Cynthia A. Gardner

Dynamics of seismogenic volcanic extrusion at Mount St Helens in 2004-05.

The 2004–05 eruption of Mount St Helens exhibited sustained, near-equilibrium behaviour characterized by relatively steady extrusion of a solid dacite plug and nearly periodic shallow earthquakes. Here we present a diverse data set to support our hypothesis that these earthquakes resulted from stick-slip motion along the margins of the plug as it was forced incrementally upwards by ascending, solidifying, gas-poor magma. We formalize this hypothesis with a dynamical model that reveals a strong analogy between behaviour of the magma–plug system and that of a variably damped oscillator. Modelled stick-slip oscillations have properties that help constrain the balance of forces governing the earthquakes and eruption, and they imply that magma pressure never deviated much from the steady equilibrium pressure. We infer that the volcano was probably poised in a near-eruptive equilibrium state long before the onset of the 2004–05 eruption.

Mount St. Helens: A 30-Year Legacy of Volcanism

The spectacular eruption of Mount St. Helens on 18 May 1980 electrified scientists and the public. Photodocumentation of the colossal landslide, directed blast, and ensuing eruption column—which reached as high as 25 kilometers in altitude and lasted for nearly 9 hours—made news worldwide. Reconnaissance of the devastation spurred efforts to understand the power and awe of those moments (Figure 1). The eruption remains a seminal historical event—studying it and its aftermath revolutionized the way scientists approach the field of volcanology. Not only was the eruption spectacular, but also it occurred in daytime, at an accessible volcano, in a country with the resources to transform disaster into scientific opportunity, amid a transformation in digital technology. Lives lost and the impact of the eruption on people and infrastructure downstream and downwind made it imperative for scientists to investigate events and work with communities to lessen losses from future eruptions.

Field-trip guide to Mount Hood, Oregon, highlighting eruptive history and hazards

Mount Hood, Oregon, an archetypal subduction zone stratovolcano, is dominated by extrusive eruptions of lava flows and domes, coupled with a high degree of homogeneity in erupted lava compositions. Over the last ~500,000 years— the age of the current edifice—the volcano has repeatedly erupted crystal-rich andesites and low SiO2 dacites, with SiO2 contents largely between 55 and 65 weight percent. Lavas also show similar phenocryst mineralogy, compositions, and textures, and are dominated by plagioclase together with pyroxene, amphibole, and occasional olivine. The presence of quenched mafic inclusions, bimodal populations of plagioclase and amphibole, mineral zoning, and a range of other evidence also shows that Mount Hood magmas are produced by quasi-binary mixing between relatively mafic (basaltic) and silicic (rhyodacitic-rhyolitic) parental magmas. Mineral zoning shows that magma mixing occurred very late in the petrogenetic history, within weeks to months of eruption. Mount Hood is a volcanic system driven by mafic recharge, where hot mafic magmas ascending from the mantle or lower crust interact with silicic magmas to produce mixed intermediate compositions. Evidence suggests that the silicic parental magma is stored within the shallow crust (3–6 kilometers) beneath the volcano as cool, crystalrich mush for long periods (>>10 ka) prior to eruptions. Mafic recharge provides both the impetus to erupt and produces the intermediate compositions, resulting in the long-term eruptive output of a homogeneous series of intermediate magmas.