Mountain's end a memorable one

People tend to remember the dates of important events in their lives. In addition to anniversaries and birth dates, volcanologists remember the dates of eruptions they witnessed — at least the big ones. June 12 stirs such memories in some HVO staff members. In 1991, the first explosive eruption of Mount Pinatubo occurred on June 12. This was a precursor to the climactic eruption of June 15 — the second largest eruption of the twentieth century.

Pinatubo had been sleeping for about 500 years before it began to stir. Aside from a few geologists, most people who lived around Mount Pinatubo in Luzon, Philippines, didn’t even know it was a volcano. The first vague hint of trouble arose in July 1990, when a small earthquake shook the Pinatubo area just a few hours after a magnitude-7.8 earthquake occurred about 100 km (62 miles) away. Other small earthquakes were felt around the volcano in succeeding days; a few weeks later a landslide was reported.

These early phenomena were not interpreted as eruption precursors, but as effects of the magnitude-7.8 earthquake. In mid-March of 1991, numerous small earthquakes were felt near Pinatubo, and on April 2 explosions occurred along an old fracture on the volcano’s north flank. These explosions ejected only old rocks and ash — there was no evidence of new magma. Fumaroles near the center of Mount Pinatubo began to belch dense jets of white steam. Portable seismometers brought in to monitor the activity revealed that tens to hundreds of small earthquakes were occurring near the volcano each day.

Volcanic unrest accelerated on June 3, when a small explosion occurred in the vicinity of the fumaroles. The steam plumes from the fumaroles became progressively darker as they carried an increasing quantity of ash. Seismicity escalated. A recently installed tiltmeter indicated that the volcano was tilting outward: it was inflating.

Seismicity and outward tilt increased until late afternoon on June 7, when a plume of steam and ash rose about 7 to 8 km above the volcano. At this point, the seismicity diminished and the tilting stopped. The next morning, a small lava dome was observed growing on the volcano’s north flank. This new lava was piling up where it exited the ground because it was too viscous to flow away. The growth of the lava dome was accompanied by increasing ash emissions, swarms of shallow earthquakes, and periods of continuous rhythmic seismicity known as harmonic tremor.

On the morning of June 12, a large explosive eruption — lasting 38 minutes — propelled a vertical column of ash more than 19 km (11 miles) above the volcano. Three more eruptions produced vertical ash columns: one late on June 12th, one on the 13th, and one on the 14th. Each had a shorter duration, and each produced less ash than its predecessor.

Each of the first four eruptions sent ash to great heights above the volcano. The ash was then dispersed by winds as it fell through the atmosphere, depositing relatively thin layers of ash over a very broad area.

Yet another eruption occurred on the afternoon of June 14, but its character was different than the first four. Instead of rising to a great height and producing vertical fallout of dispersed ash, the eruption column formed a fountain of ash and gas. As the mixture of ash and hot gas poured down around the vent, it formed ground-hugging clouds that moved away from the volcano in all directions at high speed. This phenomenon — known as a pyroclastic surge — incinerates and blasts everything in its path.

Twelve more surge-producing eruptions occurred, with successively shorter repose intervals between them, each successively smaller than its predecessor.

All of this had just been the warm-up act for the main event, which began in the early afternoon of June 15. In a little more than three hours, about 5 cubic km (1.2 cubic miles) of ash was erupted — about half of it as pyroclastic flows within 17 km (10.5 miles) of the volcano and the other half as widely dispersed ash. As material erupted from the magma chamber, the volcano collapsed, producing countless earthquakes, many large enough to be felt in Manila, 90 km (56 miles) to the southeast. The eruption cloud reached a height of 35-40 km (22-25 miles) above sea level and released about 20 megatons of sulfur dioxide into the stratosphere. This gas forms an aerosol that reflects sunlight. As a result, global warming was slowed for several years.

Now, 14 years later, what had been a mountain is a crater lake 2.5 km (1.5 miles) in diameter.

Activity Update

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater and on the southwest side of the cone.

The PKK lava tube continues to produce intermittent surface flows from above the top of Pulama pali to the ocean. Three ocean entries were active as of June 9. The largest is at East Lae`apuki, with smaller entries at Kamoamoa and East Kamoamoa. Surface flows are intermittently active inland of the entries. The East Lae`apuki entry is the closest activity to the end of Chain of Craters Road, in Hawai`i Volcanoes National Park, and is located about 4.5 km (3 miles) from the ranger shed. Expect a 2-hour walk each way and bring lots of water.

Stay well back from the sea cliff, regardless of whether there is an active ocean entry or not. Remember — the beaches that sometimes form next to an active bench are just as dangerous as the bench itself. Stay off both, and heed the National Park warning signs.

During the week ending June 8, no earthquakes were reported felt on Hawai`i Island.

Mauna Loa is not erupting. During the week ending June 8, eight earthquakes were recorded beneath the summit area. Four were deep and long-period in nature. Inflation continues, but at a slightly reduced rate over the last few weeks.

This article was written by scientists at the U.S. Geological Survey’s Hawaii Volcano Observatory and is republished by with permission.

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