krakatau

The Child Grows

Volcanoes typically build up in fits and starts. After 1883, Krakatau was dormant until 1927, when a new cone began to build at the edge of the 1883 caldera. In 1928 alone, 22,358 earth tremors were registered by a temporary volcanological observatory on the island of Panjang, while eruption plumes were seen to reach 1,200 meters high. However, the first three attempts to break the surface formed only temporary ash-cone islands, which were destroyed by marine erosion and submarine sliding on the steep slope of the submerged caldera. The second of these islands was visited by a party of scientists in May 1929, shortly before it slipped back into the sea. They collected several insects, including a large black cricket and a brown ant. The present island, the fourth, emerged in August 1930, and over the next few years, it alternated between periods of quiet and rapid growth. At times, particularly in the early 1930s, the eruptions produced huge quantities of fine ash, which, aided by the alternating monsoon winds, have been deposited on the developing forests of Panjang and Sertung islands, a few kilometers distant (in opposite directions), causing extensive damage. By September 1932, the island was 47 meters high and had a typical low wide crater some 700 meters in diameter.

By 1933, the height had increased to 67 meters, and by 1941, it had doubled again to reach 132 meters. The 1940s was a period of relatively little activity, but in 1952 and 1953, the volcano sprang back into action, producing large quantities of ejecta. This was produced in such quantities and to such heights (clouds reaching 4,000 meters at times) that once again it was carried across to Panjang and Sertung islands, damaging as much of 90 percent of their forests to a degree visible from boats passing through the group. On Anak Krakatau itself, the colonization of plants and animals was abruptly halted by these eruptions, as the vegetation was more or less eliminated. A new, high inner cone formed, that was about 500 meters in diameter, enclosing a new crater lake. The records then indicate a further period of inactivity, until 1958, with further ash eruptions occurring over the period to 1963, during which some traces of ash reached sites on the coast of Java, some 40 kilometers distant.

krakatau

So, the emergence of Anak Krakatau is part of a repeating pattern. But why at this point in the Sunda Strait? The answer to this question is that Krakatau is just one of a series of volcanoes that run down through Sumatra and Java. They mark the meeting point of two tectonic plates -- great slabs of the Earth's crust -- in a destructive plate margin. Here, the Indo-Australian plate is subducted under the Asian plate, and as it descends below the overriding plate, some of the rock melts, and the molten magma rises through the crust above, forming a line of volcanic mountains. It has long been appreciated that the orientation of the line of volcanoes running along the long axes of Java and Sumatra changes sharply at Krakatau. In addition, a shorter line of volcanoes and volcanic islands can be found within the Sunda straits running from the southern tip of Sumatra to the western tip of Java.

The interpretation for these features is that the subduction of crust is faster along the Sumatra trench than along the Javan trench, with the change occurring abruptly at the Sunda Strait. In other words, it marks a major tectonic break, a hinging point by which Sumatra is gradually rotating clockwise out of line with Java. Krakatau thus marks a focal point in a bigger tectonic picture, and one that is liable to continue to be a center of cycles of volcanic activity long into the future.

krakatau

Anak Krakatau

Verbeek, in his report on the eruption, predicted that any new activity would manifest itself in the region which had been between Perboewatan and Danan. This prediction came true on 29 December 1927, when evidence of a submarine eruption was seen in this area (an earlier event in the same area had been reported in June 1927). A new island volcano, named Anak Krakatau ("Child of Krakatoa"), rose above the waterline a few days later. The eruptions were initially of pumice and ash, and that island and the two islands that followed were quickly eroded away by the sea. Eventually a fourth island named Anak Krakatau broke water in August 1930 and produced lava flows faster than the waves could erode them. Of considerable interest to volcanologists, this has been the subject of extensive study.

Current activity

Anak Krakatau has grown at an average rate of five inches (13 cm) per week since the 1950s. The island is still active, with its most recent eruptive episode having begun in 1994. Quiet periods of a few days have alternated with almost continuous Strombolian eruptions since then, with occasional much larger explosions.

The most recent eruption began in April 2008, when hot gases, rocks, and lava were released. Scientists monitoring the volcano have warned people to stay out of a 3 km zone around the island. On 6 May 2009 the Volcanological Survey of Indonesia raised the eruption alert status of Anak Krakatau to Level Orange.

Krakatau

At some point in prehistory, an earlier caldera-forming eruption occurred, leaving as remnants Verlaten, Lang, Poolsche Hoed, and the base of Rakata. Later, at least two more cones (Perboewatan and Danan) formed and eventually joined with Rakata, forming the main island of Krakatoa. The dating of these events is currently unknown; the Sunda Strait was first mentioned by Arab sailors around 1100 AD.

416 AD event

The Javanese Book of Kings (Pustaka Raja) records that in the year 338 Saka (416 AD):
“ A thundering sound was heard from the mountain Batuwara [now called Pulosari, an extinct volcano in Bantam, the nearest to the Sunda Strait] which was answered by a similar noise from Kapi, lying westward of the modern Bantam [Bantam is the westernmost province in Java, so this seems to indicate that Krakatoa is meant]. A great glowing fire, which reached the sky, came out of the last-named mountain; the whole world was greatly shaken and violent thundering, accompanied by heavy rain and storms took place, but not only did not this heavy rain extinguish the eruption of the fire of the mountain Kapi, but augmented the fire; the noise was fearful, at last the mountain Kapi with a tremendous roar burst into pieces and sank into the deepest of the earth. The water of the sea rose and inundated the land, the country to the east of the mountain Batuwara, to the mountain Rajabasa [the most southerly volcano in Sumatra], was inundated by the sea; the inhabitants of the northern part of the Sunda country to the mountain Rajabasa were drowned and swept away with all property[8] ... The water subsided but the land on which Kapi stood became sea, and Java and Sumatra were divided into two parts. ”

There is no geological evidence of a Krakatoa eruption of this size around that time; it may describe loss of land which previously joined Java to Sumatra across what is now the narrow east end of the Sunda Strait; or it may be a mistaken date, referring to an eruption in 535 AD, for which there is some corroborating historical evidence.[9]

535 AD event

David Keys, Ken Wohletz, and others have postulated that a violent volcanic eruption, possibly of Krakatoa, in 535 may have been responsible for the global climate changes of 535-536.[9] Keys explores what he believes to be the radical and far-ranging global effects of just such a putative 6th-century eruption in his book Catastrophe: An Investigation into the Origins of the Modern World. Additionally, in recent times, it has been argued that it was this eruption which created the islands of Verlaten, Lang, and the beginnings of Rakata—all indicators of early Krakatoa's caldera's size. To date, however, little datable charcoal from that eruption has been found.

Thornton (p. 47) mentions that Krakatoa was known as "The Fire Mountain" during Java's Cailendra dynasty, with records of seven eruptive events between the 9th and 16th centuries. These have been tentatively dated as 850, 950, 1050, 1150, 1320, and 1530 (all AD/CE).

1680

In February 1681, Johann Wilhelm Vogel, a Dutch mining engineer at Salida, Sumatra (near Padang), on his way to Batavia (modern Jakarta) passed through the Sunda Strait. In his diary he wrote:
“ ...I saw with amazement that the island of Cracketovv, on my first trip to Sumatra [June 1679] completely green and healthy with trees, lay completely burnt and barren in front of our eyes and that at four locations was throwing up large chunks of fire. And when I asked the ship's Captain when the aforementioned island had erupted, he told me that this had happened in May 1680 ... He showed me a piece of pumice as big as his fist. ”

Vogel spent several months in Batavia, returning to Sumatra in November 1681. On the same ship were several other Dutch travelers, including Elias Hesse, who would be called a travel writer nowadays. Hesse's journal reports that on
“ the 19th [of November 1681] we again lifted anchor and proceeded first to the north of us to the island of Sleepzie [ Sebesi ], uninhabited, ...[here he tells of a legend about crying ghosts, which actually were orangutangs ], and then still north of the island of Cracatou, which erupted about a year ago and also is uninhabited. The rising smoke column of this island can be seen from miles away; we were with our ship very close to shore and we could see the trees sticking out high on the mountain, and which looked completely burned, but we could not see the fire itself. ”

Vogel returned to Amsterdam in 1688 and published the first edition of his journal in 1690.

These reports of an eruption in 1680-81 pose somewhat of a puzzle. These are the only two reports of an eruption that have been found to date, yet at the time, the Sunda Strait was one of the heaviest-traveled waterways in the world. Records for this time period are particularly detailed, because there was an intense effort to wipe out pirates that were preying on vessels in the Strait. Neither Vogel nor Hesse mention Krakatoa in any real detail in their other passings, and no other travelers at the time mention an eruption or evidence of one. (In November 1681, a pepper crop was being offered for sale.) Both Van den Berg and Verbeek conclude from this that Vogel must have exaggerated the extent of the eruption he saw. Even so, there must have been an eruption around this time, since in 1880, Verbeek investigated a fresh unweathered lava flow at the northern coast of Perboewatan, which could not have been more than a couple of centuries old.

[edit] Visit by HMS Discovery

In February 1780, the crews of HMS Resolution and HMS Discovery, on the way home after Captain James Cook's death in Hawaiʻi, stopped for a few days on Krakatoa. They found two springs on the island, one fresh water and the other hot. They described the natives who then lived on the island as "friendly" and made several sketches. (In his journal, John Ledyard calls the island "Cocoterra".)

Dutch activity

In 1620 the Dutch set up a naval station on the islands and somewhat later a shipyard was built. Sometime in the late 1600s an attempt was made to establish a pepper plantation on Krakatoa but the islands were generally ignored by Dutch colonial authorities. In 1809 a penal colony was established at an unspecified location which was in operation for about a decade. By the 1880s the islands were without permanent inhabitants; the nearest settlement was the nearby island of Sebesi (about 12 km away) with a population of about 3000.

Several surveys and charts were made, but mainly for the purpose of mariners, and the islands were little explored or studied. An 1854 map of the islands was used in an English chart, which shows some difference from a Dutch chart made in 1874. In July 1880, Rogier Verbeek made an official survey of the islands but he was only allowed to spend a few hours there. He was able to collect samples from several places and his investigation proved important in judging the geological impact of the 1883 eruption.[10]:9

Wikipedia.org

krakatau

The renowned volcano Krakatau (frequently misstated as Krakatoa) lies in the Sunda Strait between Java and Sumatra. Collapse of the ancestral Krakatau edifice, perhaps in 416 AD, formed a 7-km-wide caldera. Remnants of this ancestral volcano are preserved in Verlaten and Lang Islands; subsequently Rakata, Danan and Perbuwatan volcanoes were formed, coalescing to create the pre-1883 Krakatau Island. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan volcanoes, and left only a remnant of Rakata volcano. This eruption, the 2nd largest in Indonesia during historical time, caused more than 36,000 fatalities, most as a result of devastating tsunamis that swept the adjacent coastlines of Sumatra and Java. Pyroclastic surges traveled 40 km across the Sunda Strait and reached the Sumatra coast. After a quiescence of less than a half century, the post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former cones of Danan and Perbuwatan. Anak Krakatau has been the site of frequent eruptions since 1927.

krakatau

Krakatau volcano lies in the Sunda strait between the islands of Java and Sumatra. In about 416 A.D., caldera collapse destroyed the volcano and formed a 4-mile (7-km) wide caldera. The islands of Krakatau, Verlaten, and Lang are remnants of this volcano. The eruption and collapse of the caldera in 1883 produced one of the largest explosions on Earth in recorded time (VEI=6) and destroyed much of Krakatau island, leaving only a remnant. Since 1927, small eruptions have been frequent and have constructed a new island, Anak Krakatau (Child of Krakatau). Image courtesy of the Landsat Pathfinder Project.

In January of 1960 a group of scientist 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 from ash to boulders in size. This photograph shows a column of ash rising above the island. Rakata, a remnant of the volcano prior to the 1883 eruption, is in the background.

In 1960, Anak Krakatau had a minimum diameter of about one mile (1.5 km) and was 545 feet (166 m) tall. A crater on the south side of the island was 2,000 feet (600 m) in diameter and contain a growing cinder cone 300 feet (100 m) in diameter and 150 feet (50 m) high. The new cinder cone formed in about a month and can be seen just below the ash column. Verlaten Island, near the top of this photo, is a remnant of the volcano that was destroyed by the 416 A.D. caldera collapse.

Explosive, Vulcanian-type eruptions occurred at 1/2- to 10-minute intervals. The largest explosions produced turbulent clouds of ash and lapilli that rose 4,000 feet (1,200 m) above the vent. In this photo a column of ash is rising about 1,500 feet (450 m) above the crater. This episode of activity, which began in December of 1959, ended in 1963. Anak Krakatau has had at least nine episodes of activity since 1963, most lasting less than one year. The most recent episode began in March of 1994 and has continued to at least March of 1995 (last reported observation). Activity is similar to the 1959-1963 eruption.

krakatau

The Krakatau edifice grew as one or more stratovolcanoes of dominantly hypersthene-augite andesite composition. The geology of Krakatau has been described by Effendi and others (1985, 1986), who identified five main evolutionary periods. Period 1 was an early growth phase that included accumulation of lavas and pyroclastics. Period 2 was marked by caldera formation, accompanied by pyroclastic flows and partly welded tuffs (ignimbrites). The cones of Rakata, Danan, and Perbuwatan grew during the third period and were largely destroyed during the fourth period, which included the paroxysmal eruption of August 1883. Another growth phase (period 5) began by December 1927, when Anak Krakatau was first noticed in submarine eruption. ...

Several years of regional seismicity culminated in the famous caldera-forming eruption of August 1883 (Verbeek, 1886; Judd, 1888). Earthquakes were felt in west Java and eastern Sumatra 5-6 years before the 1883 eruption, and in northern Australia in the 3 years before the eruption. On 1 September 1880, a strong earthquake damaged the First Point lighthouse on the west end of Java (70 kilometers south-southwest of Krakatau). more earthquakes were felt at First Point on 9-10 May 1883 and at Katimbang (40 kilometers north-northeast) during 15-20 May 1883. None of the above-mentioned earthquakes is known to have been of volcanic origin and centered beneath Krakatau; more likely, they reflect a buildup and release of tectonic stress in the sunda Strait area.

A mild ash and steam eruption began from Perbuwatan on 20 May 1883, and similar eruptions continued for 3 months. At times several vents were in eruption simultaneously. On 11 August, vigorous ash-laden gas columns rose from the main crater of Perbuwatan, the former vent of Danan, and from the foot of Danan; in addition, "no fewer than eleven other foci of eruption could be observed on the visible portions of the island, from which smaller steam-columns issued and ejections of ash took place" (Judd, 1888, p.13). At least some of the early ash was basaltic, suggesting that intrusion of basaltic melt into the silicic reservoir may have played a role in initiating these eruptions (Francis and Self, 1983). The small eruptions intensified on 24 August 1883 and culminated in the famous, climactic explosion of 27 August 1883 (Verbeek, 1886; Judd, 1888; Simkin and Fiske, 1983).

Earthquakes and rumbling, not demonstrably from Krakatau, were noted in 1898 (Koninklijk Magnetisch en Meteorologisch Observatorium te Batavia, 1899). On 13 August 1913, a strong tectonic earthquake occurred in the Bantam-Krakatau region, damaging the First Point lighthouse. Fishermen where were on the islands of Krakatau reported that part of the caldera wall collapsed. They also reported that at 1000 hours a "column of fire" rose out of the crater, whereupon they heard a cannonlike sound and fled (Visser, 1920). There are no independent reports of eruptions of Krakatau in 1913, and we suspect that only a landslide occurred.

Eruptions from December 1927 to 1981, mostly of basaltic andesite, built the Anak Krakatau ("child of Krakatau") cone in the center of the 1883 caldera ...

The August 1883 eruption of Krakatau is often cited as a classic example of caldera formation by collapse following eruption of large volumes of pumice (Williams, 1941; Williams and McBirney, 1979; Self and Rampino, 1981, 1982; Francis and Self, 1983). However, other workers have suggested alternate mechanisms for formation of the Krakatau Caldera. Yokoyama (1981, 1982) concluded that the caldera formed by explosive destruction and reaming of the preeruption edifice, and Camus and Vincent (1983) and Francis (1985) favored an origin by large-scale collapse of the northern part of Krakatau Island (similar to the volcanic landslide at Mount St. Helens on 18 May 1980). Regrettable, much of the evidence is sumbmarine and inaccessible, but we are impressed by the similarity of Krakatau and other, better-exposed calderas (for example, Crater Lake) that are thought to have formed by simple collapse following voluminous pumice eruptions. The volume of magma erupted in the plinian eruption (9 cubic kilometers) is adequate to explain the caldera without invoking a landslide origin. ...