krakatau sunda strait tour

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PYROCLASTIC AIRFALL AND DEVASTING PYROCLASTIC FLOWS

Tsunamis were clearly responsible for most of the fatalities at Krakatau. However, ~4,500 deaths (over 10% of the total) have been attributed to falling tephra and hot pyroclastic flows. The amount of tephra generated is thought to be about 20 cubic kilometers, or twenty times that of the destructive Mt. St. Helens eruption in 1980. Near Sumatra, the Sunda Straits were clogged with so much debris that it looked like solid ground. Relief ships were unable to reach coastal communities like Telok Betong for weeks. Over the ensuing months, storms and high-tides would disperse thick banks of floating pumice beyond the Straits, into the Java Sea and Indian Ocean. Ships thousands of kilometers from Krakatau would report huge fields of this floating debris for months after the eruption. One such accumulation floated 8,170 km, until it reached Durban, South Aftica in September, 1884.

About 2000 of the corpses in southern Sumatra had severe burns, indicating that they had been scorched to death, peresumably from pyroclastic flows. Although the behavior of pyroclastic flows and surges over water is poorly contrained by direct observations, the evidence suggests that they can travel great distances over open water. One compeling feature of the Krakatau eruption is that the pyroclastic flows appear to have travelled an incredible 40 km across the Sunda Straits, where they remained hot enough to cause the burn-related fatalities on Sumatra. These same flows, however, were also recorded by several ships located at greater distances. On August 27, the Louden (see above) was located ~65 km north-northeast of Krakatau when it was struck by severe winds and tephra, and the W.H. Besse was located at ~80 km east-northeast of Krakatau when it was hit by hurricane-force winds, heavy tephra, and the strong smell of sulfur. At these greater distances, the pyroclastic flows were at lower temperatures so that the ships and crew survived.

How is it possible for pyroclastic flows to travel such great distances? Pyroclastic flows are hot mixtures of solid particles and expanding volcanic gases. While advancing over water, the base of the flow will conert the water to steam. The rapid expansion of water to vapor greatly enhances flow fluidization and inhibits the deposition of particles, particularly the low-density pumiceous particles, thus allowing the flow to travel tens of kilometers over flat oceanic waters. This mobility was first recognized during the 1902 eruption of a pyroclastic flow from Mt. Pelée, which destroyed the coastal city of St. Pierre, only to continue across open waters to overturn and burn ships anchored several kilometers offshore.

After travelling 40 km over the Sunda Straits, pyroclastic flows struck southern Sumatra with a vengence, remaining hot enough to incinerate entire villages and burn all vegetation before loosing impetus on the highly forested mountainsides. The wife of Controller Beyerinck from the Sumatra village of Ketimbang described her expereince on the morning of August 27, when the outermost edges of a pyroclastic flow enveloped her family and their acquantainces, killing some and sparing others:

"Suddenly, it became pitch dark. The last thing I saw was the ash being pushed up through the cracks in the floorboards, like a fountain. I turned to my husband and heard him say in dispair ' Where is the knife?' . . . I will cut all our wrists and then we shall be released from our suffering sooner.' The knife could not be found. I felt a heavy pressure, throwing me to the ground. Then it seemed as if all the air was being sucked away and I could not breathe. . . . I felt people rolling over me . . . No sound came from my husband or children . . . I remember thinking, I want to . . . go outside . . . . but I could not straighten my back . . . I tottered, doubled up, to the door . . . I forced myself through the opening . . . I tripped and fell. I realized the ash was hot and I tried to protect my face with my hands. The hot bite of the pumice pricked like needles . . . Without thinking, I walked hopefully forward. Had I been in my right mind, I would have understood what a dangerous thing it was to . . . plunge into the hellish darkness . . . I ran up against . . . branches and did not even think of avoiding them. I entangled myself more and more . . . My hair got caught up . . . I noticed for the first time that [my] skin was hanging off everywhere, thick and moist from the ash stuck to it. Thinking it must be dirty, I wanted to pull bits of skin off, but that was still more painful . . . I did not know I had been burnt." -- From A. Scarth, 1999.