Event Name : Pavlof 1986/4
|Start:||April 16, 1986 ||Observed|
|Stop:||August 13, 1988 ||Observed|
|Lava flow: ||
|Lahar, debris-flow, or mudflow: ||
|Pyroclastic flow, surge, or nuee ardente: ||
|Flank eruption: ||
|Tephra plume: ||
|Central eruption: ||
|MaxVEI: ||3 ||
|Duration: ||About 28 months ||
|ColHeight: ||15000 m ||
|Eruption Product: || basaltic andesite ||
From McNutt and others (1991): "The visual observations of the 1986 eruptions, both aerial and on the ground, provide information generally lacking from previous historic eruptions. The physical characteristics of of the 1986 eruption are probably similar to other historic summit eruptions, although the 1986 activity was more explosive and of longer duration.
"The 1986 activity was chiefly Strombolian, characterized by sporadic emissions of dark ash to heights of up to 5 km; one exceptionally strong (probably Vulcanian) eruptive event sent an ash column to over 15 km on 18-19 April. The initial phase of the eruption appears to have involved the summit vent on the north side of the volcano which has been the site of all Pavlof eruptions since the mid-1960s. This eruptive phase lasted from 16 to 26 April and included a hot rootless agglutinate flow that extended down the northwest flank of the volcano. The deposit was actively degassing steam along its entire length in late June. It had an estimated volume of about 4x10^6 cubic meters and, at an elevation of 760 meters, was 20-30 meters thick and 40-50 meters wide. The deposit was clast-supported and fines-depleted, consisting entirely of glassy, slightly vesicular andesitic basalt (SiO2 = 53.5%) bombs and irregular masses of spatter with a maximum diameter of about 1.2 meters. It was probably emplaced during the initial and relatively violent vent-clearing phases of the eruption on 16-28 April. Associated hot lahars caused melting of snow and ice and extensive flooding in the Cathedral River drainage north of the volcano. Heavy ash fall occurred north and west of the volcano during the April activity; 2-3 mm of ash fell on the nearby communities of Cold Bay, 35 km to the west, and King Cove, 48 km to the southwest, but caused no damage.
"Eruptive activity began again in late May and was highlighted by sporadic, but vigorous Strombolian eruptive activity and by the formation of a new vent high on the east flank of the volcano. Comparison of aerial photography indicated that this is the first major change in the vent geometry of Pavlof since the early 1960s. The period of activity was characterized by repeated small bursts of ash and cinder to a few hundred meters above the vent and spatter tossed a few tens of meters all accompanied by explosive, thunderlike reports. The explosions characterizing the Strombolian eruption occurred at intervals of 5-15 seconds during the half dozen times the new vent was observed between 14 June and 30 June, suggesting a moderately rapid rate of magma rise in the conduit. Only minor ash emission was observed associated with the explosive ejection of incandescent bombs from the new vent. Activity from the old north vent during this time consisted of steam emission with little or no ash.
"Close airborne examination of the new vent on 15 June revealed that a steep-sided, asymmetrical spatter rampart, 50-75 m across, had been constructed on the east, or downslope, side of the vent. A steep chute, directly below the spatter rampart, contained a steaming, rootless rubble flow. Further down the volcano at an elevation of about 750 m, this flow was about 100 m wide and consisted of bread-crust-like bombs and irregular masses of lava and spatter, up to 4 m in diameter, in an ash-rich, moderately inflated matrix (in contrast to the flow on the northwest flank). It was actively degassing, exhibiting abundant steam fumaroles and occasional phreatic eruptions. The flow, with an estimated volume of 3.8 x 10^6 cubic meters, also generated a number of mudflows that continued downslope to about 600 m where the rubble flow-mudflow complex widened into three broad lobes. Below this elevation, debris-laden water from the flows was contained in a steep-sided small canyon.
"The nature of the material in the flow at this elevation and the physical characteristics of the spatter rampart at the vent strongly suggest that much of the lower part of the flow may have resulted from similar pyroclastic flow activity following partial collapse of the oversteepened spatter rampart. Throughout the course of the eruption, the steep spatter rampart may have periodically become unstable and collapsed, either in whole or part, forming hot, disaggregated pyroclastic flows cascading down the chute. Where ice and snow were overrun by the hot debris, mudflows were generated that continued to travel down and fan out on the volcano's lower slopes. In support of this hypothesis, a pyroclastic flow was observed on 19 June moving down the same chute from about 1400 m elevation to about 900 m; the upper 1100 m of the volcano was cloud-covered at this time.
"An alternative scenario is that the pyroclastic flow observed on 19 June resulted from a 'boil-over' of the magma column at the vent following a larger-than-usual explosion. Although no seismic evidence for such an explosion was observed, the obscuring cloud cover prevents an exact explanation for the origin o fthe observed pyroclastic flow.
"The April-August eruption is similar to, but somewhat stronger than most of the other Pavlof eruptions of this century (Simkin and others, 1981; McNutt 1987a) in terms of its Strombolian character. The duration of the eruption, as indicated by lava fountain activity and spatter ejection, however, appeared to be much longer than for most modern eruptions. This eruption also altered the physiography of the summit area by forming a new vent. In contrast to most recent eruptions (i.e., 1973-1983) that occurred in the fall of the year, the 1986 eruption was concentrated in the spring and summer."
McNutt and others (1991) also give detailed information about the seismological data collected during this eruption.
Information on the continuing volcanic activity in 1987 and 1988 is in other sources. From Reeder (1990, pg. 53): "Numerous observations of tephra and steam emissions were made during most of 1987 from NE and SE near summit vents and less from a NE flank vent. Lava flows occurred on both the NE flank and the SE flank of the volcano from the summit vents down to at least a 1,000 m a.s.l. elevation in January and February, form the NE summit vent in May and June, and again from both summit vents in August." Please see the rest of this text for detailed eruption observations in 1987.
From Reeder (1991): "Pavlof volcano has been fairly active during 1986 and 1987 with tephra and lava emissions occurring form several summit and flank vents. During 1988, eruptive activity was restricted to only small tephra and steam emissions from NE near summit vent up through August 13, except for one minor steam emission from a NE flank vent on March 2. Since August 13, 1988, no eruptive activity was observed for the rest of the year." Please see the rest of this text for detailed eruption observations in 1988.