On the emplacement of large-volume evolved lavas: Case studies in southwest Idaho
Semple Domagall, Abigail Mary
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The high viscosity common of evolved lavas as a result of high SiO 2 contents and low eruption temperatures tends to favor explosive eruptions and only small-volumes effusive deposits. However, globally, tens of evolved lava flows have been described of unusually large volumes (e.g. the 26 km 3 Chao dacite, Chile [de Silva et al., 1994]; the 75 km 3 Bracks Rhyolite, Texas [Henry et al., 1990]; the 200 km 3 Sheep Creek Rhyolite, Idaho [Bonnichsen and Kauffman, 1987]). With aspect ratios and geometries more typical of ignimbrites it is important to use a variety of characteristics to confirm a lava flow origin for these units. A lack of pyroclastic textures (such as bubble wall shards, pumice fragments, broken phenocrysts and lithics); presence of a basal breccia and abrupt, sometimes lobate flow margins are all taken as evidence of emplacement as a lava flow rather than an ignimbrite that has undergone rheomorphism. Two field areas in southern Idaho were studied to better understand how these large-volume evolved lava flows emplace and what may be unique about them. The Reynolds Creek Rhyolite lava flow is a high silica rhyolite (∼75 wt. % SiO 2 ) that erupted during the formation of the western Snake River Plains graben ∼11.5 Ma. At its present volume of ∼1 km 3 (and an original volume of ≤1.8 km 3 ) it is not as extensive as some of the Idaho lava flows, however it has the advantage of having a surface exposure and an exposed vent region. The Reynolds Creek Rhyolite lava flow was emplaced as a non-particulate viscous fluid from a linear vent and traveled > 5 km from its vent to the NE and towards the western Snake River Plain graben. The lava flow also flowed NW into the pre-cursor of today's Reynolds Creek river valley. Petrography and chemistry suggests the Reynolds Creek Rhyolite had a high eruption temperature of >1000°C and a corresponding low viscosity of 10 6 -10 7 Pa s. Distal outcrops of the Dorsey Creek Rhyolite were studied near Murphy Hot Springs where the lava flow displays elliptical outcrops with a distinct textural pattern. The jointing within the outcrops illustrate that the elliptical shape of the outcrops is a result of the original shape of the cooling lava rather than an erosional feature. The jointing patterns also suggest movement of a more-liquid lava through a more-solid crust in the form of a protruding lobe from the flow front. Lobe outcrops get larger and start to merge with one another with distance from the flow margin suggesting that adjacent lobes coalesce in a manner similar to basaltic pahoehoe, and the main body of the flow closer to the source remains insulated by a crust. The overall conclusion is that an unusually low viscosity is a factor in the Reynolds Creek Rhyolite flows and may be in many of the large-volume evolved lava flows; high temperatures or a favorable chemistry may account for the low viscosity. A linear vent, as noted at the RCR may also play a role in the emplacement of large volumes of evolved lava as it would allow for a high mass eruption rate but still allow an effusive eruption. The Dorsey Creek Rhyolite shows flow lobes at its distal margins suggesting it emplaced similar to a basaltic pahoehoe, thus insulating the lava flow from the source to the flow front. This insulation would reduce the decrease in viscosity experienced by the cooling lava flow as it advanced and may be key to emplacing voluminous lava flows providing the large source is available in the first place.