AERIAL PHOTOGRAMMETRIC MONITORING OF THE 2004-2005 ERUPTION OF MOUNT ST. HELENS

In October 2004, a new period of dome growth began that dramatically changed the topography of the Mount St. Helens crater. From October 2004 through April 2005, more than 55,000,000 m³ of lava extruded onto the crater floor immediately south of the 1980-1986 lava dome, intensely deformed and cut a new glacier in two, and resulted in spectacular crevassing and rapid advance of the east arm of the glacier. Time-sequential vertical and oblique aerial photography of morphologic change allowed construction of a series of detailed 2 m resolution digital elevation models (DEMs) created between October 4, 2004 and April 19, 2005. Vertical aerial photographs, flown at a nominal 1:12000 scale were acquired at three-week time intervals, scanned at approximately 12-micron resolution, and rectified using a soft-copy (i.e., digital image) photogrammetric workstation. Aerotriangulated models were constructed using ground control outside the area of active deformation, derived from pre-eruption GPS and photogrammetric data, and passed to subsequent model sets. Resulting location accuracy was of the order of decimeters. Oblique stereo photographs were taken from a helicopter to augment data collection in areas occasionally obscured on vertical photos by clouds, deep shadows, or a persistent eruption plume. The DEMs were used to quantify volumetric changes associated with dome emplacement, growth, and collapse, as well as deformation of glacial ice. Additionally, the rectified stereo imagery enabled quantitative geologic analysis. Each DEM surface was compared to pre-eruption reference surfaces from 2000 and 2003 and to the preceding DEM surface to extract volumetric changes. As of April 2005, the new dome was approximately 600 m in length (NW-SE) and 550 m in width (SW-NE). The elevation of the highest point was 2336 m. As of mid-April, the volume of the total surface deformation was greater than half the volume change associated with the previous episode of dome growth from 1980-1986. The volumetric growth rate ranged from about 9 m³/sec very early in the eruption to 1-3 m³/sec during most of the eruption.