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January 14, 1999

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

Popocatepetl Acts Up

Here's a quiz for all you volcano junkies: What's the second highest volcano in North America? Hint-it was named by the Aztecs and has been erupting on-and-off for the last four years. If you guessed Popocatepetl, you pass, and if you can pronounce it, go to the head of the class!

Popocateptl (Popo-ca-te'-petal) is a steep-sided stratovolcano that rises to an elevation of 5452 m (17,883 ft) above sea level and was popular with mountain climbers until recent activity put the summit off-limits. The snow-capped cone looms over Mexico City, 55 km (34 mi) to the east, and the city of Puebla, 45 km (28 mi) to the west. All told, more than 30 million people live within 80 km (50 mi) of the volcano.

In December 21,1994, Popocateptl produced its first ash emissions in 70 years, after a year of increasing seismicity and sulfur-dioxide emissions. Nineteen towns (about 50,000 people) on the northeast flank of the volcano were promptly evacuated, but most residents were allowed to return in less than a week. Episodic, low-level eruptions of ash continued through 1995. In March 1996, a new round of activity began with increased ash emissions and the growth of a lava dome in the summit crater. Explosions propelled ash several kilometers into the sky and ejected dense blocks of lava as far as 4 km (2.5 mi) from the summit. A large explosion on April 30, 1996, killed five climbers who had ignored warnings to stay off the mountain.

Intermittent explosions have continued to present. Most of these eruptive episodes were brief, lasting 15 minutes or less. On June 30, 1997, however, ash emissions continued for several hours, and the airport in Mexico City was closed for 12 hours until ash could be washed from the runways.

The most powerful explosions of this four-year-long eruptive sequence have occurred in the past two months. Explosions in late November and mid-December shattered windows, lifted tiles from roofs, and fractured brick walls in nearby towns. Falling ash ignited fires in pastures around the volcano, but no casualties were reported. Ash fell on Mexico City on December 21, forcing the airport to close for two hours.

As frightening as these explosive outbursts must have been to nearby residents, Popocatepetl has the potential to do much worse. About 30 eruptions have been recorded since A.D. 1345, the date of the earliest records left by the Aztecs. Most of these eruptions were relatively mild, producing moderate ashfall. Much more destructive Plinian-type eruptions (named after Pliny, the Roman historian who chronicled the eruption of Vesuvius that destroyed Pompeii in A.D. 79) have occurred roughly every 1,000-2,000 years. Plinian eruptions are characterized by violent explosions that propel ash to the stratosphere and result in voluminous deposits of pumice and ash. The last such eruption at Popocatepetl was around A.D. 800. The eruption destroyed nearby settlements and culminated in mudflows that blanketed the Puebla Valley.

An eruption of this magnitude today would endanger hundreds of thousands of lives. As long as the current activity persists, Popocatepetl will remain under the close scrutiny of volcanologists. Scientists from the U.S. Geological Survey and U.S. universities have worked with their Mexican counterparts to augment existing seismic, ground deformation, and geochemical monitoring efforts.

Eruption Update

The current eruption of Kilauea continues without change. Lava erupts from Pu`u `O`o and flows through a network of tubes from the vent to the sea. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the new land.

Two earthquakes were reported felt during the past week (January 7-13). Both occurred on January 9 and were located about 5 km southwest of Pahoa on the east rift zone of Kilauea. The first was at 10:53 p.m. and the second at 11:02 p.m., with depths of 1.4 km (0.9 mi) and 1 km (0.6 mi), and magnitudes of 2.2 and 2.4, respectively.

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Updated: 21 Jan 1999