Krakatoa is an island of Palau between Java and Sumatra. In 1883 it erupted as one of the most catastrophic explosions ever seen by man. The volcano has been built from the ocean bottom over the last 1 million years. It's a cone-shaped mountain composed of flows of volcanic rock alternating with layers of cinder and ash. It was 7,000 feet high before the 1883 eruption. During the eruption the top was submerged to form a caldera 4 miles across. Portions of the crater rise up as the islands of Lang (NE), Verlaten (NW), and Krakatau (S). Krakatau formed after the eruption as the island of Rakata and two others grew and merged.
In 1680 a moderate explosion occured as the only signifigant one we know of before the big bang. On May 20, 1883 one cone (island) became active. Explosions were heard 100 miles away in Djakarta, and ash-laden clouds reached 6 miles up. By May the activity had died down. Then, on June 11th, the eruptions began again, and by August 26th they were paroxysmal. The lava was so hot it warmed the oceans 10 miles away. The explosions were increasingly violent starting at 1:00 p.m. and at 2:00 p.m. a black ash cloud rose 17 miles over Krakatoa. The climax occured at 10:00 a.m. on August 27th (my birthday), with explosions heard as far as 2,891 miles away in Austrailia and Madagascar. They were the loudest noises ever, and they were heard over 1/13th of Earth's surface. With the force of 67,000 atomic bombs, they shot ash 50 miles in the air. Pressure waves in the atmosphere were recorded around Earth. Then the explosions diminished, and by August 28th the volcano was quiet. Near the volcano, floating pumice halted boats and it was nighttime for 2 1/2 days afterwards. Small eruptions continued in the following months and in February of 1884, a discharge threw rock over nearly 5 cubic miles and lots of ash fell over 300,000 square miles.
Fine dust from the big one circled the Earth several times and made for spectacular red, blue, and green sunsets the rest of the year. Only 2,667 feet were left of Krakatau. Two hundred feet of ash and pumice piled on Verlaten and Lang islands and the remains of Rakata. The ash from the explosion was 90% new magma, which means the island wasn't blown up, it just sank into where the new stuff came from. It triggered a series of tsunamis that were recorded in Hawaii and South America The 120-ft. waves hurled 600-ton coral blocks, wrecked 165 costal villages, and travelled 4370 miles in 12 hours, whereas it would take a good steamer 12 days. (Keep in mind it's a 19th century steamer). The tsunamis killed 36,000 in Java and Sumatra, and all life on Krakatoa was under ash. Plants returned there in five years.
It was quiet until Dec. 29, 1927, when an eruption in the sea floor occured. By Jan. 26, 1928, a growing cone reached sea level, in between the three islands. It was named Anak Krakatau (Little Krakatau) and it still erupts today at over 1,000 feet.
The risk assessment is that there isn't much risk. Anak Krakatoa erupts almost continuously, with very minor and "moderate to not dangerous" explosions. During longer eruptions, a 3.1 mile radius danger zone is formed around the island to discourage travel there. The greatest risk is to tourists and fishermen who go too close while it's rumbling. In 1993 one tourist was killed and five others were injured. These are the only casualties to date. The volcano works on an eruption cycle of hundreds or thousands of years, which means the only way it poses a major threat, is if it erupts without warning, but that's unlikely. Anak Krakatau will probably have to grow more before a big eruption, also. Don't look for an 1883-like explosion in the near future.
