Lava extrusion continued to extend the active lobe SSE through November. Lava advanced about 100 m across the dome's broad, gently sloping summit area during the month, approaching the break in slope at the top of its steep upper flank. The rate of outflow appeared to be roughly the same as in previous months. A new depression, about 50 m x 80 m in horizontal dimensions and 30-40 m deep, formed about 50 m SE of the "spreading center" that fed the lobe in October. The "spreading center" had stagnated and the new depression may have been the source of some of the November lava. Just N of the stagnant "spreading center," the spine extruded in October was crumbling, but was still the high point on the dome at the end of November.

Collection of deformation data was hampered by the loss or inaccessibility of targets on the most active parts of the dome. Outward movement of accessible targets on the dome's SE flank was about 55 cm/day between late October and early November, 35 cm/day 8-21 November, and 48-50 cm/day after 21 November. If deformation patterns remained similar to those measured before the most active targets were lost, the most rapidly moving areas on the dome may have been expanding at roughly 90-100 cm/day, down slightly from the 120 cm/day of early October. Deformation of the dome's NE side had been rapid as the active lobe advanced down the NE flank from early May through late September but was negligible in November.

Gas-and-ash plumes continued to be emitted 3-6 times daily. The plumes, dominantly gas but sometimes containing some tephra, rose a few hundred meters to 1 km above the dome. Poor weather limited airborne gas measurements to five in November. The rate of SO₂ emission averaged 70 ± 45 t/d, nearly identical to October values. However, the 21 November rate of 150 t/d may have been measured in the remnants of a gas-and-ash plume; without this figure, November SO₂ emission averaged 50 ± 20 t/d.

For most of November, seismicity continued at approximately the October rate. Both seismic energy release and the number of events increased at the end of the month to values higher than in October. Average daily earthquake counts ranged from 5-13/day through 18 November, 12-15/day 19-25 November, and 24-33/day 26 November-1 December.

Geological Summary. Prior to 1980, Mount St. Helens was a conical volcano sometimes known as the Fujisan of America. During the 1980 eruption the upper 400 m of the summit was removed by slope failure, leaving a 2 x 3.5 km breached crater now partially filled by a lava dome. There have been nine major eruptive periods beginning about 40-50,000 years ago, and it has been the most active volcano in the Cascade Range during the Holocene. Prior to 2,200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older edifice, but few lava flows extended beyond the base of the volcano. The modern edifice consists of basaltic as well as andesitic and dacitic products from summit and flank vents. Eruptions in the 19th century originated from the Goat Rocks area on the N flank, and were witnessed by early settlers.

Information Contacts: D. Swanson, C. Mullins, USGS CVO, Vancouver, WA; R. Norris, University of Washington.