Santa Clara scoria cone and its leaky plumbing system (southwest Utah, USA)
摘要
Estimating magma overpressure from intrusion geometry remains a challenge especially when dikes are not directly exposed. Santa Clara volcano (~27 ka; USA) is an example of a small scoria cone (~ 0.01 km3) with an associated lava field (~0.2 km3) without a direct exposure of its feeder dike. The ~145 m high cone was built at the end of a northeast-trending, ~350 m wide sandstone ridge that rises ~115 m above the surrounding terrain. Its crater floor lies ~100 m above the cone base. Lava leaked from the western ridge side at ~90 m height, forming a ~1.5-m-thick `a`ā lava flow. On the eastern ridge side, at ~95 m height, a 50-m-long, 0.4–0.8-m-thick sill transitions laterally into a vent for agglomerates. The similar elevations of the crater floor, sill, and lava vent suggest concurrent venting under hydrostatic equilibrium during the final eruptive stages. We infer that a vertical dike propagated beneath and parallel to the ridge axis, forming the main vent where it first intersected the surface on the ridge base. The cone grew until its vent reached a height determined by magma overpressure in the dike at ridge-base level, at which point, instead of continuing upward growth, magma moved laterally within the ridge to form the sill and lava leak. This scenario implies a magma overpressure of 1–3 MPa at the base of the ridge. We compare two dike-length scenarios and combine the inferred overpressure estimates with elastic theory. These calculations indicate that dike thickness is likely underestimated, suggesting that inelastic deformation is important in accommodating dikes in the upper crust, and that combining observed dike geometries and elastic theory might overestimate magma overpressures.