We used observations and modeling of Siple Dome, West Antarctica, a ridge ice divide, to infer the importance of linear deformation mechanisms in ice-sheet flow. We determined the crossover stress (a threshold value of the effective deviatoric stress below which linear flow mechanisms dominate over nonlinear flow mechanisms) by combining measurements of ice properties with in situ deformation rate measurements and a finite-element ice flow model that accounts for the effects of viscous anisotropy induced by preferred crystal-orientation fabric. We found that a crossover stress of 0.18 bar produces the best match between predicted and observed deformation rates. For Siple Dome, this means that including a linear term in the flow law is necessary, but generally the flow is still dominated by the nonlinear (Glen; n = 3) term. The pattern of flow near the divide at Siple Dome is also strongly affected by crystal fabric. Measurements of sonic velocity, which is a proxy for vertically oriented crystal fabric, suggest that a bed-parallel shear band exists several hundred meters above the bed within the Ice Age ice.

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Axial Seamount on the Juan de Fuca Ridge is a site of ongoing volcanic activity. The vertical component of the deformation can be observed with ambient seawater pressure gauges, which have excellent short-term resolution. However, pressure gauge drift adds additional and significant uncertainty in estimates of long-period deformation; drift rates equivalent to 20-30 cm/yr have been observed. One way to circumvent gauge drift is to make differential pressure measurements relative to a distant and presumably stable seafloor reference site. Such measurements require a remotely operated vehicle and can only be made infrequently. Another approach is to incorporate a piston gauge calibrator in the seafloor pressure recorder to generate an in situ reference pressure that, when periodically applied to the drift-susceptible gauge, can be used to determine and remove gauge drift from the time series. We constructed a self-calibrating pressure recorder and deployed it at Axial Seamount in September 2013. The drift-corrected record from that deployment revealed an uplift of the volcano summit of 60 cm over 17 months.