Minor activity — early October. A gradual increase in activity began 7 October with barely detectable harmonic tremor that started a few minutes after midnight and lasted less than an hour. During the day on the 7th, a few plumes, containing no appreciable ash, rose to about 3 km altitude before drifting NE. No harmonic tremor was recorded 8 October, but shallow earthquakes of magnitudes 1.6 and 1.8 occurred at 1535 and 1537. Some 8 October plumes contained smaller amounts of ash. Several minor gas clouds containing a little fine ash were emitted 10 October between 0915 and 1100, accompanied by intermittent low-level harmonic tremor. The initial column on the l0th reached 4.5-6 km altitude and drifted NNE. Two more gas plumes were emitted during the afternoon, again accompanied by minor seismicity. Gas emissions decreased in frequency and intensity 11-13 October. Seismic records during poor weather on the 14th indicated that occasional minor gas emission continued. On 15 October, some cracks that had not been present the preceding week were visible on the crater floor. Three small volcanic earthquakes were recorded, but no harmonic tremor was detected.

Explosive eruptions 16-18 October. Shallow earthquakes, most with magnitudes less than 1, became increasingly frequent on 16 October, reaching one event every few minutes by early evening. A M 3 earthquake, centered about 1 km beneath Mt. St. Helens, occurred at 1902. An hour later, the USGS and the USFS notified state and local officials of the possibility of an eruption, prompting the successful evacuation of 92 persons from around the volcano.

Observers in a USFS aircraft saw strong incandescence in the inner crater area at 2157, then an explosion 1 minute later. Tephra was ejected for only 5-10 minutes, accompanied by strong, regular seismicity probably related to explosive gas release. The top of the eruption column reached 13.5 km, as measured by Portland airport's radar. Ash and pea-sized pumice fell on Cougar. Pumice was reported at Amboy (40 km SW) and a trace of ash fell on the Portland-Vancouver area. A few flights to Portland were rerouted or cancelled and an air pollution alert was issued, but there were no serious disruptions of auto traffic the next morning.

Seismic activity stopped immediately after the 16 October explosion, but low-level seismicity resumed about 0900 the next morning. At 0928, an ash-laden eruption cloud began to rise rapidly from the crater, reaching 9 km altitude at 0932 and its maximum altitude of 14.3 km by 0938. This explosion produced a single pyroclastic flow, at about 0935, that traveled 3-4 km down the N flank to the break in slope, coming to rest after about 5 minutes. Ash emission began a gradual decline about 0940 then decreased abruptly at 0954, although sporadic eruptive activity continued until about 1015, when seismicity ceased. Wind directions differed with altitude, but the main plume drifted SE to SSE, reaching north-central Oregon. Ash from the explosion of late 16 October and early 17 October fell as far away as Eugene Oregon, 280 km SSW. These explosions destroyed the lava dome that was extruded after the 7 August eruption.

Weak seismicity resumed at about 2045 on the 17th, gradually increasing in intensity. Observers in a USFS aircraft noted strong incandescence in the inner crater at 2108, and the emergence of an eruption cloud at 2112 as tremor amplitude increased sharply. An incandescent pyroclastic flow descended the N flank at 2116. The eruption column reached its maximum altitude of 13.7 km at 2119. Eruption intensity declined gradually after 2119, with activity becoming intermittent by 2200. Emission of steam and ash pulses continued until about 2350, accompanied by intermittent seismicity. Ash blew SE.

Occasional seismic activity, consisting of very low-level signals lasting a few seconds to a few minutes, continued until the early afternoon of 18 October. A seismic episode that began on the 18th at 1232 abruptly intensified 3 minutes later as an eruption cloud emerged from the crater, reaching a maximum altitude of 6 km at 1239. A second burst, ejected at 1246, reached 7.6 km altitude by 1249. Vigorous tephra ejection stopped l minute later, but intermittent weak emissions continued for another 15 minutes. Eruptive activity gradually weakened, but sporadic low-level seismicity continued. A new tephra plume was ejected at 1428, reaching 6 km altitude at 1432, but activity declined quickly and the episode had essentially ended by 1500. Light ashfalls from the 18 October explosions were reported from as far as north-central Oregon.

Lava dome growth. By the time visibility returned at 1520, a new lava dome about 30 m across and 6 m high was growing in a saucer-shaped depression in an area formerly occupied by the inner crater and the recently destroyed, post-7 August lava dome. An hour later, the dome was 40 m in diameter and 9 m high. By the next morning it had grown to an elliptical feature 270 m in largest dimension and 50 m high, but it had essentially ceased increasing in volume, with sagging of the summit compensating for continuing increases in width. Fragments continued to spall from the dome's irregular breadcrust-like surface. Noisy, episodic gas emission occurred around the base of the dome. Occasional low-level seismicity continued, but these episodes became briefer and less frequent through 19 and 20 October.

Minor activity — late October. No significant seismicity was recorded during the next several days. When visible through heavy weather clouds the summit of the lava dome appeared to have sagged further and the margins widened slightly, but there was no apparent increase in volume. On 25 October, a series of small, shallow seismic events between 1100 and 1130 accompanied the ejection of individual vapor plumes that reached 3.6 km altitude. The next day at 1720, a shallow volcanic earthquake of about M 2 was followed by several smaller shocks in the next few hours but no plumes were ejected. When weather conditions permitted in late October, measurements showed continued outward movement of the N crater rampart. Gas vents near the dome fumed vigorously through the end of October and cracks in the surrounding crater floor continued to widen very slowly.

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. Peterson, J. Dvorak, D. Mullineaux, C. Newhall, USGS, Vancouver, WA; R. Tilling, S. Russell-Robinson, USGS, Reston, VA; S. Malone; R. Crosson; E. Endo, University of Washington.