Sun, 14 Jan 2001

Krakatau, an ancient volcano that is still active

By Ronald G. Pate

JAKARTA (JP): Krakatau is an ancient volcano located in the Sunda Strait between the islands of Java and Sumatra in the Indonesian archipelago.

Geological studies indicate that a massive antecedent volcano was destroyed by violent eruptions in 416 AD leaving a seven kilometer wide caldera. The present day islands of Krakatau, Verlaten, and Lang are remnants of that massive volcano.

Krakatau sits near the Java Trench, an active subduction zone where the oceanic plate beneath the Indian Ocean is moving northward, plunging beneath the overriding continental plate.

The collision of the two plates produces continuous seismic activity (earth-quakes) and heat. When the earthquakes are large enough, volcanism results, often with the extrusion of magma and gases.

The earliest recorded eruptions on Krakatau were in the years 1680 to 1681.

Then on May 20, 1883, a three-month long volcanic episode began which culminated in four gigantic explosions on Aug. 27. The third of these was the most violent explosion recorded in modern times.

Every seismograph in the world recorded the explosion, and the shock waves from the blast reverberated around the world thirteen times.

Before 1883, Krakatau was a virtually unknown island with a history of violent volcanic activity. At its peak, Krakatau reached an elevation 790 meters above sea level. The eruption blew away the northern two-thirds of the island and produced tidal waves up to 37 meters high, resulting in 36,000 deaths. A dust cloud exploded 50 miles (80 km) into the atmosphere and by Sept. 9, it had encircled the Earth, causing atmospheric effects that lasted for over a year.

The total energy from the explosion was equivalent to 200 megatons of TNT and it is estimated that 21,000 cubic meters of debris was ejected. When the eruption ended the next day, Krakatau had been reduced to a deep 250-meter crater and a remnant island.

Anak Krakatau

Since 1927, small eruptions have been frequent and a new volcanic island, Anak Krakatau (Child of Krakatau), has emerged. In January 1960 a group of scientists visited Anak Krakatau to record its renewed activity and to measure changes in the size and shape of the island.

They observed explosive eruptions of pyroclasts ranging in size from ash to boulders. At that time, the volcano had a minimum diameter of approximately 1.5 km and an elevation of 166 m. A crater was present on the south side of the island which was 600 meters in diameter and contained a growing cinder cone 100 meters in diameter and 50 meters high. That volcanic episode ended in 1963.

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Image courtesy of Landsat

Since then, there have been at least 10 volcanic episodes, most lasting less than one year.

Krakatau has been the object of much scientific scrutiny because of its history and continued activity. It has also become a site for adventure tours of curious tourists who make excursions to the island from Anyer and Merak.

In 1993, an eruption caused an explosion that killed one tourist and injured five more. For this reason, tourists interested in visiting the island should inquire at the Indonesian Geological Survey about the current levels of seismic activity before venturing to the area.

In May 1997, Mike Lyvers, a volcanologist, visited Anak Krakatau and observed relatively quiet periods lasting for a couple of days, punctuated by periods of nearly continuous eruption.

He reported that eruptions consisted of minor ash emissions, accompanied at times by a few bombs (pyroclasts). He reported occasional larger explosions that sent incandescent ash high into the sky, generating spectacular displays of volcanic lightning and covering the cone with glowing bombs.

Interested observers do not have to visit the island to witness these phenomena because they are often visible from beaches on the west end of Java.

More recently, Anak Krakatau erupted on Feb. 5, 1999 with an explosion that could be heard up to 60 km away. The next day, four more explosions were heard and smoke from a vent rose up to 300 meters. On Feb. 7, smoke emissions continued, and numerous explosions (as many as 20) were heard during the day's activity.

Visitors to the island should select their transportation carefully. Make sure there are life preservers onboard for everyone. Take plenty of water, sun block and a lunch.

On a good boat and with calm seas, the trip should take less than two hours from Anyer. On rough seas or with an underpowered vessel, the trip can take from three to four hours. However, should the boat's engine malfunction, you could spend a very uncomfortable night, depending on the season (whether there are other boats making the trip).

Sources of Information: Tectonics of the Indonesian Region by Warren Hamilton, Geological Survey Professional Paper, 1078; Bulletin of Volcanology, V.20, no.3. Results of the 1960 Expedition to Krakatau by Robert Decker and Djajadi Hadikusumo, 1961; Krakatau, Bulletin of the Global Volcanism Network,1995; Seismic Bulletin, Smithsonian Institute.