Jon J. Major

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.

Snow and ice perturbation during historical volcanic eruptions and the formation of lahars and floods

Historical eruptions have produced lahars and floods by perturbing snow and ice at more than 40 volcanoes worldwide. Most of these volcanoes are located at latitudes higher than 35°; those at lower latitudes reach altitudes generally above 4000 m. Volcanic events can perturb mantles of snow and ice in at least five ways: (1) scouring and melting by flowing pyroclastic debris or blasts of hot gases and pyroclastic debris, (2) surficial melting by lava flows, (3) basal melting of glacial ice or snow by subglacial eruptions or geothermal activity, (4) ejection of water by eruptions through a crater lake, and (5) deposition of tephra fall. Historical records of volcanic eruptions at snow-clad volcanoes show the following: (1) Flowing pyroclastic debris (pyroclastic flows and surges) and blasts of hot gases and pyroclastic debris are the most common volcanic events that generate lahars and floods; (2) Surficial lava flows generally cannot melt snow and ice rapidly enough to form large lahars or floods; (3) Heating the base of a glacier or snowpack by subglacial eruptions or by geothermal activity can induce basal melting that may result in ponding of water and lead to sudden outpourings of water or sediment-rich debris flows; (4) Tephra falls usually alter ablation rates of snow and ice but generally produce little meltwater that results in the formation of lahars and floods; (5) Lahars and floods generated by flowing pyroclastic debris, blasts of hot gases and pyroclastic debris, or basal melting of snow and ice commonly have volumes that exceed 105 m3.The glowing lava (pyroclastic flow) which flowed with force over ravines and ridges...gathered in the basin quickly and then forced downwards. As a result, tremendously wide and deep pathways in the ice and snow were made and produced great streams of water (Wolf 1878).

Geomorphic Change and Vegetation Development on the Muddy River Mudflow Deposit

Geomorphic disturbances are widely recognized as important processes that influence plant-community development and landscape-scale vegetation patterns [e.g., Veblen and Ashton (1978), Garwood et al. (1979), Swanson et al. (1988), and Malanson (1993)]. In volcanically active areas such as the PacificNorthwest,mudflows are locally important geomorphic disturbance events governing shortand long-term ecological conditions. Volcanic mudflows can scour and inundate river valleys with large volumes of debris (Janda et al. 1981; Pierson 1985; Vallance and Scott 1997; Scott 1988; Vallance 2000; Kovanen et al. 2001) and influence plant succession tens of kilometers downstream from their points of origin (Halpern andHarmon 1983; Adams andDale 1987;Wood and delMoral 1987; Frenzen et al. 1988). In addition to altering plant succession, large volcanic mudflows can initiate a cascading chain of secondary disturbances that further modify the landscape and affect subsequent ecological responses (see Swanson and Major, Chapter 3, this volume). The comparatively high disturbance intensity but spatially variable nature of volcanic mudflows provide unique opportunities to study complex interactions between geomorphic processes and ecological succession (Beardsley and Cannon 1930; del Moral 1998; Kroh et al. 2000). Nonetheless, few studies examined plant succession on mudflow deposits before the 1980 eruption of Mount St. Helens (Frehner 1957). Research subsequent to that eruption has shown that plant succession on mudflow deposits is highly variable in response to local substrates, plant reproductive strategies, distances to seed sources, and chance dispersal events (Halpern and Harmon 1983; del Moral 1998). “Biological legacies,” such as floated logs, remnant snags, and shallowly buried residual plants, also play important roles in vegetation development on mudflow deposits (Frehner 1957; Franklin et al. 1985; Frenzen et al. 1988; Halpern and Harmon 1983; del Moral 1998; Kroh et al. 2000; Weber 2001). Vegetation succession on mudflow deposits can follow an initialor relay-floristics model (sensu Egler 1954) or some combination of the two (del Moral 1998) and can lead to the compositional convergence or divergence of neighboring communities (Franklin et al. 1985;Wood and delMoral 1987;Kroh et al. 2000). Although earlier studies of vegetation recovery on mudflowdeposits provide important insights into the dynamics of herbaceous plant communities, few of these studies examined succession over decades after a disturbance (Frenzen et al. 1988; Kroh et al. 2000). In this chapter, we present a case study of geomorphic and vegetation responses at four sites along the Muddy River to large (up to 107 m3) mudflows triggered by the May 18, 1980 eruption of Mount St. Helens. Our objective is to describe and qualitatively compare geomorphic changes and vegetation development along distinct reaches that represent a range of mudflow-induced disturbance intensities and environmental settings. We address three questions: