# Volcano Watch — Mount St. Helens back in the news

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As most readers probably know, Mount St. Helens has reawakened after 18 years of repose. Here are some of the many questions HVO has received in the past few days, and the corresponding answers.

What has happened so far?

Earthquake activity (seismicity) rose above normal levels on September 23 and continued to increase over a period of days. A part of the crater floor-between the lava dome formed in the 1980s and the south crater wall-has been uplifted to about the height of the dome. Small steam and ash emissions have occurred from two small vents on the margin of the uplifted area. As of this writing, earthquakes have decreased. Magma is probably very near the surface.

Ok, so what is likely to happen? Monitoring tells us when a dormant volcano is reawakening, and when an eruption is imminent. But volcanologists can't predict the exact times, types, or magnitudes of eruptions that will occur. So they tabulate the nature and magnitude of various types of past eruptions and use the information to calculate the probabilities of various outcomes.

The range of possibilities at St. Helens is broad. There's about an 80 percent chance that a magmatic eruption will occur. If a magmatic eruption occurs, there's a 10 percent chance that there would be non-explosive dome growth and a 10 percent chance that a powerful explosive eruption would occur like the one on May 18, 1980, after the lateral explosion. There's also a 20 percent chance that activity could stop without an eruption.

What happened last time?

On May 18, 1980, after about a month-and-a-half of bulging and increased seismicity, the north flank became over steepened and it collapsed, producing an enormous landslide.

As the north flank slid away, it depressurized the magma that had accumulated inside the volcano. Gases dissolved in the magma suddenly expanded, and the magma exploded producing a huge, ground-hugging blast of rock particles and hot gas that swept out to the north, devastating 600 square kilometers (150 square miles) of mountainous, densely forested terrain. Just 5 minutes after the landslide began, the devastation was complete.

Shortly after the blast, a vertical eruption began that lasted about 9 hours. Ash rose to as much as 24 kilometers (15 miles) above the volcano, carried to the east-northeast by wind. Thousands of square kilometers in Washington, Idaho, and Montana were blanketed with ash. Pumice and ash flows reached Spirit Lake, 7 kilometers (4.5 miles) to the northeast.

The eruption blew the insides out of what had been a beautiful snow-capped volcanic cone, producing a crater more than a mile wide. Fifty-seven people were killed including David Johnston, a USGS scientist who was measuring the bulge rate from the first ridge north of the volcano about 9 kilometers (5.5 miles) away.

Smaller vertical eruptions occurred later in 1980 and eventually gave way to dome-building eruptions. The last of these occurred in October 1986.

Is there going to be another devastating blast?

A large lateral explosion like the one that occurred on May 18, 1980, is not likely because much of the pre-1980 volcano is gone precluding another flank collapse. Furthermore, deformation measurements show almost no change of the volcano's flanks but, rather on the crater floor. Explosive eruptions are likely to be directed upward instead of outward.

Some upward-directed explosions produce lethal ground-hugging clouds of ash and gas, when hot ash and pumice fountain back onto the volcano's flanks and flow down slope. Unlike lateral explosions, vertical explosions usually send out material in all directions. A given quantity of material erupted vertically typically does not go as far as the same quantity of material erupted laterally.

Stay tuned, we will most likely see a magmatic eruption at Mount St. Helens in coming days to months.

### Volcano Activity Update

Eruptive activity at Puu Oo continues weakly. The Banana flow is no longer active. Scattered breakouts are taking place within a wide expanse of the PKK flow east of the Banana flow, and one small tongue of lava has been moving down Pulama pali since September 22. The eruptive activity in Puu Oo's crater is weak, with sporadic minor spattering.

No earthquakes were reported felt on the island during the week ending October 6.

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity continues at a slightly higher level than during the past several weeks. Only 47 earthquakes were recorded beneath the summit area during the past week. Nearly all of the earthquakes of this ongoing activity are of long-period type, have magnitudes less than 3, and are deep, 40 km (23 miles) or more.