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.

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We are indebted to many colleagues in the USGS Volcano Hazards Program and the Pacific Northwest Seismic Network who contributed to hazard assessment and study of the ongoing eruption at MSH.

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Author notes

  1. Anthony I. Qamar: Deceased.


  1. US Geological Survey, Cascades Volcano Observatory, 1300 SE Cardinal Ct. #100, Vancouver, Washington, 98683, USA

    • Richard M. Iverson
    • , Daniel Dzurisin
    • , Cynthia A. Gardner
    • , Terrence M. Gerlach
    • , Richard G. LaHusen
    • , Michael Lisowski
    • , Jon J. Major
    • , Seth C. Moran
    • , John S. Pallister
    • , Steven P. Schilling
    •  & James W. Vallance
  2. Earth and Space Sciences, University of Washington, Seattle, Washington, 98195, USA

    • Stephen D. Malone
    •  & Anthony I. Qamar
  3. US Geological Survey, Denver Federal Center, Box 25046, Lakewood, Colorado, 80225, USA

    • James A. Messerich


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Correspondence to Richard M. Iverson or Daniel Dzurisin or Cynthia A. Gardner.

Supplementary information

  1. Supplementary Notes

    This file contains Supplementary Figures, Supplementary Methods, Supplementary mathematics (Equations), and Supplementary Notes, as well as legends for the Supplementary Movies listed below. (PDF 1733 kb)

  2. Supplementary Movie 1

    MSH DEM sequence. This movie shows a sequence of vertical shaded relief images illustrating dome growth and accompanying glacier deformation at Mount St. Helens, 2004-2005. Images were produced from digital elevation models (DEMs) constructed from stereoscopic aerial photographs taken roughly once a month. (MPG 9361 kb)

  3. Supplementary Movie 2

    MSH timelapse photo sequence. This movie shows a sequence of terrestrial photographs depicting dome growth at Mount St. Helens, 2004-2005. The movie was compiled from selected digital photographs taken about once every three minutes by an automated camera positioned near the mouth of the MSH crater. (MPG 9981 kb)

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