End of May at Mt. St. Helens.

Profound morphological changes occurred at St. Helens following the eruption of May 18, 1980.

The debris avalanche, the lateral blast and the vertical explosion created a crater, pierced to the north, with an NS dimension of approximately 3 km and an EW dimension of approximately 1.5 km (slightly wider at the base of the breach).

The summit was destroyed. The maximum elevation of the volcano, on the edge of the crater, was about 350 m less than previously at 2,975 m. The lower end of the breach extended downward almost to the 1,500 m level.

Many lakes north of the volcano have been affected by the trees and ashes that have fallen there. Others formed: The Coldwater leke and the Castle lake were created when the waters supplying the northern branch of the Toutle river were dammed up by the avalanche of debris.

The Spirit lake experienced all the problems being directly in the focus of the blast, debris avalanches and pyroclastic flows. It has the water temperature rise to more than 32 ° C and its level of 70 meters. It can therefore be considered a completely different lake from the one before the eruption.

At the time of the 1980 eruption, 11 named glaciers were radiating along the sides of the volcano, as well as two small unnamed glaciers and many perennial snow fields. The largest glaciers extend about 2.5 (1.5 mi) from the ice-filled summit crater. The landslide and cataclysmic eruption of May 18, 1980 largely destroyed the glaciers that had existed on the slopes of Mount St. Helens, removing about 70% of the volcano's ice mass.

Eruptive activity decreased after May 18, 1980, and on May 21 was limited to episodic ejections from the crater, mainly vapor. Large fumaroles and secondary explosions were generated from the debris flow depot, sometimes producing columns of material up to 2 km. Between May 19 and 24, only a few earthquakes of magnitude greater than 3 were recorded, unlike the dozens of events that have occurred every day since the end of March. However, the harmonic tremor started during this period (exact date not reported).
 

At 2:32 a.m. on May 25, the amplitude of the harmonic tremor began to increase. Within minutes, an ash-rich eruption column was seen from a surveillance aircraft. At 2:45 a.m., Portland's NWS radar recorded the top of the plume nearly 14 km in height. A swarm of small earthquakes, centered about 8 km below the volcano, started at 2:49 am and continued at a rate of 1-2 / hour.

The density of ash in the eruption column began to decrease in 5 minutes and the height of the column decreased in the first hour. The winds were fairly variable, but much of the ash was blowing towards the W half of the compass. At 6 a.m., ash fell in the Portland-Vancouver area (80 km SW). The ash fall darkened early in the morning in the Kelso-Longview area (55 km W) and the ash cloud extended to the Olympic peninsula in northwest Washington. Heavy rains during the eruption mixed with the ashes to drop the mud over much of the affected area. Many airports have been closed and travel on the ground has been made difficult.

At 8 a.m., the amplitude of the harmonic tremors decreased and the swarm of earthquakes began to subside. However, the eruption continued for most of the day, with the altitude at the top of the column varying from 4 to 6 km. The rash subsided during the evening.

Most of the tephra ejected on May 25 is juvenile material. Certain pyroclastic flow deposits have been put in place on the northern flank.

Small glowing areas were seen on the bottom of the crater during the night of May 28-29 and several times thereafter. Careful inspection showed that the glow was caused by the heating of parts of the bottom of the crater by ventilation gases, not by the presence of magma on the surface.

The harmonic tremor continued, at variable amplitudes, until the beginning of June, but the seismic activity remained at very low levels.

To be continued next month

Source: CVO - USGS