Extrusion of a new lobe onto the E flank of the composite lava dome stopped by 2 March. However, a renewed increase in seismicity was evident by 4 March and an eruption was expected by the end of the month. Poor weather hampered observations throughout February and early March, denying geologists access to the crater and views of the dome on most days.

Because of the weather, it was difficult to determine exactly when the February lava extrusion began. Between 30 January and 4 February, 21 gas and ash explosions were observed on seismic records and by FAA radar at Portland airport. Infrared photographs from an overflight during the night of 4-5 February did not appear to show new lava in the notch in the dome's upper E flank, but at 0930 on the 5th it contained a large smooth-sided creased rock (about 10 x 20 m in lateral dimension and 10 m high), probably the new lobe. An overflight during the night of 5-6 February showed that a substantial amount of new lava had been extruded. University of Washington seismologists note that for most extrusion episodes at Mt. St. Helens, surface events (principally rockfalls) have begun to dominate the seismic record at about the time that lava extrusion started. Surface events began to increase noticeably late 4 February, although most were small and the number of surface events did not exceed the number of subsurface earthquakes until early 6 February. Gas-emission events were also recorded [seismically], some of which were associated with [observations of] minor vapor-and-ash plumes. By the afternoon of 7 February, subsurface earthquake activity had decreased to one to five events per day and remained at that level through the end of the month.

After observing the new lobe on 7 February, geologists were next able to see it on the 11th, when it appeared to have grown in size by about 30%. A well-developed, smooth-sided crease was oriented along the long axis of the lobe's surface. Similar features observed during late stages of extrusions in December 1980, and February, June, and September 1981 were thought to represent the last material extruded from a vent and not disrupted by later flow. Although rockfall seismicity continued, indicating that the lobe was advancing, little growth was apparent between observations 11 and 15 February. Measurements on 23 February showed that the new lobe had advanced 23.5 m to the E since the 11th.

A type of seismicity not previously recorded at Mt. St. Helens was first detected 14 February. Hundreds of tiny events that were remarkably similar to each other (many were identical for as much as 20 cycles) occurred at an average interval of 40 seconds. In the 24 hours beginning at 0830, 559 of these events were recorded, but none was large enough to locate and their origin is uncertain.

Television footage 21 February showed a spine, not present on the 19th, growing from the center of the lobe. On 24 February, the spine was roughly 30 m tall, and by the 28th it had roughly doubled in height, extending about 20 m above the dome's summit. The relatively undisturbed growth of the spine indicated that little downslope movement of the lobe was occurring during this period. On 28 February, geologists noted that no rocks had fallen from the lobe front onto the previous night's snowfall. Observations 1 March indicated that extrusion had ended. The new lobe had filled all but 10-15 m of the 60-100 m-deep notch, oozed out its E end, and reached the E foot of the dome (figure 23). Geologists estimated that it was roughly the same size as previous lobes. Total seismic energy released during the February extrusion episode was comparable to that associated with previous extrusions, but occurred over a longer time span.

Figure 23. Sketch showing the Mt. St. Helens composite lava dome in the crater as viewed from the NNE in late February 1983. The February 1983 lobe is shown with its spine, as are lobes extruded in May and August 1982. Sketch by Bobbie Myers.

Deformation data were limited, but indicated that little swelling of the dome was associated with the extrusion. Although the W side of the dome has usually been the area of most rapid outward movement, none was measured until 7 February, and only 0.7 cm of expansion occurred between then and 1 March. During the same period, the N side of the dome moved outward 7-8 cm, but deformation there began before the extrusion (SEAN 08:01) and shortening of measured lines totaled about 20-25 cm. Visual observations indicated substantial deformation of the dome's E side, but no instrumental measurements were possible.

SO₂ emission peaked on 15 February, reaching 400 t/d, about twice the early February rate (SEAN 08:01). By late February, SO₂ emission had dropped to slightly more than 100 t/d and the average for the month was about 170 t/d.

The number of surface seismic events remained steady through early March, but an increase in subsurface earthquakes was evident by 4 March. Six subsurface events were recorded on 1 March and ten on the 3rd; the twelve events on 4 March were larger, so energy release was substantially higher. Energy release continued to accelerate significantly 5-6 March, and 25 subsurface events were recorded on the 6th. Because of the increased seismicity, the USGS and University of Washington issued an advisory notice 6 March stating that renewed eruptive activity could be expected. Poor weather prevented deformation measurements that have previously been successfully used to predict the time of eruption onset.

Seismic energy release declined late 6 March, and only twelve to fifteen events were recorded daily 7-9 March, but both values remained significantly above background levels. An updated advisory notice issued 8 March suggested that an eruption would begin within the next 3 weeks. On 9 March, observers in a helicopter saw that most of the spine had fallen, but did not report the presence of any additional new lava. Measurements on the N and W sides of the dome 10 March did not show large acceleration of displacement rates.

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: T. Casadevall, C. Newhall, D. Swanson, B. Myers, S. Brantley, USGS CVO, Vancouver, WA; S. Malone, University of Washington.