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Volcanowatch

March 11, 2004

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


Chronology of a Volcanic Disaster

The worst volcanic disaster of the 20th century occurred in 1902 on Martinique, an island in the French West Indies.

The north end of the island is dominated by Mount Pel_e, whose name??bald? or ?peeled? mountain?refers to the scarcity of vegetation at its summit when French colonists arrived in 1635. Its baldness was in marked contrast to the lush vegetation that characterized the rest of the island. Sugar cane thrived in the rich volcanic soil and became the foundation of the island?s economy. Twenty large sugar mills and 113 rum distilleries were in operation by 1902.

The first eruption of Mount Pel_e witnessed by colonists was phreatic (steam-driven) and occurred in 1792.

Other volcanic activity?also phreatic but more vigorous and more sustained?occurred from the summer of 1851 into January 1852. Sulfurous fumes were more common, explosions were larger and more numerous, water appeared in the previously dry crater, and the flow of water in a local river increased. Saint-Pierre, the chief city of Martinique, was dusted with ash.

On the scale of volcanic magnitude, the 1792 and 1851-52 activity barely registered. When Mount Pel_e reawakened in 1902, citizens of Saint-Pierre expected the volcano to mimic its past sluggish behavior, and they assumed that their city offered a safe haven from any lethal volcanic activity.

In late April 1902, earthquakes were felt in Saint-Pierre, and phreatic explosions began on Mount Pel_e. Within days, the vigor of the explosions exceeded anything witnessed since the island was settled. The intensity then subsided for a few days. Such ?roller coaster? behavior is common when long dormant volcanoes reawaken.

The roller coaster explosions increased again?to a higher level?as the eruptions returned on May Day. Lightning laced the eruption clouds, and trade winds dumped ash on villages to the west. Heavy ashfall at times caused total darkness, breathing was difficult, and domestic animals cried out in terror. Some of the afflicted residents panicked and headed for the perceived safety of larger settlements, especially Saint-Pierre, about 10 km (6 miles) south of Pel?e?s summit. Saint-Pierre received its first ashfall on May 3.

Mount Pel_e was relatively quiet for most of the next two days. But on the afternoon of May 5, a mudflow swept down a river on the southwest flank of the volcano, destroying a sugar mill. The massive flow crushed 23 people and generated a series of three tsunamis as it hit the sea. The tsunamis swept along the coast, damaging buildings and boats.

The explosions resumed the night of May 5. The following morning, parts of the eruption plume became incandescent, signifying that the character of the eruption had changed. The phreatic explosions had finally given way to magmatic explosions as magma reached the surface. The explosions continued though the next day and night.

A brief lull was shattered by a tremendous explosion at about 8:00 a.m. on May 8. A ground-hugging cloud of incandescent lava particles suspended by searing turbulent gases moved at hurricane speed down the southwest flank of the volcano, reaching Saint-Pierre at 8:02 a.m. Escape from the city was virtually impossible. Almost everyone within the city proper?about 26,000 people?died horrifically, burned or buried by falling masonry. The hot ash ignited a firestorm, fueled by smashed buildings and countless casks of rum. Only two survived within the city, along with a few tens of people caught within the margins of the cloud. All survivors were badly burned.

The phenomenon that destroyed Saint-Pierre?unknown to science in 1902?is now called a pyroclastic flow and has been witnessed at many other volcanoes around the world. Pyroclastic flows are usually produced by volcanoes whose lavas have a high proportion of silica. Fortunately, Hawaiian lavas have rather low silica content and do not produce pyroclastic flows.

Activity update

Eruptive activity at the Pu`u `O`o vent of Kilauea Volcano continued unabated during the past week. Most lava flows have been at the lower and upper ends of the rootless shield complex along the Mother?s Day lava tube south of Pu`u `O`o. On March 4, a lava flow issued from vents at the south base of Pu`u `O`o and moved some 3 km (2 miles) southward; this flow has been sporadically active throughout the week. Vents within the crater of Pu`u `O`o remain incandescent. No active flows are on Pulama pali or the coastal flat below Paliuli, and no lava is entering the ocean.

No earthquakes were reported felt on the island during the past week.

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains very low, with no earthquakes located in the summit area since early February 19.

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Updated: March 16, 2004 (pnf)