A new phase of the ongoing activity began with increasing deformation and seismicity in late January 1984, followed by the extrusion of a new lobe near the dome's summit.

Numerous rockfalls occurred in December after the active lobe had reached the top of the dome's steep S and SE flanks. Rockfalls from that area were infrequent in January, suggesting that the advance of the lobe had nearly stopped. Deformation of the dome's SE flank accelerated briefly in late December to more than 50 cm/day, but slowed by an order of magnitude in early January and remained at 5-6 cm/day through the end of the month. On the NE flank, rates of outward movement remained relatively high through 4 January. On 9 January, when weather next allowed access to the crater, NE flank deformation had slowed and a new mound was perched just E of the dome's summit (SEAN 08:12). The surface of the mound was old material, but it was apparently cored by magma. It reached its maximum elevation 20 January, then subsided slowly. Earthquakes remained relatively numerous through 9 January, but the period 10-17 January was the quietest seismically since late September, with the number of events per day dropping from about fourteen to two to six and energy release declining to near background level.

In mid-January, points on the floor of the crater's breach, about 1 km N of the dome, began to move outward, and medium-frequency events started to appear on seismic records. Energy release remained low but the number of medium-frequency events increased gradually through the end of January. Gas ejection episodes increased noticeably in duration and amplitude on seismic records. By 23 January, some were followed by several minutes of weak harmonic tremor. Vigorous plumes were observed and small blocks were deposited on the crater floor. SO₂ emission averaged 90 ± 40 t/d during the first 3 weeks of January (as compared to 105 ± 25 t/d in December), but dropped to 35 ± 20 t/d for the remainder of the month and was below detection limits 26-28 January. A fairly large gas-and-ash ejection on the 28th was not followed by the typical temporary several-fold increase in SO₂ flux.

New cracks were observed on top of the dome 29 January. A graben was evident on its SW side by 1 February, and radial cracks had appeared on the W side. By 3 February, the graben was a few tens of meters wide and a few meters deep, extending across the summit crater. Deformation began to accelerate rapidly. A point halfway up the N flank of the dome that had moved outward no more than a few centimeters per day through most of January showed rates of 11.6 cm/day on 30 January, 46 cm/day on 3 February and about 1 m/day by the 5th. Deformation changes on the SE flank were less dramatic, but rates also increased, from 6 cm/day through 3 February to 20 cm/day on 5 February (all rates are average daily changes since the previous measurement). This activity was accompanied by a rapid increase in the number of earthquakes and seismic energy release beginning 1 February. SO₂ emission increased to 55 t/d 1 February and reached 140 t/d on the 6th.

When geologists arrived at the crater 6 February they observed a new mound filling the NW part of the dome's summit crater. The early January mound, about 100 m to the E, was subsiding. That evening, a small landslide from the E side of the dome moved 50-100 m to the main crater wall, causing minor snowmelt. The next day, the new mound had elongated and extended a short distance down the dome's N flank, while the January mound continued to subside. The surface of the new mound, like the January mound, was old material. No glow from the new mound was observed at night. N flank displacement was 2.4 m between 5 and 6 February, and rates of 3 m per day were measured on the 6th. SO₂ emission increased to 140 t/d on 6 February and 170 t/d the next day. Seismicity peaked during the evening of 7 February, dropping sharply in the next 24 hours to only slightly elevated levels. The number of rockfall events increased but they were smaller and less frequent than during previous extrusion episodes. Poor weather prevented access to the crater after 6 February. A brief glimpse of the dome on 10 February revealed a new lobe perched on its summit. . . . The new lobe appeared to be hot; snow had accumulated on the rest of the dome, but not on the new lobe.

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: R. Holcomb, T. Casadevall, USGS CVO, Vancouver, WA; S. Malone, University of Washington.