated tuyas is significantly lower (∼2 %) than reported for other glaciovolcanic systems (commonly ∼45 – 77 %) but similar to that found for block-lava flows and lava tubes.