The laterally directed pyroclastic density current (blast surge) that erupted from Mount St. Helens (Washington State, USA) on 18 May 1980 devastated ∼600 km2 of thick conifer forest, with most large trees uprooted or broken off near the base. Throughout this area, at least 51 patches of trees were left standing, mostly where the blast surge had reached 95 ± 5% of its distance traveled. Analysis of post-eruption digital topography and topographic profiles aligned in the direction that the surge traveled (defined by orientations of locally toppled trees) show that the stands of trees are on lee sides of hills and, rarely, on downstream sides of plateaus. All hills are higher than neighboring (within 500 m) hills that lack tree stands. In all cases, toppled trees upstream are orientated toward the patches of standing trees, arguing that the surge was not blocked. The patches thus reflect where the base of the surge temporarily left the ground. Because the stands are only found near the end of the surge runout, yet where inferred speeds of the surge were as slow as 40 m s–1 and as fast as 115 m s–1, the most likely reason the surge briefly detached from the ground was that its density had locally decreased enough that it approached that of the atmosphere. Numerical modeling shows that the distance the surge traveled before reattaching to the ground is controlled by its density and trajectory (i.e., hill height and stoss-side rise). This study shows that the path of destruction left by pyroclastic density currents can provide clues about their internal mechanics.

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