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December 21, 2000

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Residents on slopes of Popocatepetl Volcano heed evacuation notice

For the past two weeks, numerous reports of increased seismic activity heralding a possible large eruption at Popocatepetl Volcano in Mexico have been in the news. A disturbing commentary in the reports was that residents living on the slopes of the volcano were hesitant to comply with the government's order to evacuate.

The reluctance of the residents to evacuate is understandable. They fear that they will lose their crops and animals if they are not tended. Popocatepetl, located 60 km (36 mi) southeast of Mexico City, came back to life six years ago after nearly 70 years of dormancy and has been erupting intermittently since then. The last large eruption occurred a year ago and caused the airport in Mexico City to close, but thus far, no casualties, except for five hikers near the 5,452-meter (17,883-ft) summit in 1996, have been reported. Many residents living on the lower slopes and base of the second highest volcano in North America feel that they are not in danger.

On Monday, December 18, the largest eruption of Popocatepetl in 1,200 years occurred and convinced people to leave immediately. Over 56,000 evacuees of the 40 villages within 12 km (7.2 mi) of the volcano crowded the roads heading away from the eruption.

Popocatepetl Volcano, locally called "Popo," is one of more than 400 active volcanoes surrounding the Pacific Basin. These volcanoes are located where two tectonic plates collide with one plate subducting beneath the other. Magma is generated in the subduction zone and rises through the mantle and crust to erupt and form a range of volcanoes. Many of these stratovolcanoes have the classical steep-sided and symmetrical shape that we envision for a volcano. Eruptions from these volcanoes are usually explosive and violent.

The shape and explosiveness of the Pacific Rim volcanoes can be attributed to the high silica content of the lavas they erupt. The higher the silica content of the magma, the more viscous it is. The more viscous the magma, the more difficult it is for gases to escape from it. The more gas within a magma, the more explosive is the eruption.

Explosive eruptions create great clouds of ash that can collapse and race down the steep slopes of the volcano as a suspension of gas and ash, attaining speeds up to 150 km/hr (90 mph). Such pyroclastic flows are also generated by the crumbling of a volcanic dome or by directed explosions.

Another hazard at Popo is the possibility of the eruptive material melting glacial ice on the volcano and causing a large mudflow or lahar. This is what killed over 25,000 people in Colombia during the eruption of Nevado del Ruiz Volcano in 1985.

The high viscosity of the magma increases the internal pressure of the volcano, and this higher pressure can cause the volcano's flank to fail. The climactic eruption of Mount St. Helens on May 18, 1980, started with a slope failure and a debris avalanche.

Unlike Hawaiian eruptions, Pacific Rim eruptions pose many life-threatening hazards. Residents of Mexico are wise to heed the evacuation warnings of government officials. They gamble with their own lives by not evacuating.

Eruption Update

Last week we reported that there was a slow deflation of the Kilauea summit region on December 13 and 14. The deflation, which lasted 17 hours and totaled nearly 5 microradians, was accompanied by intensified tremor in the caldera. Although there was no immediate effect of the subsidence on the eruption at Pu`u `O`o, a pause in activity was observed from late December 15 to the early morning hours of December 17. The pause in activity was nearly coincident with the reinflation of the summit region which started at 5:00 p.m. on December 15 and continued until December 17. The renewal of activity on December 17 caused lava to breach the tube system above Pulama pali and produce surface flows. Four incandescent tongues of lava are flowing down Pulama pali and presenting visitors at the end of the Chain of Craters road in Hawai`i Volcanoes National Park with a spectacular view. The flows have advanced up to nearly a kilometer (0.6 mi) from the base of the pali toward the sea coast, and no lava is entering the ocean at this time.

No earthquakes were reported felt during the week ending on December 21.

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Updated: January 4, 2001