Lidar measurement of topographic change during the 2004 eruption of Mount St. Helens, WA

Volcanic unrest at Mount St. Helens in the autumn of 2004 is captured by high-resolution lidar surveys in September 2003 and October 2004. The 2003 survey, by EarthData International under USGS contract, provides a baseline for an October 4-5 survey by TerraPoint USA, Inc., under contract to NASA. Differences between the 9/03 and 10/04/04 surveys quantify volcanic deformation, illustrate non-volcanic processes, and demonstrate the feasibility--and limits--of change detection with commercial lidar. Volcanic deformation within the crater, as of 10/4/04, consisted of a strong central uplift with a basal area of 0.097 km² surrounded by a 50- to 80-m wide zone within which ice of the crater glacier is gently uplifted. In this zone of gentle uplift, rock beneath the crater glacier may have risen, or the glacier itself may have thickened as it was shouldered aside by the central uplift. Extrapolation from outside the zone of gentle uplift suggests that since 9/03 about 4 m of snow had accumulated in the vicinity of the new uplift. Snow and ice have melted to create an elliptical 120 m x 70 m vent in the crater glacier on the west side of the central uplift. Mean increase of elevation of the central uplift is 50.75 m, suggesting a rock volume increase (as of 10/4/04) of 4.9 x 10⁶ m³, thus emplacement of magma at an average rate of 0.5 x 10⁶ m³ day since the 9/23/04 onset of high-level seismicity. Non-volcanic processes are also illuminated by elevation changes from 9/03 to 10/4/04. Permanent snowfields on the outer flanks of the edifice thinned, typically by 2 to 4 m, whereas the crater glacier, as mentioned above, thickened. Several large rockfalls (10⁴ to 10⁵ m³) onto the glacier are apparent, as well as smaller debris flow accumulations. The NE snout of the crater glacier advanced and thinned. A 1.4 x 10⁵ m² rock glacier at the NW margin of the crater has advanced 7-8 meters and grown by 2.5 x 10⁵ m³ over the last year. It has been fed by rockfall from the collapsing crater rim and deflation of talus slopes, as well as snowfall and avalanche. Two smaller rock glaciers at the NE margin of the crater are also moving. Outside the area of concentrated change within the crater the mean elevation bias between the 9/03 and 10/4/04 surveys is about 0.9 meters. Correlation of elevation differences with aspect and slope indicates that the bias largely reflects different geodetic frameworks for the 9/03 and 10/4/04 surveys. We expect that translation of the base station for the 10/4/04 survey from ITRF2000 to NAD83/NAVD88 and re-processing will significantly reduce the elevation bias, making it possible to quantify elevation changes as small as a several decimeters.