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Santa Clara scoria cone and its leaky plumbing system (southwest Utah, USA)

Maria Clara Murta , Greg Valentine

Abstract

Santa Clara volcano (~27 ka; Utah, USA) is a monogenetic volcano with a "leaky" plumbing system. Its products include a lava field (~0.1 km3) and a scoria cone (~0.01 km3). The ~145 m high (with respect to surroundings) cone was partially built on the shoulder of a ridge, and its crater floor is at a height of ~100 m. The ridge is elongated northeast-southwestward and consists of cross-bedded sandstone. Lava leaked from the western side of the ridge at a height of ~77 m and flowed downslope, forming a tabular-shaped, ~1.5-meter-thick outcrop. The steep slope favored lava acceleration and rupture of its surface, imparting 'a'a texture. On the eastern side of the ridge at a height of ~99 m, there is a 50 m long, 0.40-0.80 m thick sill which transitions laterally into a vent for partially welded agglomerates. Similar heights of the crater floor (~100 m), sill, and lava vent indicate concurrent vent activities under hydrostatic equilibrium during the latest eruptive stages. We propose a model where a vertical dike propagating beneath and parallel to the axis of the ridge, formed the main vent where the dike first intersected the surface low on the shoulder of the ridge. Magma continued rising as the cone grew until hydrostatic pressure equaled magma overpressure. There the magma spread into bedding planes to form a sill that leaked on both sides of the ridge. This model implies a magma overpressure between 1.1 and 2.8 MPa, consistent with overpressures estimated elsewhere for basaltic dikes.