An episode of strong seismic tremor (a continuous release of seismic energy) occurred during the build-up to the 2004-2008 eruption of Mount St. Helens. This episode was remarkable because no explosion or eruption immediately followed. Research found that in this case, the tremor occurred as gas-poor magma under Mount St. Helens was slowly forced upward for about 49 minutes, breaking and tearing the fabric of the rough conduit walls. The seismic waves produced by this movement resonated within the conduit to form hybrid waves that were recorded in far-field seismic stations as tremor. The conduit resonance masked the source of the tremor, making it difficult to determine if the tremor was leading to an explosion. An understanding of the different ways tremor can be generated requires that information from other sources, such as visual observations, deformation, geology and gas geochemistry, be used in order to interpret events that could lead to an eruption. Read more at Volcanic tremor masks its seismogenic source: Results from a study of non-eruptive tremor recorded at Mount St. Helens, Washington.